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Android User Interface
Development
Beginner's Guide

Quickly design and develop compelling user interfaces for
your Android applications

Jason Morris

BIRMINGHAM - MUMBAI

Android User Interface Development
Beginner's Guide

Copyright © 2011 Packt Publishing

All rights reserved. No part of this book may be reproduced, stored in a retrieval system,
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Packt Publishing has endeavored to provide trademark information about all of the
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Packt Publishing cannot guarantee the accuracy of this information.

First published: February 2011
Production Reference: 1160211

Published by Packt Publishing Ltd.
32 Lincoln Road
Olton
Birmingham, B27 6PA, UK.
ISBN 978-1-849514-48-4
www.packtpub.com

Cover Image by Charwak A (charwak86@gmail.com)

Credits
Author
Jason Morris
Reviewers
David J. Groom
Martin Skans
Acquisition Editor
Chaitanya Apte
Development Editor
Reshma Sundaresan
Technical Editor
Harshit Shah
Copy Editor
Neha Shetty
Indexer
Tejal Daruwale

Editorial Team Leader
Akshara Aware
Project Team Leader
Priya Mukherji
Project Coordinator
Shubhanjan Chatterjee
Proofreader
Joel T. Johnson
Graphics
Nilesh R. Mohite
Production Coordinators
Kruthika Bangera
Aparna Bhagat
Cover Work
Kruthika Bangera

About the Author
Jason Morris has worked on software as diverse as fruit tracking systems, insurance
systems, and travel search and booking engines. He has been writing software for as long
as he can remember. He is currently working as a Software Architect for Travelstart in South
Africa. He works on multiple front-end and middleware systems, leveraging a variety of Java
based technologies.
The people I'd like to thank most for their direct, or indirect help in writing
this book are my wife Caron Morris, my father Mike Morris, my mom Jayne
Morris, and the rest of my family for their love and support. I'd also like
to thank Wayne, Stuart, Angela, and James, and everyone on my team at
Travelstart. Finally a very big thanks to Martin Skans for his invaluable input.

About the Reviewer
Martin Skans graduated from Lund University in Sweden, with a Master's degree in
Computer Science. After a couple of years in the online marketing industry, he moved on to
become a developer for Travelstart, an online travel agency. He relocated to Cape Town and
is currently working on Travelstart's African travel platform which has been recently launched
for the mobile market.

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Table of Contents
Preface
Chapter 1: Developing a Simple Activity
Developing our first example
Creating the project structure
Time for action – setting up the Android SDK
Time for action – starting a new project
Examining the Android project layout
Time for action – running the example project
The screen layout
The layout XML file
Resource selection qualifiers

Time for action – setting up the question activity
Populating a View and a ViewGroup
Time for action – asking a question
Time for action – adding a space for answers
Time for action – adding more buttons
Defining common dimensions
Limitations of the layout XML format
Populating the QuestionActivity
Time for action – writing more Java code
Dynamically creating widgets
Time for action – putting the questions on the screen
Handling events in Android
Summary

Chapter 2: Presenting Data for Views
Listing and selecting data
ListView choice modes
No selection mode – CHOICE_MODE_NONE
Single selection mode – CHOICE_MODE_SINGLE
Multiple selection mode – CHOICE_MODE_MULTIPLE

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Table of Contents

Adding header and footer widgets
Creating a simple ListView
Time for action – creating a fast food menu
Styling the standard ListAdapters
Defining standard dimensions

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Time for action – improving the restaurant list
Creating custom adapters
Creating a menu for The Burger Place

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Time for action – creating a Burger item layout
Time for action – presenting Burger objects
Creating TheBurgerPlaceActivity class
Time for action – implementing TheBurgerPlaceActivity
Registering and starting TheBurgerPlaceActivity
Using the ExpandableListView class
Creating ExpandableListAdapter implementations
Using the GridView class
Time for action – creating the fruit icon
Displaying icons in a GridView
Time for action – building the fruit menu
Time for action – creating the FourBucketsActivity
Summary

Chapter 3: Developing with Specialized Android Widgets
Creating a restaurant review application
Time for action – creating the robotic review project structure
Building a TabActivity
Creating tab icons
Android tabs and icons
Implementing the ReviewActivity
Time for action – writing the ReviewActivity class
Time for action – creating the Review layout
Working with switcher classes
Time for action – turning on the TextSwitcher
Creating a simple photo gallery
Time for action – building the Photos tab
Creating a thumbnail widget
Implementing a GalleryAdapter
Time for action – the GalleryAdapter
Time for action – making the gallery work
Building the reservation tab
Time for action – implementing the reservation layout
Time for action – initializing the reservation tab
[ ii ]

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Table of Contents

Time for action – listening to the SeekBar
Time for action – selecting date and time
Creating complex layouts with Include, Merge, and ViewStubs
Using Include tags
Merging layouts
Using the ViewStub class
Summary

Chapter 4: Leveraging Activities and Intents
Exploring the Activity class
Using Bundle objects
Time for action – building an example game: "guess my number"
Creating and consuming intents
Defining Intent actions
Passing data in an Intent
Adding extra data to an Intent
Using advanced Intent features
Getting data back from an Intent
Time for action – viewing phone book contacts
Summary

Chapter 5: Developing Non-linear Layouts
Time for action – creating a layouts example project
FrameLayout
Common uses
Time for action – developing a FrameLayout example
Table Layout
Common uses
Using TableLayout for a memory game
Time for action – developing a simple memory game
AbsoluteLayout/Custom Layouts
Developing your own Layouts
Time for action – creating a custom layout
Using the CircleLayout
Time for action – finishing the CircleLayout example
RelativeLayout
Common uses
Integrating the RelativeLayout
Time for action – creating a contact editor
Time for action – integration with the layout example
SlidingDrawer
Common uses
[ iii ]

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Table of Contents

Creating a SlidingDrawer example
Time for action – creating a SlidingDrawer
Time for action – sliding drawer integration
Summary

Chapter 6: Validating and Handling Input Data
Dealing with undesirable input
Correctly labeling input
Signaling undesirable input
Recovering from undesirable input
Giving users direct feedback
Avoiding invalid input entirely
Capturing date and time
Using spinners and ListView for selection
Changing the data set
Disabling selections

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Capturing text input
Autocompleting text input
Building activities for results
Generic filtering search Activity
Time for action – creating the ListItemSelectionActivity
Time for action – creating an ArrayAdapter
Time for action – creating the CursorAdapter
Time for action – setting up the ListView
Time for action – filtering the list
Time for action – returning the selection
Using the ListItemSelectionActivity
Summary

Chapter 7: Animating Widgets and Layouts
Using standard Android animations
Time for action – animating a news feed
Using flipper and switcher widgets
Using the ImageSwitcher and TextSwitcher implementations
Animating layout widgets
Time for action – animating a GridView
Creating Custom Animations
Time for action – writing a custom animation
Time for action – making a Button vanish
Summary

Chapter 8: Designing Content-centric Activities
Considering design options when displaying content on an Android device
[ iv ]

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Table of Contents

Considering user behavior

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Drawing user attention

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Displaying content with the WebView class
Using a WebView object
Time for action – creating a recipe viewer application
Taking WebView further
Creating relative layouts for content display
Taking full advantage of RelativeLayout
Considering Android layout constraints
Styling TextView objects
Time for action – developing specialized content views
Developing an online music store
Designing the music store
Developing the music store
Time for action – building a track item
Time for action – developing the main user interface layout
Time for action – developing the main user interface Java code
Summary

Chapter 9: Styling Android Applications
Working with style resources
Using shape resources
How shapes behave

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Rendering lines

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Time for action – drawing a broken line
Rendering rectangles
Time for action – creating a rounded border
Rendering ovals
Time for action – applying a gradient to an oval shape
Rendering rings
Time for action – rendering a spinner ring
Defining layers
Stretching using nine-patch images
Creating nine-patch images
Using bitmap images in Android
Handling different screen sizes
Handling different screen densities
Handling configuration changes
Providing landscape layouts
Providing text input on a landscape layout
Altering screen content
Summary
[v]

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Table of Contents

Chapter 10: Building an Application Theme
Creating a basic calculator layout
Designing a standard calculator
Time for action – building the standard calculator
Building the calculator styling
Time for action – creating the button images
Time for action – styling the calculator buttons
Time for action – styling the display
Scientific landscape layout
Defining string resources for the scientific layout
Styling the scientific layout
Building the scientific layout
Time for action – coding the scientific layout
Handling the Activity restart
Supporting hardware keyboards
Adding in display animations
Time for action – animating the display
Summary

Appendix: Pop quiz answers

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Chapter 1
Layouts as XML fles
Populating an activity
Chapter 2
List views and adapters
Chapter 3
Gallery objects and ImageViews
Chapter 4
Intents & Activities
Chapter 5.
Custom layouts
Chapter 6
Text input
Chapter 8
The WebView widget
WebView versus native layouts
Chapter 10
Layout resources
Nine-Patch Images
Android resources

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Index

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[ vi ]

Preface
On 9th January, 2007, Apple officially launched the iPhone, and the world of user interface
design shifted. While tablet PCs had been around for a while, the iPhone was the first device
to give so many people a portable touchscreen, and people loved it. Just over a year later,
Google and the Open Handset Alliance announced Android which in many ways is the direct
competitor to iPhone.
What is it about touchscreen phones that we love? The answer is simple—feedback.
Touchscreens offer a way to directly manipulate on-screen objects, which in the past had to
be driven through a keyboard, mouse, joystick, or other input device. The touchscreen model
of direct manipulation has a large impact on the way we think about our user interfaces as
developers, and changes the expectations a user has for the application. Touchscreen devices
require us to stop thinking in terms of forms, and start thinking about object-oriented user
interfaces.
Android is used as the primary operating system for a rapidly expanding range of consumer
electronics, including:
‹‹

Smartphones

‹‹

Netbooks

‹‹

Tablets

‹‹

Some desktop systems

While all of these devices have different purposes and specifications, all of them run
Android. This is unlike many other operating environments which are almost always have a
special purpose. The services and the APIs they provide to developers generally reflect their
target hardware. Android on the other hand makes the assumption that a single application
may be required to run on many different types of devices, with very different hardware
capabilities and specifications, and makes it as easy as possible for developers to handle the
differences between these devices simply and elegantly.

Preface

New challenges

As Android and the touchscreen devices it powers become increasingly common, they will
bring a new set of challenges to user interface design and development:
‹‹

You generally don't have a mouse

‹‹

You may have more than one pointing device

‹‹

You often don't have a keyboard

‹‹

Any keyboard that does exist may be a software keyboard

‹‹

A software keyboard may consume some of your application's screenspace

The software keyboard reduces the amount of screen space available to your application,
and in much the same vein, if there is a hardware keyboard present it may or may not always
be exposed to the user. Therefore, not only are different Android devices different, but they
may also appear to change features while your application is running.

The rule of finger
Most Android devices have touchscreens (although this is not a requirement). The first
restriction placed on any touchscreen user interface is the size of the human forefinger,
which of course varies widely from one person to another. If a widget is too small on the
screen, it won't be clear what the user is trying to touch. You'll notice that most Android
widgets take up plenty of space, and have more than the normal amount of padding around
them. On a touchscreen device, you can't rely on pixel-perfect precision. You need to make
sure that when the user touches a widget, they make contact, and they don't accidentally
touch another widget.

The magic touch
Another impact touchscreens have on user interface design is that an application and all the
widgets that it uses must be entirely self-explanatory (even more than usual). Far too often,
we substitute good user interface planning and design with a roll-over or tooltip to indicate
a widget's function. On a touchscreen device, there is no mouse or pointing device. The first
interaction it has with the user is when they touch it, and they will expect something to happen.
A touchy subject
Most Android devices have a touchscreen, but it's not a requirement. The quality of
a touchscreen also varies wildly from device to device. The category of touchscreens
and their capabilities will also vary from one device to the next, depending on the
intended use of the device and often its intended market segment.

[2]

Preface

A smaller view on the world
Most Android devices are small, and as a result have smaller screens and generally fewer
pixels than a normal PC or laptop. This lack of size limits the size of the widgets. Widgets
must be big enough to touch safely, but we also need to pack as much information onto the
screen as possible. So don't give your users information that they don't want, and also avoid
asking them for information you don't need.

Classic user interface principals

Here are some core guidelines which every user interface should follow. These guidelines
are what will keep your users happy, and ensure your application is successful. Throughout
the rest of the book, we'll be walking through these guidelines with practical examples of
improvements that can be made to a user interface.

Consistency
This is the cornerstone of good user interface design. A button should look like a button.
Make sure that the layout of each screen has a relationship with every other screen in your
application. People often mistake this principle for "stick to the platform look and feel". Look
and feel is important, consistency mostly applies to the layout and overall experience of the
application, rather than the color scheme.

Recycling your interface
The easiest way to maintain a consistent user interface, is to recycle as much of it as possible.
At first glance, this suggestion looks merely like a "good object-oriented" practice. However,
a closer look will reveal ways to reuse graphical widgets in ways you hadn't thought of. By
changing the visibility of various widgets, or you can reuse an edit screen to view list items
of the intended type.

Simplicity
This is especially important in a phone-based application. Often, when a user encounters a
new application, it's because they are looking for something. They may not have the time
(or more often patience) to learn a new user interface. Make sure that your application
asks for as little as possible, and guides the user to the exact information they want in as
few steps as possible.

[3]

Preface

The Zen approach
Generally, when you are using a mobile device, your time is limited. You may also be using
an application in less-than-ideal circumstances (perhaps, in a train). The lesser information
a user needs to give an application, and the lesser they need to absorb from it, the better.
Stripping away options and information also leads to a shorter learning-curve.

Android's hidden menu
A very useful feature of Android is the hidden menu structure. The menu is only visible
when the user presses the "Menu" button, which would generally mean, they're looking
for something that isn't currently on the screen. Typically, a user shouldn't need to open a
menu. However, it's a good way of hiding advanced features until they are needed.

Feedback
Feedback is what makes a touchscreen device exciting. When you drag an object, it sticks to
your finger across the screen until you let go of it. When the users puts their finger on your
application, they expect some reaction. However, you don't want to get in their way—instead
of showing an error message when they touch a button, disable the button until it's valid to
use, or don't show it at all.

Location and navigation
When you're in a place you've never been to previously, it's easy to get disoriented, or lost.
The same is true for a piece of software. Just because the application makes sense to you,
the developer, it doesn't mean it seems logical to your user. Adding transition animations,
breadcrumbs, and progress gauges help the user to identify where in the application they
are, and what's happening.

The road to recovery
A common way to tell users that something is wrong on a desktop application, or on the web
is to open an error dialog. On a mobile device, people want smoother use of an application.
While in a normal application you may inform the user that they selected an invalid option,
in a mobile application, you generally want to make sure they can't select that option in the
first place. Also, don't make them scroll through huge lists of options. Instead, allow them to
filter through the list using an auto-complete or something similar.
When something goes wrong, be nice, and be helpful—don't tell the user, "I couldn't find any
flights for your search". Instead tell them, "There were no available flights for your search,
but if you're prepared to leave a day earlier, here is a list of the available flights". Always
make sure your user can take another step forward without having to go "Back" (although
the option to go backwards should always exist).
[4]

Preface

The Android way

The Android platform is in many ways similar to developing applications for the web.
There are many devices, made by many manufactures, with different capabilities and
specifications. Yet as a developer, you will want your users to have the most consistent
experience possible. Unlike a web browser, Android has built-in mechanisms for coping with
these differences, and even leveraging them.
We'll be looking at Android from the point of view of a user rather than having a purely
development-centric approach. We'll cover topics such as:
‹‹

What user interface elements Android provides

‹‹

How an Android application is assembled

‹‹

Different types of Android layouts

‹‹

Presenting various types of data to the user

‹‹

Customising of existing Android widgets

‹‹

Tricks and tools to keep user interfaces looking great

‹‹

Integration between applications

We're about to take a jump into building user interfaces for Android devices—all Android
devices, from the highest speed CPU to the smallest screen.

What this book covers

Chapter 1, Developing a Simple Activity introduces the basics of building an Android
application, starting with a simple user interface. It also covers the various options available
to you when implementing your design as code.
Chapter 2, Views With Adapters shows us how to leverage Adapter-based widgets, Android's
answer to the Model-View-Controller (MVC) structure. Learn about these widgets, and
where they will best serve you.
Chapter 3, Specialized Android Views takes a close look at some of the more specialized
widgets that the Android platform provides, and how they relate to the mundane widgets.
This chapter covers widgets such as the gallery and rating-bar, and how they can be used and
styled.
Chapter 4, Activities and Intents discusses more about how Android runs your application,
and from that point-of-view, how best to write its user interfaces. This chapter takes a look at
how to make sure that your application will behave the way users expect it to, with minimal
effort on your part.

[5]

Preface

Chapter 5, Non-Linear Layouts takes a look at some of the advanced layout techniques which
Android offers. It talks about the best way to present different screens to the user while
taking into account the wide discrepancy in the screens on Android devices.
Chapter 6, Input and Validation provides tips regarding taking input from a user, and how
to keep this experience as painless as possible. This chapter investigates the different input
widgets Android provides and how to configure them best, depending on the situation. Also,
when everything else fails, how best to inform your users that what they are doing is wrong.
Chapter 7, Animating Widgets and Layouts will inform the reader as to where, when,
why, and how to animate your Android user interfaces. It also sheds light on what kind of
animations are provided by default, how to compose them together, and how to build your
own. This chapter looks at the importance of animations in a mobile user interface and
demonstrates how complex animations are made easy by Android.
Chapter 8, Content-centric Design details how to go about designing the screen layout, when
presenting the user with information on the screen. This chapter looks at the pros and cons
of some of the different display techniques which Android offers.
Chapter 9, Styling Android Applications shows us how to keep the look of our entire
application consistent, in order to make our application easier to use.
Chapter 10, Building an Application Theme looks at the design process, and how applicationwide themes can be applied to help your application stand out.

What you need for this book

Please have a look at "System Requirements" mentioned on the Andriod Developers website
at http://developer.android.com/sdk/requirements.html.
The code for this book was tested on Ubuntu Linux 10.04 and Mac OS X.

Who this book is for

This book is aimed at developers with at least some Java experience who want to build
applications on the Android platform. It will also be of use to people who have developed
applications on the Android platform and would like to gain additional knowledge about
its user interface design. It will also be a helpful reference for the numerous widgets and
resource structures that the Android platform provides.

[6]

Preface

This book will also be helpful to:
‹‹

Java developers learning Android development

‹‹

MIDP developers looking to broaden their skill-set

‹‹

iPhone developers wanting to port applications

‹‹

Entrepreneurial Android developers wanting to widen their user base

Conventions

In this book, you will find several headings appearing frequently.
To give clear instructions of how to complete a procedure or task, we use:

Time for action – heading
1.

Open the res/layout/main.xml layout resource in an editor or IDE.

2.

Remove the default content within the LinearLayout element.

Instructions often need some extra explanation so that they make sense, so they are
followed with:

What just happened?
This heading explains the working of tasks or instructions that you have just completed.
You will also find some other learning aids in the book, including:

Pop quiz – heading
These are short multiple choice questions intended to help you test your own understanding.

Have a go hero – heading
These set practical challenges and give you ideas for experimenting with what you have learned.
You will also find a number of styles of text that distinguish between different kinds of
information. Here are some examples of these styles, and an explanation of their meaning.
Code words in text are shown as follows: "We'll start off by creating a selector Activity,
and a simple NewsFeedActivity".

[7]

Preface

A block of code is set as follows:







When we wish to draw your attention to a particular part of a code block, the relevant lines
or items are set in bold:







Any command-line input or output is written as follows:
android create project -n AnimationExamples -p AnimationExamples -k com.
packtpub.animations -a AnimationSelector -t 3

New terms and important words are shown in bold. Words that you see on the screen, in
menus or dialog boxes for example, appear in the text like this: "Generally users are more
inclined to feel a sense of trust if they pick the Buy Music button and are not suddenly
whisked off to their web browser".
Warnings or important notes appear in a box like this.

[8]

Preface

Tips and tricks appear like this.

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Downloading the example code for this book
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Although we have taken every care to ensure the accuracy of our content, mistakes do
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Errata section of that title. Any existing errata can be viewed by selecting your title from
http://www.packtpub.com/support.
[9]

Preface

Piracy
Piracy of copyright material on the Internet is an ongoing problem across all media. At Packt,
we take the protection of our copyright and licenses very seriously. If you come across any
illegal copies of our works, in any form, on the Internet, please provide us with the location
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Questions
You can contact us at questions@packtpub.com if you are having a problem with any
aspect of the book, and we will do our best to address it.

[ 10 ]

1

Developing a Simple Activity
In the world of Android, an Activity is the point at which you make contact
with your users. It's a screen where you capture and present information to the
user. You can construct your Activity screens by using either: XML layout files
or hard-coded Java.

To begin our tour of Android user interfaces, we need a user interface to start with. In this
chapter, we will begin with a simple Activity. We will:
‹‹

Create a new Android project

‹‹

Build the Activity layout in an application resource file

‹‹

Tie the resource file to an Activity class

‹‹

Dynamically populate the Activity with a series of multiple-choice questions

Developing our first example
For our first example, we're going to write a multiple-choice question and answer Activity.
We could use it for applications such as "Who wants to be a millionaire?", or "What type of
a monkey are you?". This example will pose questions in order to answer a very important
question: "What should I have to eat?" As the user answers the questions, this application
will filter a database of food ideas. The user can exit the process at any time to view a list of
suggested meals, or just wait until the application runs out of questions to ask them.
Since it's a user interface example, we'll skip building filters and recipe databases. We'll just
ask our user food preference-related questions. For each question, we have a list of preset
answers which the user can select from (that is, multiple-choice questions). Each answer
they give will allow us to narrow the list of suitable recipes.

Developing a Simple Activity

Creating the project structure
Before we can start writing code, we need a project structure. An Android project
is made up of far more than just its Java code—there are also manifest files, resources,
icons, and more. In order to keep things easy, we use the default Android toolset
and project structure.
You can download the latest version of the Android SDK for your favorite operating system from
http://developer.android.com. A single Android SDK may be used to develop against any
number of target Android versions. You will need to follow the installation instructions on the
website at http://developer.android.com/sdk/installing.html to install the latest
SDK "starter package" and one or more platform targets. Most of the examples in this book will
work on Android 1.5 and higher. The Android website also maintains a very useful chart where
you can see what the most popular versions of Android are.

Time for action – setting up the Android SDK
Once you have downloaded the Android SDK archive for your operating system, you'll need
to install it and then download at least one Android Platform package. Open a command-line
or console and complete the following steps:

1.

Extract the Android SDK archive.

2.

Change directory to the root of the unpackaged Android SDK.

3.

Change directory to the tools directory of the Android SDK.

4.

Update the SDK by running the following command:
android update sdk

5.

Create a new Virtual Device by going to the Virtual Devices screen and clicking on
the New button. Name the new Virtual Device default.

6.

Specify its target as the most recent version of Android downloaded by the SDK. Set
the size of the SD Card to 4096 MiB. Click on the Create AVD button.

What just happened?
The above command tells the new Android SDK installation to look for available packages
and install them. This includes installing a Platform Package. Each Platform Package that you
install can be used to create an Android Virtual Device (AVD). Each AVD you create is much
like buying a new device on which tests can be performed, each with its own configuration
and data. These are virtual machines that the Android emulator will run your software on
when you wish to test.
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Download from Wow! eBook 

Chapter 1

Time for action – starting a new project
The Android SDK provides a handy command-line tool named android which can be used
to generate the skeleton of a new project. You'll find it under the tools directory of your
Android SDK. It's capable of creating a basic directory structure and a build.xml file (for
Apache Ant) to help get you started with your Android application development. You will
need to make sure that the tools directory is in your executable path for this to work. Open
a command-line or console.

1.

Create a new directory in your home directory or desktop named
AndroidUIExamples. You should use this directory for each of the examples
in this book.

2.

Change the directory to the new AndroidUIExamples.

3.

Run the following command:
android create project -n KitchenDroid -p KitchenDroid -k com.packtpub.
kitchendroid -a QuestionActivity -t 3

What just happened
We just created a skeleton project. In the preceding command line, we used the following
options to specify the structure of the new project:
Option
-n

Description

-p

Gives the base directory for the project. In this case use the same name as that of the
project. The android tool will create this directory for you.

-k

Specifies the root Java package for the application. This is a fairly important concept
since it defines our unique namespace on the Android client devices.

-a

Gives the tool a name for a "main" Activity class. This class will be populated
with a skeleton layout XML, and serves as a base point to build your application from.
The skeleton project will be pre-configured to load this Activity when it's started.

Gives the project a name, in our case, KitchenDroid. This is really just an internal
identifier for the project.

If you run the command android list targets and it presents you with an empty list of
possible targets, then you have not downloaded any of the Android Platform packages. You
can generally run the android tool by itself and use its graphical interface to download and
install Android Platform packages. The previous example uses API Level 3 which corresponds
to Android Platform version 1.5.

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Developing a Simple Activity

Examining the Android project layout
A typical Android project has almost as many directories and files as an enterprise Java
project. Android is as much of a framework as it is an operating environment. In some ways,
you can think of Android as an application container designed for running on phones and
other limited devices.
As part of the new project structure, you will have the following important files and directories:
Folder name
bin
gen
res
src
AndroidManifest.xml

Description
Your binary files will be placed in this directory by the compiler.
Source code generated by various Android tools.
Application resources go here, to be compiled and packaged with
your application.
The default Java source code directory, where the build script
will look for source code to compile.
Your application descriptor, similar to a web.xml file.

Resource Types and Files
Most types of application resources (placed in the res directory) receive special
handling by the Android application packager. This means these files consume
less space than they usually would (since XML is compiled into a binary format
instead of being left as plain text). You access resources in various ways, but
always through an Android API (which decodes them into their original form for
you).
Each subdirectory of res indicates a different file format. Therefore, you cannot
put files directly into the root res directory since the package tool won't know
how to handle it (and you'll get a compile error). If you need to access a file in its
raw state, put it in the res/raw directory. Files in the raw directory are copied
byte-for-byte into your application package.

Time for action – running the example project
The android tool has given us a minimal example of an Android project, basically a "Hello
World" application.

1.

In your console or command-line, change directory to KitchenDroid.

2.

To build and sign the project, run:
ant debug

3.

You will need to start the emulator with the default AVD you created earlier:
emulator -avd default
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Chapter 1

4.

Now install your application in the emulator:
ant install

5.

In the emulator, open the Android menu and, you should see an icon named
QuestionActivity in the menu. Click on this icon.

What just happened?
The Android emulator is a full hardware emulator including the ARM CPU, hosting the entire
Android operating system stack. This means software running under the emulator will run
exactly how it will on bare-metal hardware (although the speed may vary).
When you use Ant to deploy your Android applications, you will need to use the install
Ant target. The install Ant target looks for a running emulator and then installs the
application archive on its virtual memory. It's useful to note that Ant will not start the
emulator for you. Instead, it will emit an error and the build will fail.
Application Signatures
Every Android application package is digitally signed. The signature is used to
identify you as a developer of the application, and establish permissions for the
application. It's also used to establish permissions between applications.
You will generally use a self-signed certificate, since Android doesn't require that
you use a certificate authority. However, all applications must be signed in order
for them to be run by the Android system.

The screen layout
While Android allows you to create a screen layout in either Java code, or by declaring the
layout in an XML file, we will declare the screen layout in an XML file. This is an important
decision for several reasons. The first is that, using the Android widgets in Java code requires
several lines of code for each widget (a declaration/construction line, several lines invoking
setters, and finally adding the widget to its parent), while a widget declared in XML takes up
only one XML tag.

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Developing a Simple Activity

The second reason for keeping layouts as XML is that it's compacted into a special Android
XML format when it's stored in the APK file. Therefore your application uses less space on
the device, takes less time to download, and its in-memory size is also smaller since less byte
code needs to be loaded. The XML is also validated by the Android resource packing tool
during compilation, and so is subject to the same type safety as Java code.
The third reason XML layouts are a "good idea" is that they are subject to the same resource
selection process as all the other external resources. This means that a layout can be varied
based on any of the defined properties, such as language, screen orientation and size, and
even the time of day. This means that you can add new variations on the same layout in
the future, simply by adding new XML files, and without the need to change any of your
Java code.

The layout XML file
All XML layout files must be placed in the /res/layout directory of your Android
project in order for the Android packaging tools to find them. Each XML file will result
in a resource variable of the same name. For example, if we name our file /res/layout/
main.xml, then we can access it in Java as R.layout.main.
Since we are building the screen layout as a resource file, it will be loaded by the application
resource loader (having been compiled by the resource compiler). A resource is subject to a
selection process, so while there is only one resource that the application loads, there may
be multiple possible versions of the same resource available in the application package. This
selection process is also what Android internationalization is built on top of.
If we wanted to build a different version of the user interface layout for several different
types of touchscreens, Android defines three different types of touchscreen properties for
us: notouch, stylus, and finger. This roughly translates to: no touchscreen, resistive
touchscreen, and capacitive touchscreen. If we wanted to define a more keyboard-driven
user interface for devices without a touchscreen (notouch), we write a new layout XML file
named /res/layout-notouch/main.xml. When we load the resource in our Activity
code, the resource selector will pick the notouch version of the screen if the device we're
running on doesn't have a touchscreen.

Resource selection qualifiers
Here is a list of commonly used qualifiers (property names) that will be taken into account
when Android selects a resource file to load. This table is ordered by precedence, with the
most important properties at the top.

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Chapter 1

Name

Description

Examples

MCC and MNC The mobile-country-code (MCC) and mobile-networkcode (MNC). These can be used to determine which
mobile operator and country the SIM card in the device is
tied to.
The mobile-network-code optionally follows the mobilecountry-code, but cannot be used on its own (you must
always specify country-code first).

API
Level

mcc505
1
mcc505-mnc03
mcc238
mcc238-mnc02
mcc238-mnc20

en
en-rUS
es
es-rCL
es-rMX

1

small
medium
large

4

Screen aspect This is the aspect type of the screen, based on the way
the device would "normally" be used. This value doesn't
change based on the orientation of the device.

long
notlong

4

Screen
orientation

Used to determine whether the device is currently in
portrait (port) or landscape (land) mode. This is only
available on devices that can detect their orientation.

land
port

1

Night mode

This value simply changes with the time of day.

night
notnight

8

Language and Language and region codes are probably the most
region codes commonly used resource properties. This is generally
how you localize your application to the user language
preferences.
These values are standard ISO language and region
codes, and are not case-sensitive. You cannot specify a
region without a country code (similar to java.util.
Locale).
Screen size

There are only three variations of this property: small,
medium, and large. The value is based on the amount of
screen space that can be used:
‹‹

Small: QVGA (320×240 pixel) low-density type
screens;

‹‹

Medium: WQVGA low-density, HVGA (480x360
pixels) medium-density, and WVGA high-density
type screens;

‹‹

Large: VGA (640x480 pixels) or WVGA mediumdensity type screens

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Developing a Simple Activity

Name

Description

Screen density The DPI of the device screen. There are four possible
(DPI)
values for this property:

Keyboard
status

‹‹

ldpi: Low-density, approximately 120dpi;

‹‹

mdpi: Medium-density, approximately 160dpi;

‹‹

hdpi: High-density, approximately 240dpi;

‹‹

nodpi: Can be used for bitmap resources that
shouldn't be scaled to match the screen density

Examples

API
Level

ldpi
mdpi
hdpi
nodpi

4

keysexposed
What sort of keyboard is available on this device? This
keyshidden
attribute shouldn't be used to determine whether the
keyssoft
device has a hardware keyboard, but instead whether a
keyboard (or software keyboard) is currently visible to the
user.

1

Time for action – setting up the question activity
To kick things off we're going to be working with Android's simplest layout called:
LinearLayout. Unlike Java AWT or Swing, Android layout managers are defined as
specific container types. Thus a LinearLayout is much like a Panel with a built-in
LayoutManager. If you've worked with GWT, you'll be quite familiar with this concept. We'll
lay out the screen in a simple top-to-bottom structure (which LinearLayout is perfect for).

1.

Open the file in the /res/layout directory of your project named main.xml in
you favorite IDE or text editor.

2.

Delete any template XML code.

3.

Copy the following XML code into the file:




What just happened?
We just removed the "Hello World" example, and put in an entirely empty layout structure
which will serve as the platform for us to build the rest of the user interface upon. As you can
see, Android has a special XML namespace for its resources.
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Chapter 1

All resource types in Android use the same XML namespace.

We declare our root element as LinearLayout. This element corresponds directly to
the class android.widget.LinearLayout. Each element or attribute prefixed with
the Android namespace corresponds to an attribute that is interpreted by the Android
resource compiler.
The AAPT (Android Asset Packaging Tool) will generate an R.java file into your root (or
primary) package. This file contains all of the Java variables used to reference your various
application resources. In our case, we have the main.xml package in the /res/layout
directory. This file becomes an R.layout.main variable with a constant value assigned
as its identification.

Populating a View and a ViewGroup
A widget in Android is called a View, while a container (such as LinearLayout) is a
ViewGroup. We have an empty ViewGroup now, but we need to start populating it in
order to build up our user interface. While it is possible to nest a ViewGroup inside another
ViewGroup object, an Activity has only one root View—so a layout XML file may have
only one root View.

Time for action – asking a question
In order to ask our user a question, you will need to add a TextView to the top of your
layout. A TextView is a bit like a Label or JLabel. It's also the base class for many other
Android View widgets that display text. We want it to take up all of the available horizontal
space, but only enough vertical space for our question to fit. We populate the TextView
with Please wait... as its default text. Later, on we will replace this with a dynamically
selected question.

1.

Go back to your main.xml file.

2.

Between the  and  create a  element, ending it with the empty element /> syntax since elements
representing View objects are not allowed to have child elements.

3.

Give the TextView element an ID attribute:
android:id="@+id/question"

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Developing a Simple Activity

4.

Change the layout width and height attributes to fill_parent and wrap_
content respectively (the same as the LinearLayout element):
android:layout_width="fill_parent"
android:layout_height="wrap_content"

5.

Give the TextView some placeholder text so we can see it on the screen:
android:text="Please wait..."

6.

Reinstall the application using Apache Ant from your project root folder:
ant install

7.

Run the application again in the emulator and it should look like the following
screenshot:

The code for the TextView should end up looking something like this:


What just happened
In the preceding example, we used fill_parent and wrap_content as values for the layout
width and height attributes. The fill_parent value is a special value that is always equal to
the parent size. If it's used as the value for the android:layout_width attribute (as in our
example), then it's the width of the parent view. If it's used in the android:layout_height
attribute, it would be equal to the height of the parent view instead.
The value wrap_content can be used much like a preferred size in Java AWT or Swing.
It says to the View object, "Take as much space as you need to, but no more". The only
valid place to use these special attribute values is in the android:layout_width and
android:layout_height attributes. Anywhere else will result in a compiler error.

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Chapter 1

We will need to access this TextView in our Java code later, in order to invoke its setText
method (which directly corresponds to the android:text attribute we used for the
placeholder text). A Java reference to a resource variable is created by assigning the resource
an ID. In this example, the ID is declared here as @+id/question. The AAPT will generate
an int value as an identifier for each resource of id as part of your R class. The ID attribute
is also needed for accessing resources from another resource file.

Time for action – adding a space for answers
While posing a question to the user is all very fine and well, we need to give them some
way to answer that question. We have several possibilities at our disposal: We could use a
RadioGroup with a RadioButton for each possible answer, or a ListView with an item
for each answer. However, to minimize the required interaction, and make things as clear
as possible, we use one Button for each possible answer. However, this complicates things
slightly, since you can't declare a variable number of Button objects in your layout XML file.
Instead, we will declare a new LinearLayout and populate it with Button objects in the
Java code.

1.

Under the TextView where we pose our question, you will need to add a
 element. While this element would normally have child
elements, in our case, the number of possible answers is varied, so we leave it as an
empty element.

2.

By default, a LinearLayout will place its child View objects horizontally alongside
each other. However, we want each child View to be vertically below each other, so
you'll need to set the orientation attribute of the LinearLayout:
android:orientation="vertical"

3.

We will need to populate the new ViewGroup (LinearLayout) later in our Java
code, so give it an ID: answers:
android:id="@+id/answers"

4.

Like our TextView and root LinearLayout, make the width fill_parent:
android:layout_width="fill_parent"

5.

Make the height wrap_content so that it doesn't take up more space than all the
buttons it will be populated with:
android:layout_height="wrap_content"

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Download from Wow! eBook 

Developing a Simple Activity

The resulting code should look like this:


What just happened?
You may have noticed that for this example, we have no content in our new LinearLayout.
This may seem a little unusual, but in this case, we want to populate it with a variable
number of buttons—one for each possible answer to our multiple-choice questions.
However, for the next part of the example we need some simple content Button widgets
in this LinearLayout so that we can see the entire screen layout in action. Use the
following code in your layout resource file to add Yes!, No!, and Maybe? Button widgets
to the LinearLayout:






In Android XML layout resources, any View classes extending from the ViewGroup class
are considered containers. Adding widgets to them is as simple as nesting those View
elements inside the element of your ViewGroup (as opposed to closing it with no child
XML elements).

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Chapter 1

The following is a screenshot of the preceding Yes!, No!, Maybe? options:

Time for action – adding more buttons
We have two additional buttons to add to the screen layout. One will allow the user to skip
the current question; the other will allow them to look at the short list of meals that we have
filtered through so far (based on the questions they have already answered).

1.

Start by creating an empty  element below our answers ViewGroup
 (but still within the root LinearLayout element). Assign it
the ID skip, so that we can reference it in Java:
android:id="@+id/skip"

2.

Create some padding between the answers and the new button by using a margin:
android:layout_marginTop="12sp"

3.

Give it the display label Skip Question:
android:text="Skip Question"

4.

Like all of the previous widgets, the width should be fill_parent and the height
should be wrap_content:
android:layout_width="fill_parent"
android:layout_height="wrap_content"

5.

Now create another empty  element below the Skip Question button

6.

The ID for the new button should be view:
android:id="@+id/view"

7.

We want this button to display the text: Feed Me!:
android:text="Feed Me!"

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Developing a Simple Activity

8.

Again, put a little space between the Skip Question button, and the new
Feed Me! button:
android:layout_marginTop="12sp"

9.

Finally, set the width and height of the Feed Me! button as with the other elements
we've created so far:
android:layout_width="fill_parent"
android:layout_height="wrap_content"

When you've completed these two buttons, your layout XML file should now end with:




What just happened
Separation of unrelated user interface objects is a very important part of user interface
design. Groups of items can be separated by whitespace, a border, or a box. In our case, we
chose to use whitespace, as space also helps make the user interface feel cleaner.
We created our whitespace by using a margin above each of the buttons. Margins and
padding work exactly the same way as they (should) do in CSS. A margin is spacing outside of
the widget, while padding is spacing inside the widget. In Android, a margin is the concern of
the ViewGroup, and so its attribute name is prefixed with layout_. Because padding is the
responsibility of a View object, the padding attribute has no such prefix:


The previous code would create extra space between the edge of the Button and the text in
the middle of it, as well as retaining the margin above the button.
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Chapter 1

All of the measurements in the preceding example are specified in the sp unit, which is short
for "scale independent pixels". Much like CSS, you suffix your measurement numbers
with the unit of size that you are specifying the measurement in. Android recognizes the
following measurements:
Unit suffix
px

Full name

Description and uses

Pixel

Exactly one pixel of the device screen. This unit is the most
common when writing desktop applications, but with the wide
variety of phone screen sizes, it becomes much harder to use.

in

Inch

One inch (or the closest approximation). This is based on the
physical size of the screen. This is great if you need to work with
real world measurements, but again, because of the variations in
the size of a device screen, it is not always very useful.

mm

Millimeters

Another real world measurement, made to the closest
approximation. This is just a metric version of inches: 25.4
millimeters in 1 inch.

pt

Points

Points are 1/72 of an inch in size. Much like millimeters and
inches, they are very useful for sizing things against real-world
sizes. They are also commonly used for sizing fonts, and so work
well relative to font sizes.

dp or dip

Densityindependentpixels

A single DP is the same size as a single pixel is for a 160 dpi screen.
This size is not always a direct ratio, not always precise, but is a
best approximation for the current screen.

sp

Scaleindependent
pixels

Much like the dp unit, it is a pixel scaled according to the user
selected font size. This is possibly the best unit to use, as it's
based on a user-selected parameter. The user may have increased
the font size because they find the screen hard to read. Using an
sp unit ensures that your user interface scales with it.

Defining common dimensions
Android also allows you to define your own dimension values as resource constants (note:
dimensions, not measurements). This can be useful when you want several view widgets to
be the same size, or to define a common font size. Files containing dimension declarations
are placed in the /res/values directory in your project. While the actual file name isn't
significant, a common name is dimens.xml. Dimensions can technically be included with
other value types (that is, strings), but this is not recommended since it makes it harder to
trace the dimension that are being applied at runtime.

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Developing a Simple Activity

One advantage of having your dimensions in their own file as opposed to being declared
inline) is that you can then localize them based on the size of the screen. This makes screenresolution-significant scales (such as pixels) much more useful. For example, you can place a
dimens.xml file with different values into /res/values-320x240 and another version of
the same dimensions into /res/values-640x480.
A dimensions resource file is a simple values file (much like strings.xml), but dimensions
are defined with the  tag:

160px


To access this as a size in a layout XML file, you use a resource reference (much the same way
as you access a resource string):


Building a list of common dimensions comes in handy when you want to build complex
layouts that will look good on many different screens since it avoids the need to build several
different layout XML files.

Have a go hero – improve the styling
Now we have the most basic structure for this user interface, but it doesn't really look too
great. Other than the margins between the answer buttons, and the Skip Question and Feed
Me! buttons, you can't really tell them apart. We need to let the user know that these buttons
all do different things. We also need to draw more attention to the question, especially if they
don't have a lot of time to squint at their screen. You may need the Android documentation,
which can be found online at http://developer.android.com/reference/.
We have a question at the top of our screen, but as you can see in the previous screenshots,
it doesn't stand out much. Therefore, it's not really very clear to the user what they need to
do (especially the first time they use the application).
Try making the following styling changes to the question TextView at the top of our screen.
These will only require you to add some attributes to its XML element:
1. Center the text.
2. Make the text bold.
3. Change the text size to 24sp.
4. Add 12sp spacing between the bottom of the question and the answer buttons

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Chapter 1

The Feed Me! button is also very important. This is the button that gives the user access to
the list of suggested recipes that the application has filtered based on their answers, so it
should look good.
The following styling should help the Feed Me! button to stand out nicely (hint: Button
extends TextView):
1. Make the text size 18sp.
2. Change the text color to a nice red #9d1111.
3. Style the text as bold.
4. Add a text shadow: x=0, y=-3, radius=1.5, and color=white ("#fff").
When you've finished styling the screen, it should look like the following screenshot:

Limitations of the layout XML format
One of the most obvious limitations of the layout XML format is that you can't dynamically
populate part of the Activity based on external variables—there are no loops or methods
in the XML file.
In our example, this limitation shows itself in the form of our empty LinearLayout.
Because each question has any number of possible answers, we need a varying number of
buttons in the group. For our purposes, we will create the Button objects and put them into
the LinearLayout as part of the Java code.

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Developing a Simple Activity

The other place the XML layout format falls down is dynamically referencing
external resources. This can be seen in our example, where we put placeholder text in
the android:text attribute on the TextView element—question. We could have
referenced an external string using the following syntax:


This will effectively reference a static variable from the strings.xml file. It's not suitable
for a dynamically selected question, which will change each time we initialize the Activity.

Pop quiz
1. What reason do you have for writing your layouts in XML instead of in pure Java
code?
a. Android can read the layout file externally for optimization.
b. The layout becomes part of the resource selection process.
c.

Your users could download new layouts from the App Store.

d. The layout can have custom themes applied to it.
2. How would we make the text of the Next Question button bold?
a. Use the android:typeface attribute.
b. Create a custom Button implementation.
c.

Add a CSS attribute: style="font-weight: bold".

d. Use the android:textStyle attribute.
3. What would happen if we changed the LinearLayout from vertical orientation,
to horizontal?
a. The layout would turn on its side.
b. All of the widgets would be squashed together on the screen.
c.

Only the question TextView would be visible on the screen.

d. The question, and possibly some other View objects may be visible on the
screen depending on the number of pixels available.
e. The layout would overflow, causing the widgets to appear next to each other,
over several lines.
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Chapter 1

Populating the QuestionActivity
We have a basic user interface, but right now, it's static. We may want to ask our user many
different questions, each of which have different answers. We may also want to vary which
questions we ask in some way or another. In short, we need some Java code to populate the
layout with a question and some possible answers. Our questions are made up of two parts:
‹‹

The question

‹‹

A list of possible answers

In this example, we will make use of string array resources to store all of the question and
answer data. We will use one string array to list the question identifiers, and then one string
array for each question and its answers. The advantages of this approach are very similar
to the advantages of using a layout XML file instead of hard-coding it. The res/values
directory of your project will have an auto-generated strings.xml file. This file contains
string and string-array resources that you want your application to use. Here is the start
of our strings.xml file, with two questions to ask the user:


Kitchen Droid

vegetarian
size


Are you a Vegetarian?
Yes
No
I\'m a vegan


How much do you feel like eating?
A large meal
Just a nice single serving of food
Some finger foods
Just a snack



The first item of each question array (vegetarian and size) is the question itself, while
each following item is an answer.
[ 29 ]

Developing a Simple Activity

Time for action – writing more Java code
1.
2.

Open the QuestionActivity.java file in an editor or IDE.
Import the Android Resources class below the package declaration:
import android.content.res.Resources;

3.

In order to start asking the questions from your strings.xml file, you'll need a
method to look in the questions  and find the name of the
array that contains the current question. This is not normally something you need to
do with application resources—their identifiers are generally known to you through
the R class. In this case however, we want to work in the order defined in the
questions , making things a little bit more difficult:
private int getQuestionID(Resources res, int index) {

4.

We can now look at the questions string-array, which contains the identifying
name of each question (our index string-array):
String[] questions = res.getStringArray(R.array.questions);

5.

We have the array of questions, and we need to find the identifier value. This is
much like using R.array.vegetarian for the vegetarian question, except
that it's a dynamic lookup, and therefore much slower than normal. In general, the
following line is not recommended, but in our case it's very useful:
return res.getIdentifier(
questions[index],
"array",
"com.packtpub.kitchendroid");

6.

The QuestionActivity class will display several questions to the user. We want
the application to "play nice" with the phone and its environment. For that reason,
each question will be posed in a new instance of QuestionActivity (allowing
the device to control the display of our Activity). However, this method raises
an important question: How do we know the index of the question to pose to the
user? The answer: Our Intent. An Activity is started with an Intent object, and
each Intent object may carry any amount of "extra" information (similar to request
attributes in the HttpServletRequest interface) for the Activity to use, sort of
like arguments to a main method. So, an Intent is also like a HashMap, containing
special data for the Activity to use. In our case we use an integer property named
KitchenDroid.Question:
private int getQuestionIndex() {
return getIntent().getIntExtra("KitchenDroid.Question", 0);
}
[ 30 ]

Chapter 1

These two methods form the basis for populating our question screen and navigating our
way through a defined list of questions. When complete, they should look like this:
private static int getQuestionID(
final Resources res,
final int index) {
final String[] questions = res.getStringArray(R.array.questions);
return res.getIdentifier(
questions[index],
"array",
"com.packtpub.kitchendroid");
}
private int getQuestionIndex() {
return getIntent().getIntExtra("KitchenDroid.Question", 0);
}

What just happened
The getQuestionID method is pretty straight forward. In our code we use R.array.
questions to access the  which identifies all of the questions we
are going to ask the user. Each question has a name in the form of a String, and a
corresponding resource identification number in the form of an int.
In the getQuestionID method, we make use of the Resources.getIdentifier
method, which looks for the resource identifier (the integer value) for a given resource
name. The second parameter of the method is the type of resource to look up. This
parameter is normally an inner class to the generated R class. Finally, we pass the base
package that the resource is found in. Instead of all three of these parameters, you could
also look up the resource by its full resource name:
return res.getIdentifier(
"com.packtpub.kitchendroid:array/" + questions[index],
null,
null);

The getQuestionIndex method tells us where in the questions 
we currently are, and thus, which question to ask the user. This is based on the "extra"
information in the Intent that triggered the Activity. The getIntent() method provides
you with access to the Intent that triggered your Activity. Each Intent may have any
amount of "extra" data, and that data may be any "primitive" or "serializable" type. Here
we fetch the KitchenDroid.Question extra integer value from our Intent, substituting
a value of 0 if it has not been set (that is, the default value). If the user taps our icon in the
menu, Android won't have specified that value, so we start from the first question.

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Developing a Simple Activity

Dynamically creating widgets
Up to this point we've only used the layout XML file to populate our screen. In some cases,
this is just not enough. In this simple example, we want the user to have a list of buttons that
they can touch to answer the questions posed to them. We could pre-create some buttons
and name them button1, button2, and so on, but that means limiting the number of
possible answers.
In order to create buttons from our  resources, we need to do it in
Java. We created a ViewGroup earlier (in the form of the LinearLayout that we named
answers). This is where we will add our dynamically created buttons.

Time for action – putting the questions on the screen
Your application now knows where to find the questions to ask, and knows which question
it should be asking. Now it needs to put the question on the screen, and allow the user to
select an answer.

1.

Open the main.xml file in your editor or IDE.

2.

Remove the Yes!, No!, and Maybe? Button elements from the layout resource.

3.

Open the QuestionActivity.java file in an editor or IDE.

4.

We will need a new class field to hold the dynamically-created Button objects
(for reference):
private Button[] buttons;

5.

In order to keep things neat, create a new private method to put the questions on
the screen: initQuestionScreen:
private void initQuestionScreen() {

6.

In this method, we assume that the layout XML file has already been loaded into the
Activity screen (that is, it will be invoked after we setContentView in onCreate).
This means that we can look up parts of the layout as Java objects. We'll need both the
TextView named question and the LinearLayout named answers:
TextView question = (TextView)findViewById(R.id.question);
ViewGroup answers = (ViewGroup)findViewById(R.id.answers);

7.

These two variables need to be populated with the question and its possible
answers. For that we need the  (from our strings.xml file)
which contains that data, so we need to know the resource identifier for the current
question. Then we can fetch the actual array of data:
[ 32 ]

Chapter 1
int questionID = getQuestionID(resources, getQuestionIndex());
String[] quesionData = resources.getStringArray(questionID);

8.

The first element of a question string array is the question to pose to the user.
The following setText call is exactly the same as specifying an android:text
attribute in your layout XML file:
question.setText(quesionData[0]);

9.

We then need to create an empty array to store references to our Button objects:
int answerCount = quesionData.length – 1;
buttons = new Button[answerCount];

10.

Now we're ready to populate the screen. A for loop over each of the answer values
indexed according to our arrays:
for(int i = 0; i < answerCount; i++) {

11.

Get each answer from the array, skipping the question string at index zero:
String answer = quesionData[i + 1];

12.

Create a Button object for the answer and set its label:
Button button = new Button(this);
button.setText(answer);

13.

Finally, we add the new Button to our answers object (ViewGroup), and reference
it in our buttons array (where we'll need it later):
answers.addView(button);
buttons[i] = button;

14.

Having done that, just after the setContentView calls in onCreate, we need to
invoke our new initQuestionScreen method.

What just happened?
The findViewById method traverses the tree of View objects looking for a specific
identifying integer value. By default, any resource declared with an android:id attribute
in its resource file will have an associated ID. You could also assign an ID by hand using the
View.setId method.
Unlike many other user interface APIs, the Android user interface API is geared towards XML
development than pure Java development. A perfect example of this fact is that the View
subclasses have three different constructors, two of which are designed for use with the XML
parsing API. Instead of being able to populate the Button label in a constructor (as with
most other UI APIs), we are forced to first construct the object, and then use setText to
define its label.
[ 33 ]

Developing a Simple Activity

What you do pass into the constructor of every View object is a Context object. In the
preceding example you pass the Activity object into the constructor of the answer
Button objects as this. The Activity class inherits from the Context class. The
Context object is used by the View and ViewGroup objects to load the application
resources and services that they require in order to function correctly.
You can now try running the application, in which case you'll be greeted with the following
screen. You may have noticed that there is additional styling in this screenshot. If you don't
have this, you may want to backtrack a little to the previous Have a go hero section.

Handling events in Android
Android user interface events work in much the same way as a Swing event-listener or a
GWT event-handler. Depending on the type of event you wish to receive, you implement an
interface and pass an instance to the widget you wish to receive events from. In our case we
have Button widgets that fire click-events when they are touched by the user.
The event-listener interfaces are declared in many of the Android classes, so there isn't a
single place you can go look for them. Also, unlike most event-listener systems, many widgets
may only have one of any given event-listeners. You can identify an event-listener interface
by the fact that their class names are prefixed with On (much like HTML event attributes). In
order to listen for click-events on a widget, you would set its OnClickListener using the
View.setOnClickListener method.
The following code snippet shows how a click-listener might be added to a Button object
to show a Toast. A Toast is a small pop-up box which is displayed briefly to give the user
some information:
[ 34 ]

Download from Wow! eBook 

Chapter 1
button.setOnClickListener(new View.OnClickListener() {
public void onClick(View clicked) {
Toast.makeText(this, "Button Clicked!", Toast.LENGTH_SHORT).
show();
}
});

The preceding event-listener is declared as an anonymous inner class, which is okay when
you are passing similar event-listeners to many different widgets. However, most of the time
you'll want to be listening for events on widgets you've declared in an XML layout resource.
In these cases it's better to either have your Activity class implement the required
interfaces, or create specialized classes for different event-driven actions. While Android
devices are very powerful, they are still limited when compared to a desktop computer
or laptop. Therefore, you should avoid creating unnecessary objects in order to conserve
memory. By placing as many event-listener methods in objects that will already be created,
you lower the overhead required.

Pop quiz
1. When you declare an object in a layout XML file, how do you retrieve its Java object?
a. The object will be declared in the R class.
b. Using the Activity.findViewById method.
c.

By using the Resources.getLayout method.

d. The object will be injected into a field in the Activity class.
2. What is the "best" way of listening for events in an Android application?
a. Declaring the listeners as anonymous inner classes.
b. Create a separate event listener class for each Activity.
c.

Implement the event-listening interfaces in the Activity class.

3. Why do you pass this Activity into the constructors of View objects
(that is, new Button(this)).
a. It defines the Activity screen they will be displayed on.
b. It's where event messages will be sent to.
c.

It's how the View will reference its operating environment.

[ 35 ]

Developing a Simple Activity

Summary
Android comes with some great tools to create and test applications, even if you don't
have an Android device handy. That said, there's no replacement for actually touching your
application. It's part of what makes Android such a compelling platform, the way it feels and
responds (and the emulator just doesn't convey that).
One of the most important tools in an Android developer's arsenal is the resource selection
system. With it you can build highly dynamic applications that respond to changes in
the devices, and thus, the user environment. Changing the screen layout based on the
orientation of the device, or when the user slides out the phone's QWERTY keyboard, lets
them know that you've taken their preferences into account when building your application.
When building user interfaces in Android, it's strongly recommended to build at least the
layout structure in an XML file. The XML layout files are not only considered as application
resources, but Android also strongly favors building XML user interfaces over writing Java
code. Sometimes, however, a layout XML file isn't enough, and you need to build parts of the
user interface in Java. In this case it's a good idea to define at least a skeleton layout as XML (if
possible), and then place the dynamically created View objects into the layout by using marker
IDs and containers (much like dynamically adding to an HTML document in JavaScript).
When building a user interface, think carefully about the look and feel of the outcome. In
our example, we use Button objects for the answers to questions. We could have used
RadioButton objects instead, but then the user would have needed to select an option,
and then touch a Next Question button, requiring two touches. We could also have used
a List (which interacts nicely with the fact that it needs to be dynamically populated),
however, a List doesn't indicate an "action" to the user quite the way a Button does.
When coding layouts, be careful with the measurement units that you use. It's strongly
recommend that you stick to using sp for most purposes—if you can't use one of the special
fill_parent or wrap_content values. Other values are highly dependent on the size
of screen, and won't respond to the user preferences. You can make use of the resource
selection process to build different screen designs for small, medium, or large screens.
You could also define your own measurement unit and base it on the screen size.
Always think about how your user will interact with your application, and how much
(or little) time they are likely to have with it. Keeping each screen simple and responsive
keeps your users happy.
Now that we've learned how to create a skeleton Android project, and a simple Activiy,
we can focus on the more subtle problems and solutions of Android user interface design.
In the next chapter, we will focus on working with data-driven widgets. Android has several
widgets designed specifically for displaying and selecting from more complex data structures.
These widgets form the basis of data-driven applications such as an address book or a
calendar application.
[ 36 ]

2

Presenting Data for Views
In the first chapter we covered the basic creation of a project, and how to put
together a simple user interface. We backed our first Activity with enough
code to dynamically generate some buttons that the user can use to answer our
multiple-choice questions.
So now we can capture some data, but what about displaying data? One large
advantage of software is its ability to present and filter very large volumes of
data quickly and in an easy-to-read format. In this chapter we will look at a
series of Android widgets that are designed exclusively for presenting data.

Most Android data-centric classes are built on top of Adapter objects, and thus extend the
AdapterView class. An Adapter can be thought of as a cross between a Swing Model,
and a renderer (or presenter). An Adapter object is used to create View objects for data
objects that your software needs to display to the user. This pattern allows the software to
maintain and work with a data-model and only create a graphical View for each of the data
objects when one is actually needed. This doesn't just help conserve memory, but it is also
more logical from a development point of view. As a developer you work with your own data
objects instead of trying to keep your data in graphical widgets (which are often not the most
robust of structures).
The most common AdapterView classes you'll encounter are: ListView, Spinner, and
GridView. In this chapter we'll introduce the ListView class and GridView, and explore
the various ways they can be used and how they can be styled.

Presenting Data for Views

Listing and selecting data
The ListView class is probably the most common way to display lists of data. It's backed
by a ListAdapter object, which is responsible for both holding the data and rendering the
data objects in a View. A ListView includes built-in scrolling, so there's no need to wrap it
in a ScrollView.

ListView choice modes
The ListView class allows for three basic modes of item selection, as defined by its
constants: CHOICE_MODE_NONE, CHOICE_MODE_SINGLE, and CHOICE_MODE_MULTIPLE.
The mode for a ListView can be set by using the android:choiceMode attribute in your
layout XML file, or by using the ListView.setChoiceMode method in Java.
Choice modes and items
The choice mode of a ListView changes the way the ListView
structure behaves, but not the way it looks. The look of a ListView
is defined mostly by the ListAdapter, which provides View objects
for each of the items that should appear in the ListView.

No selection mode – CHOICE_MODE_NONE
On a desktop system, this would make no sense—a list that doesn't allow the user to choose
anything? However, it's the default mode of an Android ListView. The reason is it makes
sense when your user is navigating by touch. The default mode of a ListView allows the
user to tap on one of the elements, and trigger an action. As a result of this behavior, there's
no need for a "Next" button, or anything similar. So the default mode for a ListView is to
act like a menu. The following screenshot is a default ListView object displaying a list of
different strings from a String array Java object, taken from one of the default ApiDemos
examples in Android SDK.

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Chapter 2

Single selection mode – CHOICE_MODE_SINGLE
In this mode, the ListView acts more like a desktop List widget. It has the notion of
the current selection, and tapping on a list item does nothing more than selecting it.
This behavior is nice for things like configuration or settings, where the user expects the
application to remember his or her current selection. Another place a single selection list
becomes useful is when there are other interactive widgets on the screen. However, be
careful not to put too much information in a single Activity. It's quite common for a
ListView to occupy almost an entire screen.
Single-choice selection: It doesn't directly change the way your list items
appear. The look and feel of your list items is defined entirely by the
ListAdapter object.

Android does, however, provide a collection of sensible defaults in the system resources.
In the android package you will find an R class. It's a programmatic way to access the
system's default resources. If you wanted to create a single-choice ListView with a
 of colors in it, you could use the following code:
list.setAdapter(new ArrayAdapter(
this,
android.R.layout.simple_list_item_single_choice,
getResources().getStringArray(R.array.colors)));

In this case we use the provided ArrayAdapter class from the android.widget package.
In the second parameter we referenced the Android layout resource named simple_list_
item_single_choice. This resource is defined by the Android system as a default way to
display items in a ListView with CHOICE_MODE_SINGLE. Most typically this is a label with
a RadioButton for each object in the ListAdapter.

[ 39 ]

Presenting Data for Views

Multiple selection mode – CHOICE_MODE_MULTIPLE
In multi-selection mode, the ListView replaces the radio buttons of single-selection mode
with normal checkboxes. This design structure is often used on desktops and web-based
systems as well. Checkboxes are easily recognized by users, and make it easy to go back and
turn options off again. If you wish to use a standard ListAdapter, Android provides you
with the android.R.layout.simple_list_item_multiple_choice resource
as a useful default: A label with a CheckBox for each object in the ListAdapter.

Adding header and footer widgets
Headers and footers in a ListView allow you to put additional widgets at the top and
bottom of the List. The header and footer widgets are by default treated as though they
are items in a list (as though they come from your ListAdapter). This means that you will
be able to select them as though they are data elements in the List structure. A very simple
example of a header item could be:
TextView header = new TextView(this);
header.setText("Header View");
list.addHeaderView(header);

Often you don't want your headers and footers to be items in the ListView, but instead a
label or group of labels identifying parts of the ListView, or providing other information. In
this case you need to tell the ListView that your header or footer views are not selectable
list items. This can be done by using the extended implementation of addHeaderView or
addFooterView:
TextView footer = new TextView(this);
footer.setText("Footer View");
list.addFooterView(footer, null, false);

The ListView class integrates headers and footers so tightly into the list structure
that you can also provide an Object that it will return from the AdapterView.
getItemAtPosition(index) method. In our previous example we have provided null.
Each header item will offset the index of subsequent views by one (as though you are adding
new items to the ListView). The third parameter tells the ListView whether the header
or footer should be counted as a selectable list item (in our previous example it shouldn't).
[ 40 ]

Chapter 2

If you are used to desktop widgets, the header and footer widgets on an Android ListView
will have a bit of a surprise for you. They will scroll with the rest of the list items, and won't
stay attached to the top and bottom of the ListView object.

Creating a simple ListView
To introduce the ListView class, we'll start a new example which will be enhanced by
various subsequent sections of this chapter. The first Activity we will create will use a
simple ListView populated from a  resource.

Time for action – creating a fast food menu
To continue with the food and eating theme, let's build a simple application that allows us
to order various types of fast food, and get it delivered! The user will first select where they
want to order from, and then select the various foodstuffs that they want to eat.

1.

Create a new android project using the Android command-line tool:
android create project -n DeliveryDroid -p DeliveryDroid -k com.
packtpub.deliverydroid -a SelectRestaurantActivity -t 3

2.

Open the /res/values/strings.xml file in your favorite editor or IDE.

3.

Create a string-array structure listing the various fast-food restaurants our users can
order from:

The Burger Place
Mick's Pizza
Four Buckets \'o Fruit
Sam\'s Sushi


4.

Open the /res/layout/main.xml file in your favorite editor or IDE.

5.

Remove any widget that is inside the default LinearLayout.

6.

Add a new  element.

7.

Assign the  element an ID of restaurant:


8.

Assign the width and height of the ListView to fill_parent:
android:layout_width="fill_parent"
android:layout_height="fill_parent"

[ 41 ]

Presenting Data for Views

9.

Since we have a string-array resource of the content we want to populate the
ListView with, we can reference it directly in our layout XML file:
android:entries="@array/restaurants"

10.

When you've completed the specified steps, you should have a main.xml layout file
that looks like the following:





What just happened
If you install your application into the emulator, and run it, you'll be presented with a screen
where you can select from the list of restaurants specified in your string-array resource.
Notice that the choiceMode on the ListView is left as CHOICE_MODE_NONE, making this
into a more direct menu where the user selects their restaurant and is instantly transported
to its menu.

In this example, we used the android:entries attribute in the layout XML file to specify
a reference to a string-array resource with our desired list items in it. Normally, using an
AdapterView requires you to create an Adapter object to create View objects for each of
the data objects.

[ 42 ]

Chapter 2

Using the android:entries attribute allows you to specify the data contents of the
ListView from your layout resource, instead of requiring you to write the normal Java code
associated with an AdapterView. It, however, does have two disadvantages to be aware of:
‹‹

The View objects created by the generated ListAdapter will always be the
system-specified defaults, and so cannot be easily themed.

‹‹

You cannot define data objects that will be represented in the ListView. Since
string-arrays are easily localized, your application will rely on the index locations of
items to determine what they indicate.

You may notice that at the top of the screenshot, the label Where should we order
from? is not the application default. The label for an Activity is defined in the
AndroidManifest.xml file as follows:


Styling the standard ListAdapters
The standard ListAdapter implementations require each item be represented in a
TextView item. The default single-choice and multiple-choice items are built using a
CheckedTextView, and while there are plenty of other TextView implementations
in Android, it does limit our options a bit. However, the standard ListAdapter
implementations are very convenient and provide solid implementations for the most
common listing requirements.
Since a ListView with CHOICE_MODE_NONE is a lot like a menu, wouldn't it be nice to
change the items into Button objects instead of normal TextView items? Technically, a
ListView can contain any widget that extends TextView. However, some implementations
are more suitable than others (for example, a ToggleButtonView won't maintain the
specified text-value when the user touches it).

Defining standard dimensions
In this example we'll be creating various menus for the application. In order to maintain a
consistent look and feel, we should define a set of standard dimensions which will be used
in each of our layout files. This way we can redefine the sizes for different types of screens.
There's nothing more frustrating for a user than only being able to see a partial item because
it's been sized bigger than their screen.
Create a new resource file to contain the dimensions. The file should be named res/
values/dimens.xml. Copy the following code into the new XML file:
[ 43 ]

Presenting Data for Views


48sp
52sp
45sp
24sp
15dp


We declare two height dimensions for the list items: item_outer_height and item_
inner_height. The item_outer_height will be the height of the list items, while the
item_inner_height is the height of any View object contained inside the list item.
The padding dimension at the end of the file is used to define a standard amount of
whitespace between two visual elements. This is defined as dp so it will remain constant
based on the DPI of the screen (instead of scaling according to the font size preferences of
the user).
Sizing of interactive items
In this styling, you'll notice that the item_outer_height and menu_item_
height are 48sp and 52sp, which makes the items in the ListView rather
large. The standard size of a list view item in Android is 48sp. The height of a list
item is critical. If your users have large fingers, they will struggle to tap on their
target list item if you make them too small.
This is a general "good practice" for Android user interface design. If the user
needs to touch it, make it big.

Time for action – improving the restaurant list
The list of restaurants we put together earlier is nice, but it's a menu. In order to further
emphasize the menu, the text should stand out more. In order to style a ListView with a
standard ListAdapter implementation, you will need to specify the ListAdapter object
in your Java code.

1.

Create a new file in the res/layout directory named menu_item.xml.

2.

Create the root XML element as a TextView:



3.

Import the Android resource XML namespace:
xmlns:android="http://schemas.android.com/apk/res/android"

[ 44 ]

Chapter 2

4.

Center the text in the TextView widget by setting its gravity:
android:gravity="center|center_vertical"

5.

We assign the textSize of the TextView to our standard item_text_size:
android:textSize="@dimen/item_text_size"

6.

The default color of the text of TextView is a bit gray, we want it to be white:
android:textColor="#ffffff"

7.

We want the width of the TextView to be the same as the ListView that contains
it. Since this is for our main menu, its height is menu_item_height:
android:layout_width="fill_parent"
android:layout_height="@dimen/menu_item_height"

8.

Now that we have a styled TextView resource, we can incorporate it into our
menu. Open the SelectRestaurantActivity.java file.

9.

In the onCreate method, after you use setContentView, we need a reference to
the ListView we created earlier in main.xml:
ListView restaurants = (ListView)findViewById(R.id.restaurant);

10.

Set the restaurants ListAdapter to a new ArrayAdapter containing the stringarray of restaurants we created in our values.xml file:
restaurants.setAdapter(new ArrayAdapter(
this,
R.layout.menu_item,
getResources().getStringArray(R.array.restaurants)));

What just happened
We first created a new layout XML resource containing the styled TextView that we wanted
to be used for each list item in our restaurant's ListView. The menu_item.xml file you
wrote should contain the following code:



[ 45 ]

Presenting Data for Views

Unlike our previous layout resources, menu_item.xml contained no ViewGroup (such as
LinearLayout). This is due to the fact that the ArrayAdapter will attempt to cast the
root View of the menu_item.xml file to a TextView. So, if we nested the TextView in a
ViewGroup of some sort, we'd get a ClassCastException.
We also created an ArrayAdapter instance to reference both our menu_item XML
resource, and the string-array of restaurants we created earlier. This action eliminates
the use of the android:entries attribute on the ListView in the main.xml layout
XML resource. If you want, you can remove that attribute. Your onCreate method in
SelectRestaurantActivity should now look as follows:
public void onCreate(final Bundle icicle) {
super.onCreate(icicle);
setContentView(R.layout.main);
final ListView restaurants = (ListView)
findViewById(R.id.restaurant);
restaurants.setAdapter(new ArrayAdapter(
this,
R.layout.menu_item,
getResources().getStringArray(R.array.restaurants)));
}

Try re-installing the application into the emulator with Apache Ant, and you'll now be
greeted by a screen that looks a lot more like a menu:

Have a go hero – developing a multiple-choice question application
Try going back to the multiple-choice question application we wrote in Chapter 1, Developing a
Simple Activity. It uses LinearLayout and Button objects to display the possible answers to
the questions, but it also uses string-arrays for the answers. Try modifying the application to:
‹‹

Use a ListView instead of a LinearLayout

‹‹

Style the ListView with Button objects, as we styled our restaurant menu with
TextView objects
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Chapter 2
‹‹

Make sure you have some margin between the Button list items so that they're not
too close to each other

Creating custom adapters
When we want to order food, we often want to order more than one of the same item. The
ListView implementation, and the standard ListAdapter implementations allow for
us to select a Cheese Burger item, but not for us to request 3 Cheese Burgers. In order to
display a menu of different foods that the user can order in multiple quantities, we need
a customized ListAdapter implementation.

Creating a menu for The Burger Place
For each restaurant in our main menu, we are going to build a separate Activity class. In
reality, this is not a great idea, but it allows us to investigate different ways of organizing and
displaying the menu data. Our first stop is The Burger Place, for which we present the user
with a list of burgers, and let them tap the ones they want on the screen. Each time they
tap a list item, they order another burger. We will display the number of burgers they are
ordering in bold to the left of the burger's name. Next to burgers that they aren't ordering,
there should be no number (this allows the user to see what they are ordering at a
quick glance).

The Burger class
In order to display the menu, we need a simple Burger data object. The Burger class
will hold a name to be displayed in the menu, and the number of Burger the user
is ordering. Create a Burger.java file in the root package of your project with the
following code:
class Burger {
final String name;
int count = 0;
public Burger(String name) {
this.name = name;
}
}

You'll notice that there are no getters and setters in the preceding code, and that both the
name and count fields are declared as package-protected. In versions of Android prior to
2.2, methods incurred a heavy expense when compared to a straight field lookup. Since this
class will be a small part of the rendering procedure (we will be extracting data from it for
display), we should make sure we incur as little expense as possible.

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Presenting Data for Views

Time for action – creating a Burger item layout
The first thing to do in order to create a nice looking menu for The Burger Place is to design
the menu items. This is done in much the same way as the styling of the restaurant list
with a layout XML resource. However, since we will be building the ListAdapter ourselves
this time, we are not forced to use a single TextView, but can instead build a more
complex layout.

1.

Create a new XML file in the res/layout directory named burger_item.xml.
This file will be used for each burger in the ListView.

2.

Declare the root of the layout as a horizontal LinearLayout (note the height,
which will be the height of each item in the ListView):


3.

Next, declare a TextView, which we will use as a counter for the number of
burgers being ordered. We will later access this through its ID:


4.

The counter text size is exactly the same as all of the other list items in the
application. However, it should be bold, so it can be easily identified and read:
android:textSize="@dimen/item_text_size"
android:textStyle="bold"

5.

We also want the counter to be square, so set the width and height exactly
the same:
android:layout_width="@dimen/item_inner_height"
android:layout_height="@dimen/item_inner_height"

6.

We also want to center the text inside the counter:
android:gravity="center|center_vertical"

7.

We'll also need a text space to display the name of the burger:


8.

The text size is standard:
android:textSize="@dimen/item_text_size"

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Chapter 2

9.

We want a little bit of space between the counter and the text label:
android:layout_marginLeft="@dimen/padding"

10.

The label's width should fill the ListView, but we want the size of both TextView
objects to be the same:
android:layout_width="fill_parent"
android:layout_height="@dimen/item_inner_height"

11.

The text of the label should be centered vertically, to match the location of the
counter. However, the label should be left-aligned:
android:gravity="left|center_vertical"

What just happened?
You've just built a very nice LinearLayout ViewGroup which will be rendered for each
of the burgers we sell from The Burger Place. Since the counter TextView is a separate
object from the label, it can be independently styled and managed. This makes things much
more flexible going forward if we want to apply additional styles to them independently.
Your complete burger_item.xml file should now appear as follows:






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Presenting Data for Views

Time for action – presenting Burger objects
The standard ListAdapter classes work well if your data objects are either strings or easily
represented as strings. In order to display our Burger objects nicely on the screen, we need
to write a custom ListAdapter class. Fortunately, Android provides us with a nice skeleton
class for ListAdapter implementations named BaseAdapter.

1.

Create a new class named BurgerAdapter, and have it extend from the android.
widget.BaseAdapter class:
class BurgerAdapter extends BaseAdapter {

2.

An Adapter is part of the presentation layer, but is also the underlying model of the
ListView. In the BurgerAdapter we store an array of Burger objects which we
assign in the constructor:
private final Burger[] burgers;
BurgerAdapter(Burger... burgers) {
this.burgers = burders;
}

3.

Implement the Adapter.getCount() and Adapter.getItem(int) methods
directly on top of the array of Burger objects:
public int getCount() {
return burgers.length;
}
public Object getItem(int index) {
return burgers[index];
}

4.

An Adapter is also expected to provide identifiers for the various items, we will just
return their index:
public long getItemId(int index) {
return index;
}

5.

When an Adapter is asked for a View of a list item, it may be given an existing
View object that could be reused. We will implement a simple method to handle
this case, and if required, inflate the burger_item.xml file we wrote earlier using
the LayoutInflator class from the android.view package:
private ViewGroup getViewGroup(View reuse, ViewGroup parent) {
if(reuse instanceof ViewGroup) {
return (ViewGroup)reuse;
}
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Chapter 2
Context context = parent.getContext();
LayoutInflater inflater = LayoutInflater.from(context);
ViewGroup item = (ViewGroup)inflater.inflate(
R.layout.burger_item, null);
return item;
}

6.

The most important method for us in the BurgerAdapter is the getView method.
This is where the ListView will ask us for a View object to represent each list item
it needs to display:
public View getView(int index, View reuse, ViewGroup parent) {

7.

In order to fetch the correct View for a given item, you'll first need to use the
getViewGroup method to ensure you have the burger_item.xml ViewGroup to
display the Burger item in:
ViewGroup item = getViewGroup(reuse, parent);
TextView counter = (TextView)item.findViewById(R.id.counter);
TextView label = (TextView)item.findViewById(R.id.text);

8.

We'll be populating these two TextView objects with the data from the Burger
object at the requested index. The counter widget needs to be hidden from the
user if the current count is zero:
Burger burger = burgers[index];
counter.setVisibility(
burger.count == 0
? View.INVISIBLE
: View.VISIBLE);
counter.setText(Integer.toString(burger.count));
label.setText(burger.name);
return item;

What just happened?
We just wrote a custom Adapter class to present an array of Burger objects to the user in
a ListView. When a ListView invokes the Adapter.getView method, it will attempt to
pass in the View object that was returned from a previous call to Adapter.getView. A View
object will be created for each item in the ListView. However, when the data displayed by
the ListView changes, the ListView will ask the ListAdapter to reuse each of the View
objects it generated the first time around. It's important to try and honor this behavior, since
it has a direct impact on the responsiveness of your application. In our preceding example, we
implemented the getViewGroup method so that it would take this requirement into account.
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Presenting Data for Views

The getViewGroup method is also used to inflate the burger_item.xml file we wrote.
We do this using a LayoutInflator object, which is exactly how the Activity.
setContentView(int) method loads XML layout resources. The Context object which
we fetch from our parent ViewGroup (which will generally be the ListView) defines
where we will load the layout resource from. If the user hasn't selected a Burger, we hide
the counter TextView using the View.setVisibility method. In AWT and Swing, the
setVisible method takes a Boolean parameter, whereas in Android, setVisibility
takes an int value. The reason for this is that Android treats visibility as part of the layout
process. In our case we want the counter to disappear, but still take up its space in the
layout, which will keep the text labels left-aligned with each other. If we wanted the
counter to vanish and take up no space, we could use:
counter.setVisibility(burger.count == 0
? View.GONE
: View.VISIBLE);

ListView objects will automatically handle the highlighting of a selected item. This includes
when the user holds their finger on the item, and when they use a track-pad or directional
buttons to navigate the ListView. When an item is highlighted, its background generally
changes color, according to standard UI conventions.

However, using widgets in a ListView that in some way directly captures user input
(that is, a Button or EditText) will cause the ListView to stop showing the selection
highlighting for that widget. In fact, it will stop the ListView from registering OnItemClick
events completely.
Custom separators in a ListView
If you override the isEnabled(int index) method of ListAdapter,
you can strategically disable specified items in the ListView. A common
use of this is to turn certain items into logical separators. For example, a
section separator in an alphabetically sorted list, containing the first letter of
all items in the next "section".

Creating TheBurgerPlaceActivity class
In order to put the Burger menu on the screen, and to allow the user to order items, we
need a new Activity class. We need to know when the user touches the items in the
list, for which we will need to implement the OnItemClickListener interface. When a
specific event occurs (in this case the user touches a specific item in the ListView), objects
registered as listeners will have a related method invoked with the details of the event that
occurred. Android provides a simple ListActivity class to provide some default layout
and utility methods for this scenario.

[ 52 ]

Download from Wow! eBook 

Chapter 2

Time for action – implementing TheBurgerPlaceActivity
In order to present a ListView of Burger objects with the BurgerAdapter class, we will
need to create an Activity implementation for The Burger Place. The new Activity will
also be responsible for listening to "touch" or "click" events on the items in the ListView.
When the user touches one of the items, we need to update the model and ListView to
reflect that the user has ordered another Burger.

1.

Create a new class in the root package of your project named
TheBurgerPlaceActivity, and make sure it extends ListActivity:
public class TheBurgerPlaceActivity extends ListActivity {

2.

Override the Activity.onCreate method.

3.

Invoke the super.onCreate to allow normal Android startup.

4.

Create an instance of BurgerAdapter with some Burger objects, and set it as the
ListAdapter for the ListActivity code to use:
setListAdapter(new BurgerAdapter(
new Burger("Plain old Burger"),
new Burger("Cheese Burger"),
new Burger("Chicken Burger"),
new Burger("Breakfast Burger"),
new Burger("Hawaiian Burger"),
new Burger("Fish Burger"),
new Burger("Vegatarian Burger"),
new Burger("Lamb Burger"),
new Burger("Rare Tuna Steak Burger")));

5.

Finally, implement the onListItemClicked method with the following code:
protected void onListItemClick(
ListView parent,
View item,
int index,
long id) {
BurgerAdapter burgers = (BurgerAdapter)
parent.getAdapter();
Burger burger = (Burger)burgers.getItem(index);
burger.count++;
burgers.notifyDataSetInvalidated();
}

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Presenting Data for Views

What just happened?
This implementation of TheBurgerPlaceActivity has a simple hard-coded list of Burger
objects to display to the user and creates a BurgerAdapter to turn these objects into the
burger_item View objects which we created earlier.
When the user taps a list item, we increment the count of the related Burger object
in the onItemClick method. We then call notifyDataSetInvalidated() on the
BurgerAdapter. This method will inform the ListView that the underlying data has
changed. When the data changes, the ListView will re-invoke the Adapter.getView
method for each item in the ListView.
The items in a ListView are represented by effectively static View objects. This means
that the Adapter must be allowed to update or recreate that View when the data model
is updated. A common alternative is to fetch the View representing your updated data, and
update it directly.

Registering and starting TheBurgerPlaceActivity
In order to start the new Activity class from our restaurant menu, you will need to
register it in the AndroidManifest.xml file. First, open the AndroidManifest.xml
file in an editor or IDE, and copy the following  code into the
... block:






To start the Activity, you'll need to go back to SelectRestaurantActivity
and implement the OnItemClickListener interface. After setting the
Adapter on the restaurants ListView, set SelectRestaurantActivity
as the OnItemClickListener of the restaurants ListView. You can start
TheBurgerPlaceActivity using an Intent object in the onItemClick method. Your
SelectRestaurantActivity class should now look like the following code snippet:
public class SelectRestaurantActivity extends Activity
implements OnItemClickListener {
@Override
public void onCreate(Bundle icicle) {
super.onCreate(icicle);
setContentView(R.layout.main);
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Chapter 2
ListView restaurants = (ListView)
findViewById(R.id.restaurant);
restaurants.setAdapter(new ArrayAdapter(
this,
R.layout.menu_item,
getResources().getStringArray(R.array.restaurants)));
restaurants.setOnItemClickListener(this);
}
public void onItemClick(
AdapterView parent,
View item,
int index,
long id) {
switch(index) {
case 0:
startActivity(new Intent(
this,
TheBurgerPlaceActivity.class));
break;
}
}
}

When you reinstall the application and start it up in the emulator, you'll be able to navigate
to The Burger Place and place an order for burgers. Pressing the hardware "Back" button in
The Burger Place menu will take you back to the restaurant menu.

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Presenting Data for Views

Pop quiz
1. Setting the choice mode on a ListView object to CHOICE_MODE_SINGLE will:
a. Add a RadioButton to each item.
b. Do nothing (this is the default).
c.

Make the ListView track a "selected" item.

2. A ListAdapter defines how a ListView displays its items. When will it be asked
to reuse a View for an item object?
a. When the data model is invalidated or changed.
b. On every item, for rubber-stamping.
c.

When the ListView redraws itself.

3. When a ListView is scrollable, header and footer objects will be positioned:
a. Above and below the scrolling items.
b. Horizontally alongside each other, above and below the scrolling items.
c.

Scrolling with the other items.

Using the ExpandableListView class
The ListView class is great for displaying small to medium amounts of data, but there
are times when it will flood your user with far too much information. Think about an email
application. If your user is a heavy email user, or subscribes to a few mailing lists, they
may well have several hundred emails in a folder. Even though they may not need to scroll
beyond the first few, seeing the scrollbar shrink to a few pixels in size doesn't have a good
psychological effect on your user.
In desktop mail clients, you will often group the email list by time: Today, yesterday,
this week, this month, and forever (or something similar). Android includes the
ExpandableListView for this type of grouping. Each item is nested inside a group, and a
group can be displayed or hidden by the user. It's a bit like a tree view, but always nested
to exactly one level (you can't display an item outside a group).
Massive ExpandableListView groups
There are times where even an ExpandableListView will not be enough
to keep the amount of data to a reasonable length. In these cases, consider
giving your user the first few items in the group and adding a special View
More item at the end. Alternatively, use a ListView for the groups, and a
separate Activity for the nested items.
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Chapter 2

Creating ExpandableListAdapter implementations
Since the ExpandableList class includes two levels of detail, it can't work against
a normal ListAdapter which only handles a single level. Instead, it includes the
ExpandableListAdapter which uses two sets of methods: one set for the group level and
another set for the item level. When implementing a custom ExpandableListAdapter,
it's generally easiest to have your ExpandableListAdapter implementation inherit from
the BaseExpandableListAdapter, as it provides implementations for event registration
and triggering.
The ExpandableListAdapter will place an arrow pointer on the left side of
each group item to indicate whether the group is open or closed (much like a dropdown/combobox). The arrow is rendered on top of the group's View object as
returned by the ExpandableListAdapter. To stop your group label from being
partly obscured by this arrow, you'll need to add padding to your list item View
structures. The default padding for a list item is available as the theme parameter
expandableListPreferredItemPaddingLeft, which you can make use of:
android:paddingLeft=
"?android:attr/expandableListPreferredItemPaddingLeft"

In order to keep your ExpandableListView looking consistent, it's a good idea to add the
same amount of padding to the normal (child) items of the ExpandableListView (to keep
their text aligned with that of their parent group), unless you are putting an item on the
left-hand side, such as an icon or checkbox.

Have a go hero - ordering customized pizzas
For the Mick's Pizza example, we're going to create a menu of categorized pizza toppings.
Each topping consists of a name, whether it's 'on' or 'off' the pizza, or 'extra' (for example,
extra cheese). Use two TextView objects arranged horizontally for each item. The right
TextView can hold the name of the topping. The left TextView can be empty when
toppings are not included, On when toppings are included, and Extra for toppings that the
user wants more than the usual amount.
Create an object model with ToppingCatagory objects, containing a name and an array of
PizzaTopping objects. You'll want to store some state whether each topping is ordered,
and in what quantity.
You'll also want to implement a PizzaToppingAdapter class, extending the
BaseExpandableListAdapter class. Make use of the default Android simple_
expandable_list_item_1 layout resource for the group label, and a new customized
layout resource for the item labels.

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Presenting Data for Views

When the user taps on a pizza topping, it changes its status between the three values: Off,
On, and Extra.
Using the ListView.getAdapter() method will not return your
ExpandableListAdapter implementation, but a wrapper instead.
To fetch the original ExpandableListAdapter, you will need to use
the getExpandableListAdapter() method. You will also want to
make use of the ExpandableListView. OnChildClickListener
interface to receive click events.

When your new Activity is complete, you should have a screen which looks something
like the following:

Using the GridView class
A GridView is a ListView with a fixed number of columns, arranged left-to-right,
top-to-bottom. The standard (un-themed) Android application menu is arranged like a
GridView. The GridView class makes use of a ListAdapter in the exact same format
as ListView. However, because of its fixed column count, a GridView is very well suited
for lists of icons.
Using GridViews effectively
A GridView can display significantly more information on a single screen
than a ListView, at the expense of not being able to show as much text
information. From a usability point of view, icons are often easier to work
with than text. Icons can be recognized more quickly than text, thanks to their
colors. When you have information that can be represented using icons, it's
a good idea to display it as such. However, remember that icons need to be
unique within a single screen preferably within the entire application.

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Chapter 2

For our next example, we're going to build the Four Buckets 'o Fruit menu, using GridView.
The GridView will have an icon for each item on the menu, and the name of the item below
the icon. So, when complete, it will look much like the standard Android application menu.
This next example will focus less on the implementation of the ListAdapter, since it's
largely the same as the ListAdapter we built for The Burger Place.
Icons on touchscreen devices
It's important to think about icons on a touchscreen device. They need to be
even more self-explanatory than usual, or be accompanied by some text. With
a touchscreen, it's very hard to provide any sort of contextual help, such as a
tool-tip. If the user is touching the object, it's often obscured by their finger
and/or hand, making the icon and tool-tip invisible.

Time for action – creating the fruit icon
In order to display the various types of fruits as icons, we will need to create a layout XML
file. Each icon in the GridView will be represented as an instance of this layout, in exactly
the same way as list items are represented in a ListView. We create each item as an
ImageView for the icon, with a TextView below it for the label.

1.

Create a file in the res/layout directory named fruit_item.xml.

2.

Declare the root element of the icon as a vertical LinearLayout:


3.

Create the ImageView element that will serve as our icon:


4.

Next, create the TextView element that will serve as the label:


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Presenting Data for Views

What just happened?
The fruit_item.xml file is a very simple layout for our menu icons, and could be used for
many other types of icons represented as a grid. ImageView objects will, by default, attempt
to scale their content to their size. In our previous example, the root LinearLayout has the
width and height defined as fill_parent. When placed in a GridView as a single item,
using fill_parent as a size will cause the LinearLayout to fill the space provided for
that grid item (not the entire GridView).

Displaying icons in a GridView
We need an object model and ListAdapter in order to display the fruits to the user in a
GridView. The adapter is fairly straightforward at this point. It's a normal ListAdapter
implementation built on top of an item class and the layout XML we defined for the icons.
For each item of fruit, we will need an object holding both the fruit's name and icon. Create
a FruitItem class in the root package with the following code:
class FruitItem {
final String name;
final int image;
FruitItem(String name, int image) {
this.name = name;
this.image = image;
}
}

In the preceding code, we referenced the icon image for the fruit as an integer. When we
reference application resources and IDs in Android, it's always with an integer. For this
example we're assuming that all of the different types of fruit each have an icon as an
application resource. Another option would be to hold a reference to a Bitmap object
in each FruitItem. However, this would have meant holding the full image in memory
when the FruitItem is potentially not on the screen.
In order for the Android Asset Packaging Tool to recognize and store the icons, you will need
to put them in the res/drawable directory.
Android Image Resources
Generally, it's considered a good practice in Android to store bitmap images
as PNG files. Since you will be accessing these files from your code, make
sure they have Java-friendly filenames. The PNG format (unlike JPG) is
lossless, can have various different color depths, and correctly handles
transparency. This generally makes it a great image format on the whole.

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Chapter 2

Time for action – building the fruit menu
For the Four Buckets 'o Fruit menu, we're going to need a ListAdapter implementation to
render the FruitItem objects into the fruit_item.xml layout resources. We'll also need
a layout resource for the GridView which we will load in our new Activity class.

1.

Create a new class named FruitAdapter extending BaseAdapter in the root
package of the project.

2.

FruitAdapter needs to hold and represent an array of FruitItem
objects. Implement the class using the same structure as the BurgerAdapter.

3.

In the ListAdapter.getView method, set the label and icon as defined in the
fruit_item.xml layout resource:
FruitItem item = items[index];
TextView text = ((TextView)view.findViewById(R.id.text));
ImageView image = ((ImageView)view.findViewById(R.id.icon));
text.setText(item.name);
image.setImageResource(item.image);

4.

Create a new layout resource to hold the GridView that we will use for the Four
Buckets 'o Fruit menu, and name it res/layout/four_buckets.xml.

5.

Populate the new layout resource with a three column GridView:


What just happened?
The new four_buckets.xml layout resource has nothing but a GridView. This is unlike
the other layout resources we've written so far, especially since the GridView has no ID.
For this example, the fruit menu Activity will contain nothing but the GridView, so
there's no need for an ID reference or layout structure. We also specified horizontal and
vertical spacing of 5dip. A GridView object's default is to have no spacing between its cells,
which makes for fairly squashed content. In order to space things out a bit, we ask for some
whitespace between each of the cells.

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Presenting Data for Views

Time for action – creating the FourBucketsActivity
Since we are working with a layout resource with only a GridView, and no ID reference,
we're going to walk through the creation of the Activity step-by-step. Unlike previous
Activity implementations, we will need a direct reference to the GridView defined in
four_buckets.xml, and this means loading it manually.

1.

Start by creating a new class in your project's root package:
public class FourBucketsActivity extends Activity {

2.

Override the onCreate method, and invoke the super implementation:
protected void onCreate(final Bundle istate) {
super.onCreate(istate);

3.

Get the LayoutInflator instance for your Activity object:
LayoutInflater inflater = getLayoutInflater();

4.

Inflate the four_buckets.xml resource and cast its contents directly to a
GridView object:
GridView view = (GridView)inflater.inflate(
R.layout.four_buckets,
null);

5.

Set the ListAdapter of the view object to a new instance of the FruitAdapter
class, and populate the new FruitAdapter with some FruitItem objects:
view.setAdapter(new FruitAdapter(
new FruitItem("Apple", R.drawable.apple),
new FruitItem("Banana", R.drawable.banana),
new FruitItem("Black Berries", R.drawable.blackberry),
// and so on

6.

Use setContentView to make the GridView your root View object:
setContentView(view);

7.

Register your FourBucketsActivity class in your AndroidManifest.xml.

8.

Add a case to the SelectRestaurantActivity to start the new
FourBucketsActivity when the user selects it.

What just happened?
You just completed the Four Buckets 'o Fruit menu. If you re-install the application into your
emulator, you'll now be able to go and order fruits (just be careful to have the 16 ton weight
ready in case the delivery guy attacks you).
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Chapter 2

If you look through the Activity documentation, you'll notice that while there's a
setContentView method, there's no corresponding getContentView method. Take a
closer look and you will notice the addContentView method. An Activity object may
have any number of View objects attached to it as "content". This precludes any useful
implementation of a getContentView method.
In order to get around this limitation, we inflated the layout ourselves. The
getLayoutInflator() method used is simply a shortcut for LayoutInflator.
from(this). Instead of using an ID and findViewById, we simply cast the View returned
directly to a GridView, since that's all that our four_buckets.xml file contains (much the
same way the ArrayAdapter class works with TextView objects). If we wanted to make
things a little more abstract, we could have cast it to an AdapterView, in
which case we could have swapped in implementation in the file with a ListView. However,

this wouldn't have been very useful for this example.
If you now re-install and run the application, your new FourBucketsActivity will present
you with a screen similar to the following one:

Have a go hero – Sam's Sushi
The last restaurant on the menu is Sam's Sushi. Try using the Spinner class along with
a GridView to create a composite sushi menu. Place the spinner at the top of the screen,
with options for different types of sushi:
‹‹

Sashimi

‹‹

Maki Roll

‹‹

Nigiri

‹‹

Oshi
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Presenting Data for Views
‹‹

California Roll

‹‹

Fashion Sandwich

‹‹

Hand Roll

Below the Spinner, use a GridView to display icons for each different type of fish that the
user can order. Here are some suggestions:
‹‹

Tuna

‹‹

Yellowtail

‹‹

Snapper

‹‹

Salmon

‹‹

Eel

‹‹

Sea Urchin

‹‹

Squid

‹‹

Shrimp

The Spinner class makes use of the SpinnerAdapter instead of a ListAdapter.
The SpinnerAdapter includes an additional View object which represents the
drop-down menu. This is most typically a reference to the android.R.layout.simple_
dropdown_item_1line resource. However, for this example, however, you can probably
make use of the android:entries attribute on the Spinner XML element.

Summary
Data display is one of the most common requirements of a mobile application, and Android
has many different options available. The ListView is probably one of the most commonly
used widgets in the standard Android suite, and styling it allows it to be used to display
varying amounts of data, from one line menu items to multi-line to-do notes.
The GridView is effectively a tabular version of ListView, and is well suited for presenting
the user with icon views. Icons have enormous advantages over text, since they can be
recognized much more quickly by the user. Icons can also take up significantly less space, and
in a GridView, you could easily fit four to six icons in a portrait screen without making the
user interface cluttered or more difficult to work it. This also frees up precious screen space
for other items to be displayed.

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Chapter 2

Building custom Adapter classes not only allows you to take complete control over the
styling of the ListView, but also determine where the data comes from, and how it's
loaded. You could, for example, load the data directly from a web service by using an
Adapter which generates dummy View objects until the web service responds with actual
data. Take a good look at the default Adapter implementations, they will generally serve
your requirements, especially when coupled with a custom layout resource.
In the next chapter, we will take a look at some less generic, more specialized View classes
that Android provides. As with almost everything in Android, the defaults may be specific,
but they can be customized in any number of ways to fit some very unusual purposes.

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Developing with Specialized
Android Widgets
Along with the many generic widgets such as buttons, text fields, and
checkboxes, Android also includes a variety of more specialized widgets. While
a button is fairly generic, and has use in many situations, a gallery-widget for
example, is far more targeted. In this chapter we will start looking at the more
specialized Android widgets, where they appear, and how best they can be
used.

Although these are very specialized View classes, they are very important. As mentioned
earlier (and it really can't be stressed enough) one of the cornerstones of good user interface
design is consistency. An example is the DatePicker widget. It's certainly not the prettiest
date-selector in the world. It's not a calendar widget, so it's sometimes quite difficult for
the user to select exactly which date they want (most people think in terms of "next week
Tuesday", and not "Tuesday the 17th"). However, the DatePicker is standard! So the user
knows exactly how to use it, they don't have to work with a broken calendar implementation.
This chapter will work with Android's more specialized View and layout classes:
‹‹

Tab layouts

‹‹

TextSwitcher

‹‹

Gallery

‹‹

DatePicker

‹‹

TimePicker

‹‹

RatingBar

Download from Wow! eBook 

Developing with Specialized Android Widgets

These classes have very specialized purposes, and some have slight quirks in the way they
are implemented. This chapter will explore how and where to use these widgets, and
where you need to be careful of their implementation details. We'll also discuss how
best to incorporate these elements into an application, and into a layout.

Creating a restaurant review application
In the previous chapter, we built an ordering-in application. In this chapter, we're going
to take a look at reviewing restaurants. The application will allow the user to view other
people's opinions on the restaurant, a gallery of photos of the restaurant, and finally
a section for making an online reservation. We will divide the application into three sections:
‹‹

Review: Review and ratings information for this restaurant

‹‹

Photos: A photo gallery of the restaurant

‹‹

Reservation: Request a reservation with the restaurant

When building an application where all three of these sections need to be quickly available
to the user, the most sensible option available is to place each of the sections in a tab on the
screen. This allows the user to switch between the three sections without having all of them
on the screen at the same time. This also saves screen real estate giving us more space for
each section.
The Review tab will include a cycling list of comments that people have made about the
restaurant being viewed, and an average "star" rating for the restaurant.
Displaying photographs of the restaurant is the job of the Photos tab. We'll provide the
user with a thumbnail "track" at the top of the screen, and a view of the selected image
consuming the remaining screen space.
For the Reservation tab, we will want to capture the user's name and when they would like
the reservation to be (date and time). Finally we also need to know for how many people
the reservation will be made.

Time for action – creating the robotic review project structure
To start this example we'll need a new project with a new Activity. The new layout and
Activity will be a little different from the structures in the previous two chapters. We
will need to use the FrameLayout class in order to build a tabbed layout. So to begin, we'll
create a new project structure and start off with a skeleton that will later become our tab
layout structure. This can be filled with the three content areas.

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Chapter 3

1.

Create a new Android project using the Android command-line tool:
android create project -n RoboticReview -p RoboticReview -k com.
packtpub.roboticreview -a ReviewActivity -t 3

2.

Open the res/layout/main.xml file in an editor or IDE.

3.

Clear out the default code (leaving in the XML header).

4.

Create a root FrameLayout element:


5.

Inside the new FrameLayout element, add a vertical LinearLayout:



6.

After the LinearLayout, add another empty LinearLayout element:



7.

Then, after the second LinearLayout element, add an empty ScrollView:



The FrameLayout will be used by the Android tab structures as a content area, each of
the child elements will become the contents of a tab. In the preceding layout, we've added
in two LinearLayout elements for the Review and Photos sections, and a ScrollView for
the Reservation tab.

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Developing with Specialized Android Widgets

What just happened?
We've just started the "restaurant review" application, building a skeleton for the user
interface. There are several key parts of this main.xml file which we should walk through
before continuing the example.
First, our root element is a FrameLayout. The FrameLayout anchors all of its children to its
own top-left corner. In effect, the two occurrences of LinearLayout and the ScrollView
will overlap each other. This structure can be used to form something like a Java AWT
CardLayout, which will be used by the TabHost object to display these objects when their
relative tab is active.
Second, each of the LinearLayout and the ScrollView have an ID. In order to identify
them as tab roots, we need to be able to easily access them from our Java code. Tab
structures may be designed in XML, but they need to be put together in Java.

Building a TabActivity
In order to continue, we need our Activity class to set up the three tab content
elements we declared in our main.xml file as tabs. By preference, all tabs in Android
should have an icon.
The following is a screenshot of the tabs without their icons:

The following is a screenshot of the tabs with the icons:

Creating tab icons
Android applications have a specific look and feel defined by the default widgets provided
by the system. In order to keep all applications consistent for users, there are a set of user
interface guidelines that application developers should follow. While it's important to have
your application stand out, users will often get frustrated with applications that are not
familiar or look out of place (this is one of the reasons why automatically ported applications
are often very unpopular).

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Chapter 3

Android tabs and icons
When selecting tab icons for your application, it's considered a good practice to include
several different versions of the icon for different screen sizes and densities. The anti-aliased
corners that look so good on a high-density screen, look terrible on low-density screens.
You can also provide entirely different icons for very small screens, instead of loosing all
of your icons details. Android tabs appear raised when they are selected, and lowered in
the background when they are not selected. The Android tab icons should appear in the
"opposite" etching effect to the tab that they are placed in, that is, lowered when they
are selected and raised when they are not selected. The icons therefore have two primary
states: selected and unselected. In order to switch between these two states, a tab-icon will
generally consist of three resource files:
‹‹

The selected icon image

‹‹

The unselected icon image

‹‹

An XML file describing the icon in terms of its two states

Tab icons are generally simple shapes while the image size is squared (generally at a
maximum of 32 x 32 pixels). Different variations of the image should be used for screens of
different pixel densities (see Chapter 1, Developing a Simple Activity for "Resource Selection"
details). Generally you will use a dark outset image for the selected state, since when a tab is
selected, the tab background is light. For the unselected icon, the opposite is true and a light
inset image should be used instead.
The bitmap images in an Android application should always be in the PNG format. Let's call
the selected icon for the Review tab res/drawable/ic_tab_selstar.png, and name
the unselected icon file res/drawable/ic_tab_unselstar.png. In order to switch
states between these two images automatically, we define a special StateListDrawable
as an XML file. Hence the Review icon is actually in a file named res/drawable/review.
xml, and it looks like this:





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Developing with Specialized Android Widgets

Note the android:constantSize="true" of the  element. By default,
Android will assume that each state in the resulting StateListDrawable object will cause
the image to be of a different size, which in turn may cause the user interface to re-run its
layout calculations. This can be fairly expensive, so it's a good idea to declare that each of
your states is exactly of the same size.
For this example, we'll be using three tab icons, each with two states. The icons are named
review, photos, and book. Each one is composed of three files: A PNG for the selected
icon, a PNG for the unselected icon, and an XML file defining the state-selector. From our
application, we will only make direct use of the state-selector XML files, leaving the Android
APIs to pickup the actual PNG files.

Implementing the ReviewActivity
As usual, we will want to have localized text in our strings.xml file. Open the res/
values/strings.xml file and copy the following code into it:

Robotic Review
Review
Photos
Reservations


Time for action – writing the ReviewActivity class
As already said, we will need to set up our tabbed-layout structure in our Java code.
Fortunately, Android provides a very useful TabActivity class that does much of the heavy
lifting for us, providing us with a ready-made TabHost object with which we can construct
the Activity tab structure.

1.

Open the ReviewActivity.java file generated earlier in an editor or IDE.

2.

Instead of extending Activity, change the class to inherit TabActivity:
public class ReviewActivity extends TabActivity

3.

In the onCreate method, remove the setContentView(R.layout.main) line
(generated by the android create project utility) completely.

4.

Now start by fetching the TabHost object from your parent class:
TabHost tabs = getTabHost();

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Chapter 3

5.

Next, we inflate our layout XML into the content view of the TabHost:
getLayoutInflater().inflate(
R.layout.main,
tabs.getTabContentView(),
true);

6.

We'll need access to our other application resources:
Resources resources = getResources();

7.

Now we define a TabSpec for the Review tab:
TabHost.TabSpec details =
tabs.newTabSpec("review").
setContent(R.id.review).
setIndicator(getString(R.string.review),
resources.getDrawable(R.drawable.review));

8.

Define two more TabSpec variables for the Photos and Reservation tabs using the
preceding pattern.

9.

Add each of the TabSpec objects to our TabHost:
tabs.addTab(details);
tabs.addTab(gallery);
tabs.addTab(reservation);

This concludes the creation of the tab structure for the ReviewActivity class.

What just happened?
We built a very basic tabbed-layout for our new ReviewActivity. When working with
tabs, we didn't simply use the Activity.setContentView method, instead we inflated
the layout XML file ourselves. Then we made use of the TabHost object provided by the
TabActivity class to create three TabSpec objects. A TabSpec is a builder object that
enables you to build up the content of your tab, similar to the way you build up text with
a StringBuilder.
The content of a TabSpec is the content-view that will be attached to the tab on the screen
(assigned using the setContent method). In this example, we opted for the simplest option
and defined the tab content in our main.xml file. It's also possible to lazy-create the tab
content using the TabHost.TabContentFactory interface, or even to put an external
Activity (such as the dialer or browser) in the tab by using setContent(Intent).
However, for the purposes of this example we used the simplest option.
You'll notice that the TabSpec (much like the StringBuilder class) supports chaining
of method calls, making it easy and flexible to either set up a tab in a "single shot" approach
(as done previously), or build up the TabSpec in stages (that is, while loading from an
external service).
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Developing with Specialized Android Widgets

The indicator we assigned to the TabSpec is what will appear on the tab. In the previous
case, a string of text and our icon. As of API level 4 (Android version 1.6) it's possible to use
a View object as an indicator, allowing complete customization of the tab's look and feel.
To keep the example simple (and compatible with earlier versions)
we've supplied a String resource as the indicator.

Time for action – creating the Review layout
We've got a skeleton tab structure, but there's nothing in it yet. The first tab is titled Review,
and this is where we are going to start. We've just finished enough Java code to load up the
tabs and put them on the screen. Now we go back to the main.xml layout file and populate
this tab with some widgets that supply the user with review information.

1.

Open res/layout/main.xml in an editor or IDE.

2.

Inside the  that we named review, add a new TextView that
will contain the name of the restaurant:


3.

Below the new TextView, add a new RatingBar, where we will display how other
people have rated the restaurant:


4.

Keeping this first tab simple, we add a TextSwitcher where we can display other
people's comments about the restaurant:


The Review tab only has three widgets in this example, but more could easily be added
to allow the user to input their own reviews.

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Chapter 3

What just happened
We just put together the layout for our first tab. The RatingBar that we created has a width
of wrap_content, which is really important. If you use fill_parent, then the number
of stars visible in the RatingBar will simply be as many as can fit on the screen. If you
want control over how many stars appear on your RatingBar, stick to wrap_content,
but also make sure that (at least on portrait layouts) the RatingBar has its own horizontal
line. If you install the Activity in the emulator now, you won't see anything in either the
TextView or the TextSwitcher.
The TextSwitcher has no default animations, so we specify the "in" animation as
the default fade_in as provided by the android package, while the "out" animation
will be fade_out. This syntax is used to access resources that can be found in the
android.R class.

Working with switcher classes
The TextSwitcher we've put in place is used to animate between different TextView
objects. It's really useful for displaying things like changing stock-prices, news headlines, or
in our case, reviews. It inherits from ViewSwitcher which can be used to animate between
any two generic View objects. ViewSwitcher extends ViewAnimator which can be used
as a sort of animated CardLayout.
We want to display a series of comments from past customers, fading between each of them
with a short animation. TextSwitcher needs two TextView objects (which it will ask us to
create dynamically), for our example. We want these to be in a resource file.
For the next part of the example, we'll need some comments. Instead of using a web service
or something similar to fetch real comments, this example will load some comments from its
application resources. Open the res/values/strings.xml file and add  with a few likely comments in it:

Just Fantastic
Amazing Food
What rubbish, the food was too hairy
Messy kitchen; call the health inspector.


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Developing with Specialized Android Widgets

Time for action – turning on the TextSwitcher
We want the TextSwitcher to display the next listed comment every five seconds. For
this we'll need to employ new resources, and a Handler object. A Handler is a way for
Android applications and services to post messages between threads, and can also be used
to schedule messages at a point in the future. It's a preferred structure to use over a java.
util.Timer since a Handler object will not allocate a new Thread. In our case, a Timer is
overkill, as there is only one task we want to schedule.

1.

Create a new XML file in your res/layout directory named review_comment.xml.

2.

Copy the following code into the new review_comment.xml file:


3.

Open the ReviewActivity.java file in your editor or IDE.

4.

We'll need to be able to load the review_comment resources for the
TextSwitcher, so ReviewActivity needs to implement the ViewSwitcher.
ViewFactory interface.

5.

In order to be update the TextSwitcher, we need to interact with a Handler, and
the easiest way to do that here is to also implement Runnable.

6.

At the top of the ReviewActivity class, declare a Handler object:
private final Handler switchCommentHandler = new Handler();

7.

We'll also want to hold a reference to the TextSwitcher for our run() method
when we switch comments:
private TextSwitcher switcher;

8.

In order to display the comments, we'll need an array of them, and an index to keep
track of which comment the TextSwitcher is displaying:
private String[] comments;
private int commentIndex = 0;

9.

Now, in the onCreate method, after you add the TabSpec objects to the TabHost,
read the comments string-array from the Resources:
comments = resources.getStringArray(R.array.comments);
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Chapter 3

10.

Next, find the TextSwitcher and assign it to the switcher field:
switcher = (TextSwitcher)findViewById(R.id.reviews);

11.

Tell the TextSwitcher that the ReviewActivity object will be its ViewFactory:
switcher.setFactory(this);

12.

In order to comply with the ViewFactory specification, we need to write a
makeView method. In our case it's really simple—inflate the review_comment
resource:
public View makeView() {
return getLayoutInflater().inflate(
R.layout.review_comment, null);
}

13.

Override the onStart method so that we can post the first timed event on the
Handler object declared earlier:
protected void onStart() {
super.onStart();
switchCommentHandler.postDelayed(this, 5 * 1000l);
}

14.

Similarly, override the onStop method to cancel any future callback:
protected void onStop() {
super.onStop();
switchCommentHandler.removeCallbacks(this);
}

15.

Finally, the run() method alternates the comments in the TextSwitcher, and in
the finally block, posts itself back onto the Handler queue in five seconds:
public void run() {
try {
switcher.setText(comments[commentIndex++]);
if(commentIndex >= comments.length) {
commentIndex = 0;
}
} finally {
switchCommentHandler.postDelayed(this, 5 * 1000l);
}
}

Using Handler objects instead of creating Thread objects means all of the timed tasks
can share the main user interface thread instead of each allocating a separate thread. This
reduces the amount of memory and CPU load your application places on the device, and has
a direct impact on the application performance and battery life.
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Developing with Specialized Android Widgets

What just happened?
We just built a simple timer structure to update the TextSwitcher with a rotating array of
comments. The Handler class is a convenient way to post messages and actions between
two application threads. In Android, as with Swing, the user interface is not thread-safe, so
inter-thread communication becomes very important. A Handler object attempts to bind
itself to the thread it's created in (in the preceding case, the main thread).
It's a prerequisite that a thread which creates a Handler object must have an associated
Looper object. You can set this up in your own thread by either inheriting the
HandlerThread class, or using the Looper.prepare() method. Messages sent to a
Handler object will be executed by the Looper associated with the same thread. By
sending our ReviewActivity (which implements Runnable) to the Handler object that
we had created in the main thread, we know that the ReviewActivity.run() method
will be executed on the main thread, regardless of which thread posted it there.
In the case of long-running tasks (such as fetching a web page or a long-running calculation),
Android provides a class that bares a striking resemblance to the SwingWorker class,
named AsyncTask. AsyncTask (like Handler) can be found in the android.os package,
and you make use of it by inheritance. AsyncTask is used to allow interaction between
a background task and the user interface (in order to update a progress bar or
similar requirements).

Creating a simple photo gallery
The use of the word Gallery is a little misleading, it's really a horizontal row of items with
a "single item" selection model. For this example we'll be using the Gallery class for what
it does best, displaying thumbnails. However, as you'll see, it's capable of displaying scrolling
lists of almost anything. Since a Gallery is a spinner, you work with it in much the same
way as a Spinner object or a ListView, that is, with an Adapter.

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Chapter 3

Time for action – building the Photos tab
Before we can add images to a Gallery, we need the Gallery object on the screen. To
start this exercise, we'll add a Gallery object and an ImageView to FrameLayout of our
tabs. This will appear under the Photos tab that we created at the beginning of the chapter.
We'll stick to a fairly traditional photo gallery model of the sliding thumbnails at the top of
the screen, with the full view of the selected image below it.

1.

Open res/layout/main.xml in your editor or IDE.

2.

Inside the second LinearLayout, with android:id="@+id/photos", add a new
Gallery element to hold the thumbnails:


3.

Gallery objects, by default, squash their contents together, which really doesn't
look great in our case. You can add a little padding between the items by using the
spacing attribute of Gallery class:
android:spacing="5dip"

4.

We also have tabs directly above the Gallery, and we'll have an ImageView
directly below it. Again, there won't be any padding, so add some using a margin:
android:layout_marginTop="5dip"
android:layout_marginBottom="5dip"

5.

Now create an ImageView which we can use to display the full-sized image:


6.

In order to ensure that the full display is scaled correctly, we need to specify the
scaleType on the ImageView:
android:scaleType="centerInside"

The Gallery element provides us with the thumbnail track at the top of the screen. The
image selected in the Gallery will be displayed at full-size in the ImageView widget.

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Developing with Specialized Android Widgets

What just happened?
We just populated the second tab with the standard widgets required for a basic photo
gallery. This structure is very generic, but is also well known and understood by users. The
Gallery class will handle the thumbnails, scrolling, and selection. However, you will need
to populate the main ImageView with the selected image, and provide the Gallery object
with the thumbnail widgets to display on the screen.
The spacing attribute on the Gallery element will add some whitespace, which serves
as a simple separator between thumbnails. You could also add a border into each of the
thumbnail images, border each ImageView widget you return for a thumbnail, or use
a custom widget to create a border.

Creating a thumbnail widget
In order to display the thumbnails in the Gallery object, we will need to create an
ImageView object for each thumbnail. We could easily do this in Java code, but as usual, it is
preferable to build even the most basic widgets using an XML resource. In this case, create a
new XML resource in the res/layout directory. Name the new file gallery_thn.xml and
copy the following code into it:


That's right, it has just two lines of XML, but to reiterate, this allows us to customize this
widget for many different configurations without editing the Java code. While editing the
code might not seem like a problem (the resource needs to be recompiled anyway), you
also don't want to end up with a long series of if statements to decide on exactly how you
should create the ImageView objects.

Implementing a GalleryAdapter
For the example, we'll stick to using application resources to keep things simple. We'll
have two arrays of resource IDs, thumbnails, and the full-size images. An Adapter
implementation is expected to provide an identifier for each of the items. In this next
example, we're going to provide an identifier as the resource identifier of the full-size
image, which gives us easy access to the full-size image in classes outside of the Adapter
implementation. While this is an unusual contract, it provides a convenient way for us to
pass the image resource around within an already defined structure.
In order to display your gallery, you'll need some images to display (mine are sized 480 x 319
pixels). For each of these images, you'll need a thumbnail image to display in the Gallery
object. Generally, these should simply be a scaled-down version of the actual image (mine
are scaled to 128 x 84 pixels).
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Chapter 3

Time for action – the GalleryAdapter
Creating the GalleryAdapter is much like the ListAdapter classes we created in Chapter
2, Presenting Data for Views. The GalleryAdapter however, will use ImageView objects
instead of TextView objects. It also binds two lists of resources together instead of using
an object model.

1.

Create a new Java class in your project root package named GalleryAdapter. It
should extend the BaseAdapter class.

2.

Declare an integer array to hold the thumbnail resource IDs:
private final int[] thumbnails = new int[]{
R.drawable.curry_view_thn,
R.drawable.jai_thn,
// your other thumbnails
};

3.

Declare an integer array to hold the full-size image resource IDs:
private final int[] images = new int[]{
R.drawable.curry_view,
R.drawable.jai,
// your other full-size images
};

4.

The getCount() method is simply the length of the thumbnails array:
public int getCount() {
return thumbnails.length;
}

5.

The getItem(int) method returns the full-size image resource ID:
public Object getItem(int index) {
return Integer.valueOf(images[index]);
}

6.

As mentioned earlier, the getItemId(int) method returns the full-size image
resource ID (almost exactly the way that getItem(int) does):
public long getItemId(int index) {
return images[index];
}

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Developing with Specialized Android Widgets

7.

Finally, the getView(int, View, ViewGroup) method uses a LayoutInflater
to read and populate the ImageView which we created in the gallery_thn.xml
layout resource:
public View getView(int index, View reuse, ViewGroup parent) {
ImageView view = (reuse instanceof ImageView)
? (ImageView)reuse
: (ImageView)LayoutInflater.
from(parent.getContext()).
inflate(R.layout.gallery_thn, null);
view.setImageResource(thumbnails[index]);
return view;
}

The Gallery class is a subclass of AdapterView and so functions in the same way as
a ListView object. The GalleryAdapter will provide the Gallery object with View
objects to display the thumbnails in.

What just happened
Much like the Adapter classes built in the last chapter, the GalleryAdapter will attempt
to reuse any View object specified in its getView method. A primary difference however,
is that this GalleryAdapter is entirely self-contained, and will always display the same list
of images.
This example of a GalleryAdapter is extremely simple. You could also build a
GalleryAdapter that held bitmap objects instead of resource ID references.
You'd then make use of the ImageView.setImageBitmap method instead of
ImageView.setImageResource.
You could also eliminate the thumbnail images by having the ImageView scale the full-size
images into thumbnails. This would just require a modification to the gallery_thn.xml
resource file in order to specify the required size of each thumbnail.


The adjustViewBounds attribute tells the ImageView to adjust its own size in
a way such that it maintains the aspect ratio of the image it contains. We also change the
scaleType attribute to centerInside, which will also retain the aspect ratio of the
image when it scales. Finally, we set a maximum width for the ImageView. Using the
standard layout_width or layout_height attributes is ignored by the Gallery class,
so we instead specify the desired thumbnail size to the ImageView (the layout_width
and layout_height attributes are handled by the Gallery, while the maxWidth and
maxHeight are handled by the ImageView).
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Chapter 3

This would be a standard speed/size trade-off. Having the thumbnail images takes up more
application space, but having the ImageView perform the scaling makes the application
slower. The scaling algorithm in ImageView will also not be as high-quality as the scaling
performed in an image-manipulation application such as Adobe Photoshop. In most cases
this won't be a problem, but if you have high detail images, you often get "scaling artifacts"
with simpler scaling algorithms.

Time for action – making the gallery work
Now that we've got the GalleryAdapter working, we need to connect the Gallery, the
GalleryAdapter, and the ImageView together, so that when a thumbnail is selected, the
full-view of that image is displayed in the ImageView object.

1.

Open the ReviewActivity source code in your editor or IDE.

2.

Add AdapterView.OnItemSelectedListener to the interfaces that the
ReviewActivity implements.

3.

Below the declaration of the TextSwitcher, declare a reference to the ImageView
which will hold the full-size image:
private TextSwitcher switcher;
private ImageView photo;

4.

At the end of the onCreate method, find the ImageView named photo and assign
it to the reference you just declared:
photo = ((ImageView)findViewById(R.id.photo));

5.

Now fetch the Gallery object as declared in the main.xml layout resource:
Gallery photos = ((Gallery)findViewById(R.id.gallery));

6.

Create a new GalleryAdapter and set it on the Gallery object:
photos.setAdapter(new GalleryAdapter());

7.

Set the OnItemSelectedListener of the Gallery object to this:
photos.setOnItemSelectedListener(this);

8.

At the end of the ReviewActivity class, add the onItemSelected method:
public void onItemSelected(
AdapterView av, View view, int idx, long id) {
photo.setImageResource((int)id);
}

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9.

OnItemSelectedListener requires an onNothingSelected method as well,

but we don't need it to do anything for this example.

The GalleryAdapter provides the ReviewActivity with the resource to load for the
full view of the photo through the id parameter. The id parameter could also be used as
an index or identifier for a URL if the image was located on a remote server.

What just happened?
We've now connected the Gallery object to the ImageView where we will display the fullsize image instead of the thumbnail. We've used the item ID as a way to send the resource
ID of the full-size image directly to the event listener. This is a fairly strange concept since
you'd normally use an object model. However, an object model in this example wouldn't just
introduce a new class, it would also require another method call (in order to fetch the image
object from the Adapter when the event is triggered).
When you specify an Adapter on an AbsSpinner class like Gallery, it will by default
attempt to select the first item returned from its new Adapter. This in turn notifies the
OnItemSelectedListener object if one has been registered. However, because of the
single-threading model used by the Android user interface objects, this event doesn't get
fired immediately, but rather some time after we return from the onCreate method. When
we call setAdapter(new GalleryAdapter()) on the Gallery object, it schedules a
selection change event, which we then receive. The event causes the ReviewActivity
class to display the first photo in the GalleryAdapter object.
If you now reinstall the application in your emulator, you'll be able to go to the Photos
tab and browse through a Gallery of all the images that you had populated the
GalleryAdapter with.

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Chapter 3

Pop quiz
1. What would happen in the previous example if you substituted
OnItemSelectedListener with OnItemClickListener (as done in the
ListView examples)?
a. The full size won't appear anymore.
b. The Gallery will not rotate the thumbnails when they are touched.
c.

The full-size photo won't appear until a thumbnail is clicked.

2. What is the primary difference between the ScaleType values fitXY
and centerInside?
a. The fitXY type will anchor the picture to the top-left, while centerInside
will center the picture in the ImageView.
b. fitXY will cause the picture to distort to the size of the ImageView, while
centerInside will maintain the picture's aspect ratio.
c.

centerInside will cause the larger axis to be cropped in order to fit the
picture into the ImageView, while fitXY will scale the picture so that the
larger axis is of the same size as the ImageView.

3. What dictates the size of a Gallery object containing ImageView objects when
using the wrap_content attribute?
a. The width and height of the ImageView objects, as dictated by the size of their
content image, or their maxWidth and maxHeight parameters.
b. The itemWidth and itemHeight parameters on the Gallery object.
c.

The LayoutParams set on the ImageView objects (either with the
setLayoutParams method, or layout_width/layout_height attributes).

Have a go hero – animations and external sources
Now that you have the basic example working, try improving the user experience a bit.
When you touch the images, they should really animate instead of undergoing an instant
change. They should also come from an external source instead of application resources.
1. Change the ImageView object of full-size images to an ImageSwitcher, use the
standard Android fade-in/fade-out animations.
2. Remove the thumbnail images from the project, and use the ImageView declared
in the gallery_thn.xml file to scale the images.
3. Change from a list of application resource IDs to a list of Uri objects so that the
images are downloaded from an external website.

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Building the reservation tab
While the Review and Photos tabs of this example have been concerned with displaying
information, the Reservation tab will be concerned with capturing the details of a
reservation. We really only need three pieces of information:
‹‹

The name under which the reservation needs to be made

‹‹

The date and time of the reservation

‹‹

How many people the reservation is for

In this part of the example we'll create several widgets which have formatted labels. For
example, How Many People: 2, which will update the number of people as the user changes
the value. In order to do this simply, we specify that the widget's text (as specified in the
layout file) will contain the format to use for display. As part of the initialization procedure,
we read the text from the View object and use it to create a format structure. Once we have
a format, we populate the View with its initial value.

Time for action – implementing the reservation layout
In our main.xml layout resource, we need to add the View objects which will form the
Reservation tab. Currently it consists only of an empty ScrollView, which enables
vertically-long layouts to be scrolled by the user if the entire user interface doesn't fit
on the screen.

1.

Open the main.xml file in your editor or IDE.

2.

Inside the  we had created for the Reservation tab earlier. Declare
a new vertical LinearLayout element:


3.

Inside the new LinearLayout element, create a TextView to ask the user under
what name the reservation should be made:


4.

After the TextView label, create an EditText to allow the user to input the name
under which reservation is to be made:

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5.

Create another TextView label to ask the user how many people will be going. This
includes a format element where we will place the number:


6.

Add a SeekBar with which the user can tell us about how many people are going:


7.

Use another TextView to ask the user what date the reservation will be on:


8.

Add a Button to display the date for which the reservation is made. When the user
taps this Button, we will ask him to select a new date:


9.

Create another TextView label to ask the time of reservation:


10.

Add another Button to display the time, and allow the user to change it:


11.

Finally add a Button to make the reservation, and add some margin to separate it
from the rest of the inputs in the form:

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Several of the preceding widgets include the format of their labels instead of the label literal,
the actual label will be generated and set in the Java code. This is because these labels are
subject to change when the user changes date, time, or the number of people expected for
the reservation.

What just happened?
In the Reservation tab, we ask the user how many people the reservation is for, and in order
to capture their answer, we make use of a SeekBar object. The SeekBar works in much the
same way as a JSlider in Swing, and provides the user with a way of selecting the number
of people for the reservation, as long as that number is within a range that we define.
SeekBar in Android is actually built on top of the ProgressBar class, and so inherits all of
its XML attributes, which will seem a little strange at times. Unfortunately, unlike a JSlider
or JProgressBar, the SeekBar class has no minimum value, and since you can't make a
reservation for 0 people, we work around this by always adding 1 to the selected value of the
SeekBar before display. This means that the default value is 1 (setting the displayed value
to 2 people).
Most people would make a restaurant reservation
for two people, hence the default value of 1.

In the How Many People: label, we put in a %d, which is a printf marker for where we
will put the number of people the reservation is being made for. When the SeekBar is
manipulated by the user, we'll update the label with the number the user selects using
String.format. In the "date" and "time" Button labels, we want to display the currently
selected date and time for the reservation. We set the label in the XML file to the format
that we want to display this data in, and we'll parse it later with a standard java.text.
SimpleDateFormat.
What about internationalization in our previous example? Shouldn't we have put the labels
in the strings.xml file so that the layout doesn't need to change? The answer is: Yes, if
you want to internationalize your user interface. Later, make sure you have all of your display
text in an application resource file. However, I strongly recommend fetching the format
strings directly from the layout, since it allows you to decouple the format data
one additional level.
In the preceding layout, you created Button widgets to display the date and time. Why not
use a DatePicker and TimePicker object directly? The answer is: They unfortunately
don't fit well into normal layouts. They take up a large amount of vertical space, and don't
scale horizontally. If we placed a DatePicker and TimePicker inline in this user interface,
it would look like the following screenshot on the left, while the actual user interface is the
screenshot on the right.
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As you can see, the Button objects give a much cleaner user interface. Thankfully, Android
provides us with a DatePickerDialog and TimePickerDialog for just this sort of
situation. When the user taps on one of the Button widgets, we'll pop up the appropriate
dialog and then update the selected Button label when he approves.
While the use of a Button and Dialog adds at least two more touches to the user interface,
it dramatically improves the look and feel of the application. User interfaces that are not
properly aligned will irritate users, even if they can't tell why it's irritating. Screens that users
find annoying or irritating are screens that they will avoid, or worse—simply uninstall.

Time for action – initializing the reservation tab
In the Reservation tab we made use of formatted labels. These labels shouldn't be displayed
to the user as-is, but need to be populated with data before we let the user see them. For
this, we need to go to our Java code again and build some functionality to remember the
format, and populate the label.

1.

Open the ReviewActivity Java source in your editor or IDE.

2.

Below of all the fields you've declared so far, we need to add some more for the
Reservations tab. Declare a String to remember the formatting of the How Many
People: label:
private String peopleLabelFormat;

3.

Then declare a reference to the How Many People: label:
private TextView peopleLabel;

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4.

Declare a SimpleDateFormat object for the format of the date Button:
private SimpleDateFormat dateFormat;

5.

Declare a reference to the date Button:
private Button date;

6.

Add another SimpleDateFormat for the format of the time Button:
private SimpleDateFormat timeFormat;

7.

Next, declare a Button reference for the time Button object:
private Button time;

8.

At the end of the onCreate method, we need to initialize the Reservations tab.
Start by assigning out the peopleLabel and fetching the peopleLabelFormat
using the TextView.getText() method:
peopleLabel = (TextView)findViewById(R.id.people_label);
peopleLabelFormat = peopleLabel.getText().toString();

9.

Then fetch the date Button reference and its label format:
date = (Button)findViewById(R.id.date);
dateFormat = new SimpleDateFormat(date.getText().toString());

10.

Do the same for the time Button and its label format:
time = (Button)findViewById(R.id.time);
timeFormat = new SimpleDateFormat(time.getText().toString());

11.

Now we need to populate the Button objects with a default date and time, and for
this we need a Calendar object:
Calendar calendar = Calendar.getInstance();

12.

If it's later than 4:00p.m., it's likely that the reservation should be made for the next
day, so we add one day to the Calendar if this is the case:
if(calendar.get(Calendar.HOUR_OF_DAY) >= 16) {
calendar.add(Calendar.DATE, 1);
}

13.

Now we set the default time of day for a reservation on the Calendar object:
calendar.set(Calendar.HOUR_OF_DAY, 18);
calendar.clear(Calendar.MINUTE);
calendar.clear(Calendar.SECOND);
calendar.clear(Calendar.MILLISECOND);

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14.

Set the label for the date and time button from the Calendar object:
Date reservationDate = calendar.getTime();
date.setText(dateFormat.format(reservationDate));
time.setText(timeFormat.format(reservationDate));

15.

Now we need the SeekBar so that we can fetch its default value (as declared in the
layout application resource):
SeekBar people = (SeekBar)findViewById(R.id.people);

16.

Then we can use the label format, and the SeekBar value to populate the How
Many People label:
peopleLabel.setText(String.format(
peopleLabelFormat,
people.getProgress() + 1));

Now we have the various formats in which the labels need to be displayed on the
user interface. This allows us to regenerate the labels when the user changes the
reservation parameters.

What just happened?
The Reservations tab will now be populated with the default data for a reservation, and
all the formatting in the labels has disappeared. You will probably have noticed the many
calls to toString() in the previous code. Android View classes generally accept any
CharSequence for labels. This allows for much more advanced memory management
than the String class, as the CharSequence may be a StringBuilder, or may facade
a SoftReference to the actual text data.
However, most traditional Java APIs expect a String, not a CharSequence, so we use
the toString() method to make sure we have a String object. If the underlying
CharSequence is a String object, the toString() method is a simple return this;
(which will act as a type cast).
Again, to work around the fact that the SeekBar doesn't have a minimum value, we add 1
to its current value in the last line, when we populate the peopleLabel. While the date
and time formats are stored as a SimpleDateFormat, we store the peopleLabelFormat
as a String and will run it through String.format when we need to update the label.

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Time for action – listening to the SeekBar
The user interface is now populated with the default data. However, it's not interactive at all.
If you drag the SeekBar the How Many People: label will remain at its default value of 2.
We need an event listener to update the label when the SeekBar is used.

1.
2.

Open the ReviewActivity Java source in your editor or IDE.

3.

In onCreate, after fetching the SeekBar with findViewById, set its
OnSeekBarChangeListener to this:

Add SeekBar.OnSeekBarChangeListener to the interfaces that
ReviewActivity implements.

SeekBar people = (SeekBar)findViewById(R.id.people);
people.setOnSeekBarChangeListener(this);

4.

Implement the onProgressChanged method to update peopleLabel:
public void onProgressChanged(
SeekBar bar, int progress, boolean fromUser) {
peopleLabel.setText(String.format(
peopleLabelFormat, progress + 1));
}

5.

Implement an empty onStartTrackingTouch method:
public void onStartTrackingTouch(SeekBar bar) {}

6.

Implement an empty onStopTrackingTouch method:
public void onStopTrackingTouch(SeekBar bar) {}

The String.format method is a common method of placing parameters in a localized
string in Android. While this is rather different to the normal java.text.MessageFormat
class, it's the preferred method in Android (although MessageFormat is still supported).

What just happened?
When you reinstall the application in the emulator, you'll now be able to use SeekBar
to select the number of people that the reservation is to be made for. While we didn't
implement the onStartTrackingTouch or onStopTrackingTouch methods, they can
be extremely useful if you hide the actual status value by default. For example, you could use
a Dialog containing icons of people to inform the user how many people the reservation is
for. When they touch the SeekBar—display the Dialog, and then when they release the
SeekBar—hide the Dialog again.

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Time for action – selecting date and time
We've made the SeekBar work as expected, but what about the date and time Button
widgets? When the users touch them, they expect to be able to select a different date
or time for their reservation. For this we'll need a good old OnClickListener, the
DatePickerDialog and TimePickerDialog classes.

1.

Open the ReviewActivity Java source in your editor or IDE again.

2.

Add View.OnClickListener, DatePickerDialog.OnDateSetListener,
and TimePickerDialog.OnTimeSetListener to the interfaces that
ReviewActivity implements. Your class declaration should now look
something like this:
public class ReviewActivity extends TabActivity
implements ViewSwitcher.ViewFactory,
Runnable,
AdapterView.OnItemSelectedListener,
SeekBar.OnSeekBarChangeListener,
View.OnClickListener,
DatePickerDialog.OnDateSetListener,
TimePickerDialog.OnTimeSetListener {

3.

Implement a utility method to parse a CharSequence into a Calendar object with
a specified SimpleDateFormat:
private Calendar parseCalendar(
CharSequence text, SimpleDateFormat format) {

4.

Open a try block to allow handling of parse errors if the CharSequence is not
formatted according to the SimpleDateFormat.

5.

Parse the CharSequence into a Date object:
Date parsedDate = format.parse(text.toString());

6.

Then create a new Calendar object:
Calendar calendar = Calendar.getInstance();

7.

Set the time on the Calendar object to the time in the Date object:
calendar.setTime(parsedDate);

8.

Return the parsed Calendar object:
return calendar;

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9.

You'll need to catch(ParseException) in this method. I recommend wrapping it
in a RuntimeException and re-throwing it:
catch(ParseException pe) {
throw new RuntimeException(pe);
}

10.

In the onCreate method, after setting the labels of the date and time Button
widgets, set their OnClickListener to this:
date.setText(dateFormat.format(reservationDate));
time.setText(timeFormat.format(reservationDate));
date.setOnClickListener(this);
time.setOnClickListener(this);

11.

Implement the onClick method to listen for when the user taps the date or
time Button:
public void onClick(View view) {

12.

Use the View parameter to determine if the clicked View is the date Button:
if(view == date) {

13.

If so, use the parseCalendar method to parse the current value of the date
Button widget's label:
Calendar calendar = parseCalendar(date.getText(), dateFormat);

14.

Create a DatePickerDialog and populate it with the date in the Calendar,
then show() the DatePickerDialog:
new DatePickerDialog(
this, // pass ReviewActivity as the current Context
this, // pass ReviewActivity as an OnDateSetListener
calendar.get(Calendar.YEAR),
calendar.get(Calendar.MONTH),
calendar.get(Calendar.DAY_OF_MONTH)).show();

15.

Now check if the user has clicked on View Button instead of date:
else if(view == time) {

16.

If so, parse a Calendar using the time Button widget's label value:
Calendar calendar = parseCalendar(time.getText(), timeFormat);

17.

Now create a TimePickerDialog with the selected time, then show() the new
TimePickerDialog to the user:
new TimePickerDialog(
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this, // pass ReviewActivity as the current Context
this, // pass ReviewActivity as an OnTimeSetListener
calendar.get(Calendar.HOUR_OF_DAY),
calendar.get(Calendar.MINUTE),
false) // we want an AM / PM view; true = a 24hour view
.show();

18.

Now implement the onDateSet method to listen for when the user accepts the
DatePickerDialog with a new date selected:
public void onDateSet(
DatePicker picker, int year, int month, int day)

19.

Create a new Calendar instance to populate the date into:
Calendar calendar = Calendar.getInstance();

20.

Set the year, month, and day on the Calendar:
calendar.set(Calendar.YEAR, year);
calendar.set(Calendar.MONTH, month);
calendar.set(Calendar.DAY_OF_MONTH, day);

21.

Set the label of the date Button to the formatted Calendar:
date.setText(dateFormat.format(calendar.getTime()));

22.

Implement the onTimeSet method to listen for when the user accepts the
TimePickerDialog after selecting a new time:
public void onTimeSet(TimePicker picker, int hour, int minute)

23.

Create a new Calendar instance:
Calendar calendar = Calendar.getInstance();

24.

Set the Calendar object's hour and minute fields according to the parameters
given by the TimePickerDialog:
calendar.set(Calendar.HOUR_OF_DAY, hour);
calendar.set(Calendar.MINUTE, minute);

25.

Set the label of the time Button by formatting the Calendar object:
time.setText(timeFormat.format(calendar.getTime()));

Having stored the format for the date and time objects, we can now display the values
selected by the user in the Button widgets. When the user has selected a new date or
time we update the Button labels to reflect the new selections.

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What just happened
If you install and run the application in the emulator, you can now tap on either the date or
time Button widgets, and you will be greeted by a modal Dialog allowing you to select a
new value. Beware of overusing modal Dialog widgets, because they block access to the
rest of your application. You should avoid using them for displaying status messages as they
effectively render the rest of the application useless during that time. If you do display a
modal Dialog, ensure that there is some way for the user to dismiss the Dialog without
any other interaction (that is, a Cancel button or something similar).
The first advantage to using a DatePickerDialog and TimePickerDialog comes
from the fact that both include Set and Cancel buttons. This allows the user to manipulate
the DatePicker or TimePicker, and then cancel the changes. If you used an inline
DatePicker or TimePicker widget, you could provide a Reset button, but this would
take up additional screen space, and generally would seem out-of-place (until it's
actually needed).
Another advantage of the DatePickerDialog over the DatePicker widget is that the
DatePickerDialog displays a long-format of the selected date in it's title area. This
long-format date generally includes the day of the week that the user has currently selected.
The "day of the week" is a field that is noticeably missing from the DatePicker widget,
which makes it surprisingly difficult to use. Most people think in terms of "next Thursday",
instead of "the 2nd of August, 2010." Having the day of the week visible makes the
DatePickerDialog a much better choice for date selection than an inline DatePicker.

Creating complex layouts with Include, Merge, and
ViewStubs
In this chapter we've built a single layout resource with three different tabs in it. As a result
of this, the main.xml file has become quite large and hence, more difficult to manage.
Android provides several ways in which you can break up large layout files (such as this one)
into smaller chunks.
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Using Include tags
The include tag is the simplest one to work with. It's a straight import of one layout XML
file into another. For our previous example, we could separate each tab out into its own
layout resource file, and then include each one in the main.xml. The include tag has
only one mandatory attribute: layout. This attribute points to the layout resource to be
included. This tag is not a static or compile-time tag, and so the included layout file will be
selected through the standard resource selection process. This allows you to have a single
main.xml file, but then add a special reviews.xml file (perhaps for Spanish).
The layout attribute on the include tag is not prefixed with the android XML namespace.
If you attempt to use the layout attribute as android:layout, you won't get any compiletime errors, but your application will strangely fail to run.
The include element can also be used to assign or override several attributes of the root
included element. These include the element android:id, and any of the android:
layout attributes. This allows you to reuse the same layout file in several parts of your
application, but with different layout attributes and a different ID. You can even include the
same layout file several times on the same screen, but with a different ID for each instance.
If we were to change our main.xml file to include each of the tabs from other layout
resources, the file would look something more like this:







Merging layouts
The include element is very fine and well when you want to include a single View or
ViewGroup into a larger layout structure. However, what if you want to include multiple
elements into a larger layout structure, without implying the need for a root element in the
included structure? In our example each tab needs a single root View in order that each tab
carries a single and unique ID reference.
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However, having an additional ViewGroup just for the sake of an include can adversely
affect the performance of large layout trees. In this case, the merge tag comes to the rescue.
Instead of declaring the root element of a layout as a ViewGroup, you can declare it as
. In this case, each of View objects in the included layout XML will become direct
children of the ViewGroup that includes them. For example, if you had a layout resource
file named main.xml, with a LinearLayout that included a user_editor.xml
layout resource, then the code would look something like this:





The simple implementation of the user_editor.xml looks something like this:






However, when this is included into the main.xml file, we embed the user_editor.
xml LinearLayout into the main.xml LinearLayout, resulting in two LinearLayout
objects with identical layout attributes. Obviously it would be much better to simply put
the TextView and EditView from user_editor.xml directly into the main.xml
LinearLayout element. This is exactly what the  tag is used for. If we now re-write
the user_editor.xml file using the  tag instead of a LinearLayout, it looks
like this:


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Chapter 3




Note that we no longer have the LinearLayout element, instead the TextView and
EditView will be added directly to the LinearLayout in the main.xml file. Beware of
layouts that have too many nested ViewGroup objects, as they are almost certain to give
trouble (more than about ten levels of nesting is likely to cause your application to crash!).
Also be careful with layouts that have too many View objects. Again, more than 30 is very
likely to cause problems or make your application crash.

Using the ViewStub class
When you load a layout resource that includes another layout, the resource loader
will immediately load the included layout into the memory, in order to attach it to the
layout you've requested. When main.xml is read in by the LayoutInflator, so are
the reviews.xml, photos.xml, and reservations.xml files. In situations with very
large layout structures, this can consume a huge amount of your application memory, and
even cause your application to crash. The Android API includes a specialized View named
ViewStub which allows lazy-loading of layout resources.
A ViewStub is by default a zero-by-zero sized empty View, and when it's specialized,
inflate() method is invoked. It loads the layout resource and replaces itself with the
loaded View objects. This process allows the ViewStub to be garbage-collected as soon
as its inflate() method has been called.
If we were to make use of a ViewStub in the example, you would need to lazy-initialize
the content of a tab when it is selected by the user. This also means that none of the View
objects in a tab would exist until that tab has been selected. While using a ViewStub is a
bit more work than a straight include, it can allow you to work with much larger and more
complex layout structures than would otherwise be possible.
Any layout attributes set on a ViewStub will be passed on to its inflated View object. You
can also assign a separate ID to the inflated layout. If we wanted to include each of our tabs
in a ViewStub, the main.xml file would look something like this:







Note that unlike the include tag, the ViewStub requires the android XML namespace for
its layout attribute. After you inflate() one of the ViewStub objects, it will no longer
be available by its original android:id reference. Instead, you will be able to access the
inflated layout object using the android:inflatedId reference.

Have a go hero – separate the tabs
Extract each of the tabs into its own layout resource file, and use the include tag to load
each of them. This shouldn't require any changes to the Java source code.
For more of a challenge, try using ViewStub objects instead of the include tag. This will
require you to break up the onCreate method and listen for when tabs are clicked. For
this you'll need to use TabHost.OnTabChangeListener to know when to load a specific
tab's content.

Summary
Tabs are a great way of breaking an Activity into different areas of work. With limited
screen real estate, they are a great way to make an Activity more accessible to the user.
They also have a performance impact since only one tab is rendered on the screen at a time.
The RatingBar and SeekBar are two different methods of capturing, or displaying numeric
data to the user. While they are closely related, and both function in the same way, each
class is used to address different types of data. Keep in mind the limitations of both of these,
before deciding whether and where to use them.
The Gallery class is brilliant for allowing the user to view a large number of different
objects. While in this example we used it to simply display thumbnails, it could be used as
a replacement for tabs in a web browser by displaying a list of page thumbnails above the
actual browser view. All you need to do to customize its function is to change the View
objects returned from the Adapter implementation.

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When it comes to date and time capturing, try to stick to using the DatePickerDialog
and TimePickerDialog instead of their inline counterparts (unless you have good reason).
The use of these Dialog widgets helps you conserve screen space and improve the user
experience. When they open a DatePickerDialog or TimePickerDialog, they have
better access to the editor than you can generally provide as part of your user interface
(especially on a device with a small screen).
In the next chapter, we'll take a closer look at Intent objects, the activity stack, and the
lifecycle of an Android application. We'll investigate how Intent objects and the activity
stack can be used as a way to keep applications more usable. Also, we shall learn about
improving the reuse of Activity classes.

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Leveraging Activities and Intents
In many ways Android application management appears to be inspired by
JavaScript and the web browser and rightly so! The web browser model has
proved itself as a mechanism that users find easy to work with. Android as
a system, Android has many things in common with a web browser, some of
which are obvious, and others that you will need to look a little deeper for.
The Activity Stack is much like a single-directional web browser history. When
you launch an Activity using the startActivity method, you effectively
hand control back to the Android system. When the user pushes the hardware
"Back" button on their phone, the default action is to pop the top Activity off
the stack, and display the one underneath (not always the one that started it).

In this chapter we'll explore a little of how Android runs an application and manages
Activity instances. While not strictly necessary for user interface design, it's important to
know how this works. Properly leveraging these concepts will help you ensure a consistent
user interface experience. As you will also see, it will help you improve the performance of
your application, and allow you to reuse more of your application components.
It's also important to understand how an Activity is created (and when it is created),
as well as how Android decides what Activity to create. We'll also look at some good
practices to follow when building an Activity class, and how to behave nicely within the
confines of an Android application.

Leveraging Activities and Intents

We've already encountered the "Activity Stack" in Chapter 1, Developing a Simple Activity
and Chapter 2, Presenting Data for Views where we constructed Intent objects to
launch specific Activity classes. When you used the hardware "Back" button, you were
automatically taken to the previous Activity instance, no code needed (much like a
web-browser). For this chapter we'll be looking at:
‹‹

The life cycle of an Activity object

‹‹

Using the Bundle class to maintain application state

‹‹

Exploring the relationship between an Intent and an Activity

‹‹

Passing data into an Activity through an Intent

Exploring the Activity class
The life cycle of an Activity object is much more like a Java Applet than a normal
application. It may be started, paused, resumed, paused again, killed, and then brought
back to life in a seemingly random order. Most Android devices have very good performance
specifications. However, most of them appear underpowered when compared to the top-ofthe-range devices. For those devices that do have good specifications, users tend to demand
a lot more from them than the cheaper devices. On a phone, you're never going to get away
from the fact that you have many applications and services sharing a very limited device.
An Activity may be garbage-collected any time it is not visible to the user. This means
it may be your application that is running, but because the user is looking at a different
Activity, any non-visible or background Activity objects may be shut down or garbagecollected in order to save memory. By default, the Android APIs will handle these shut down/
start up cycles elegantly by storing their state before a shut down, and restoring it when they
are re-created. A very simple diagram of the life cycle of an application with two Activity
instances is shown in the following figure. When the "Main Activity" is paused, it becomes
eligible for garbage-collection by the system. If this happens, it will first store its state in a
temporary location, restoring the state when it is brought back to the foreground.

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Storage of user interface state
If an Activity is stopped, all View objects that have an ID assigned
will attempt to store their state before they are made available for
garbage-collection. However, this state is only stored for the lifetime of
the application. When the application is shut-down, this state is lost.

While it's possible to use the setContentView method over and over again to change
the content on the screen (much the way you might build a wizard interface with an AWT
CardLayout object), it's considered a very bad idea. You are effectively trying to take the
control away from Android, which will always create problems for you. If for example, you
developed an application with only one Activity class, and used multiple layout resources
or your own custom ViewGroup objects to represent different screens, you would also have
to take control of the hardware "Back" button on the device in order to allow the user to
go backwards. Your application is released in the Android market, and a few months later a
handset manufacturer decides to put a "Forward" button onto their new phone (in the same
style as the "Forward" button on a web-browser). The Android system would be patched to
handle this change to the device, but your application would not be. As a result, your users
get frustrated with your application because "it doesn't work properly".

Using Bundle objects
In the onCreate method of the Activity class, we've been accepting a Bundle parameter
named saveInstanceState, as you may have guessed. It's where state information is
stored between stops and starts of an Activity. Despite what it looks like, a Bundle object
is not a form of persistent storage. When the configuration of a device context changes (for
example when the user selects a new language, or changes from "portrait" to "landscape"
mode), the current Activity is "restarted". For this to happen, Android requests the
Activity save its state in a Bundle object. It then shuts down and destroys the existing
instance, and then creates a new instance of the Activity (with the new configuration
parameters) with the Bundle that the state information was saved in.
The Bundle class is effectively a Map containing any number of values. Since
Bundle objects are used to store short term state (that is, the blog post a user was busy
typing), they are mostly used to store the state of View objects. They have two major
advantages over standard Java serialization in this regard:
‹‹

You are forced to implement the storage of the object manually. This requires some
thought as to how the object will be stored, and what parts of it need to be stored.
For example, most of the time in a user interface, you don't need to store the layout
information, since that can be recreated from the layout file.

‹‹

Being a key-value structure, a Bundle is more future-proof and flexible than a
serialized object. You can leave out values that are set to their defaults, reducing
the size of the Bundle.
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Leveraging Activities and Intents

A Bundle object is also a type-safe structure. If you use the putString method, only then
getString or getCharSequence will work to retrieve the object. I strongly advise that
when using the get methods of Bundle, you should always provide a default value.
Before an Activity is paused by the Android system, the system requests that it save any
state information in a Bundle object. To do this, the onSaveInstanceState method will
be invoked on the Activity. This happens before the onPause method. In order to restore
the state of the Activity, the system will invoke the onCreate method with the saved
state Bundle.
Handling Activity crashes
If an Activity class throws an uncaught exception, the user will get
the dreaded Force Close dialog box. Android will attempt to recover
from these errors by terminating the Virtual Machine, and re-opening
the root activity, providing a Bundle object with the last known state
from onSaveInstanceState.

The View class also has an onSaveInstanceState method, as well as a corresponding
onRestoreInstanceState method. As mentioned earlier, the Activity class' default
functionality will attempt to save each View object with an ID within a Bundle. This is
another good reason to stick to XML layouts instead of building your own. Having a reference
to a View object is not enough for it to be saved and restored, and while you can assign
IDs in Java code, it clutters your user interface code even more.

Time for action – building an example game: "guess my number"
We want to build a simple example that will save and restore its state from a Bundle object.
For this example, we have a very simple "guess my number" game. The Activity object
picks a number between one and ten and challenges the user to guess it.
The basic user interface layout for this example will need a label telling the user what
to do, an input area for them to input their guess, and a button to tell the application
they wish to input a guess. The following diagram is a basic idea of how the user interface
should be structured:

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Chapter 4

If the user were to get an SMS while playing this game, there's a strong chance that we will
lose the number he is trying to guess. For this reason we will store the number that he is
trying to guess in a Bundle object when the system asks us to save our state. We'll also need
to look for the stored number when starting up.

1.

From a command-prompt, create a new project named GuessMyNumber:
android create project -n GuessMyNumber -p GuessMyNumber -k com.
packtpub.guessmynumber -a GuessActivity -t 3

2.

Open the default res/layout/main.xml file in an editor or IDE.

3.

Remove the default content within the LinearLayout element.

4.

Add a new TextView to serve as a label, to tell the user what to do:


5.

Create a new EditText where the users will enter their guess. Use the
android:numeric attribute of TextView to enforce only integer input:


6.

Add a Button that the users can click on to submit their guess:


7.

Now open the GuessActivity.java file in your editor or IDE.

8.

Make the GuessActivity class implement OnClickListener:
public class GuessActivity
extends Activity implements OnClickListener {

9.

Create a field variable to store the number the user is supposed to guess:
private int number;

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Leveraging Activities and Intents

10.

Create a utility method to generate a random number between one and ten:
private static int random() {
return (int)(Math.random() * 9) + 1;
}

11.

In the onCreate method, directly after the call to super.onCreate, check
to make sure the Bundle passed in is not null:
if(savedInstanceState != null) {

12.

If the Bundle isn't null, then attempt to fetch the stored Number from it:
number = savedInstanceState.getInt("Number", random());

13.

If the Bundle is null, the Activity is running as a new instance—generate
a random number:
else {
number = random();
}

14.

Then setContentView to the main.xml layout resource:
setContentView(R.layout.main);

15.

Find the Button object you declared in the main.xml layout resource:
Button button = (Button)findViewById(R.id.guess);

16.

Set the Button object's OnClickListener to the GuessActivity object:
button.setOnClickListener(this);

17.

Now override the onSaveInstanceState method:
protected void onSaveInstanceState(Bundle outState) {

18.

Be sure to first allow the default Activity behavior:
super.onSaveInstanceState(outState);

19.

Then store the number variable in the Bundle:
outState.putInt("Number", number);

20.

We need to override the onClick method to handle the user's guess:
public void onClick(View clicked) {

21.

Find the EditText where the user enters the guessed number:
EditText input = (EditText)findViewById(R.id.number);
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Chapter 4

22.

Parse the current value of the EditText as an integer:
int value = Integer.parseInt(input.getText().toString());

23.

If the number they guessed is too low, use a Toast to tell them:
if(value < number) {
Toast.makeText(this, "Too low", Toast.LENGTH_SHORT).show();
}

24.

If the number they guessed is too high, again use a Toast to tell them:
else if(value > number) {
Toast.makeText(this, "Too high", Toast.LENGTH_SHORT).show();
}

25.

If they successfully guessed the correct number, then congratulate them:
else {
Toast.makeText(
this,
"You got it! Try guess another one!",
Toast.LENGTH_SHORT).show();

26.

Then generate a new number for the user to guess:
number = random();
}

The Toast class is used in the previous code to display the output messages for Too high,
Too low, and You got it! The Toast class is the perfect mechanism for displaying short
output messages, and they automatically disappear after a few seconds. However they're
not suitable for long messages as the user has no control over them, and cannot leave the
message open or close it on command as they are entirely non-interactive.

What just happened
In the previous example, we listened for a call to onSaveInstanceState in order to record
the number that the user is supposed to guess. We also have the current guess which the
user most recently made, in the form of an EditText. Since we assigned an ID value to
EditText in the main.xml file, the call to super.onSaveInstanceState will handle
the storage of the EditText widget's exact state (potentially including "selection" and
"focus" state).

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Leveraging Activities and Intents

In the onCreate method, the example first checks to make sure that the Bundle is not
null. If Android is attempting to create a new instance of the GuessActivity object, it
won't pass in any saved state. If however, we have a Bundle object, we invoke the Bundle.
getInt method to attempt to fetch our previously stored number value. We also pass in
a random() number as a second parameter. If the Bundle object (for whatever reason)
doesn't have a stored Number, it will return this random number, eliminating the need for
us to check such a condition.
As a quick side-note, the example made use of the android:numeric attribute of the
TextView class to enforce integer input on the EditText object. Switching to a numeric
view stops the user from entering anything except "valid" characters. It also affects the softkeyboard. Instead of displaying the full keyboard, it will only display the numbers and symbols.

Creating and consuming intents
The Intent class is Android's primary method of "late binding". It's a form of very loose
coupling which allows you to specify an action (along with some parameter data), while
not specifying how the action should be carried out. For example, you may specify browse
to http:// www.packtpub.com/ using an Intent, but you don't need to specify how
Android should carry out this action. It may use the default "browser" application, or another
web browser the user has installed, or it may even ask the user how exactly they want to get
to http:// www.packtpub.com/. There are two primary types of Intent:
‹‹

Explicit Intents

‹‹

Implicit Intents

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So far we've only made use of explicit Intent objects, where we specify the exact class
we want to run. These are very important when switching from one Activity to another,
as your application may depend on the exact implementation of an Activity. An implicit
Intent is one where instead of specifying the exact class which we want to work with, we
include an abstract name for the action we want carried out. Generally, an implicit Intent
will have much more information content, due to the following reasons:
‹‹

To allow the system to make a good selection of which component to interact with

‹‹

Intent may point to a more generic structure than we would have built ourselves

and a more generic structure often requires more information about how it is
expected to behave
Intent objects are what really make Android different from other (more traditional)

operating systems. They level the playing field between applications, and allow the user
much more choice in how they want to run their phones. It's perfectly plausible for the
user to not just install a new web browser, but also a new menu, desktop, or even
dialler application.
Each Activity instance holds onto the Intent object that started it. In Chapter 1,
Developing a Simple Activity, we made use of the Activity.getIntent() method
to fetch some parameters from the Intent object, which in turn told us which question
to ask the user.

Defining Intent actions
The first thing looked at in an implicit Intent is its action. The action defines what the
Intent "does", but not how it does it, or what it does it to. The Intent class defines a long
series of constants which represent common actions. These common actions always have
some form of backing logic, which is generally defined by the phone system. Thus they are
always available to be used by an application.
For example, you wanted to present the users with the dialler application, so they could dial
a phone number and make a call, you would use an Intent with ACTION_DIAL:
startIntent(new Intent(Intent.ACTION_DIAL));

The action value of an Intent is matched against one of the actions defined for
an Activity. An Activity may have any number of actions that it may perform,
and they're all specified as part of an application's AndroidManifest.xml file. For
example, you wanted to define an askQuestion action and bind it to an Activity,
your AndroidManifest.xml file would contain an Activity entry which would look
something like this:







An Activity may have any number of  elements, each defining a
different type of match to perform on an Intent. The Activity with the closest match
to any given Intent is chosen to perform the action requested by the Intent object.

Passing data in an Intent
Presenting the user with the dialler application in order to let them dial a phone number
is very nice, but what if we actually need them to dial a phone number? The Intent class
doesn't just work by using the action, it also provides a default space for us to tell it what
we want the action to be performed on. It's not brilliantly useful being able to open a web
browser without being able to tell the browser what URL to go to, is it?
The default data provided by an Intent is provided as a Uri object. The Uri can be made
to technically point to absolutely anything. For our earlier code snippet, we started the
dialler to let the user dial a phone number. How would we then tell the dialler: "Dial 5551234"? Simple, just take a look at the following code:
startActivity(new Intent(
Intent.ACTION_DIAL,
Uri.parse("tel://5551234")));

Adding extra data to an Intent
Sometimes a Uri doesn't allow enough data to be specified. For these cases, the Intent
class provides you with a Map space of key-value pairs, called "extra" data. The methods for
accessing the extra data correspond to the methods in the Bundle class. Back in Chapter
1, Developing a Simple Activity, we used the extra data to keep track of which question we
were asking the user.
When defining generic Activity classes (such as file viewers), it's a good idea to work on
a three phase fall-back system when looking for operational data:
‹‹

Any custom (non-standard) parameters can be passed in extra fields (and none of
them should be mandatory)

‹‹

Inspect the data Uri to see what information you should be working with

‹‹

If no data Uri is specified, fall-back gracefully to a logical default, and provide some
functionality to the user
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Chapter 4

Have a go hero – generic questions and answers
Go back to the example question and answer application from Chapter 1, Developing a
Simple Activity. Rework the QuestionActivity class to use the data Uri to specify the
question ID (by name) instead of the extra parameters.
Also, allow for the full question to be passed in using "extra" parameters—a parameter
Question for the question text to ask the user, and a parameter Answers, specifying

a string array of possible answers to the given question.

Using advanced Intent features
An Intent object is designed to indicate a single action as requested by the user. It's a selfcontained request, and in some ways it is quite similar to an HTTP request, containing both,
the action to carry out, and the resource upon which the action should be carried out, and
any additional information that may be required.
In order to find the Activity (service or broadcast receiver) that will handle an
Intent, the system makes use of intent-filters (as we discussed briefly earlier). Each
intent-filter indicates a single type of action that could be carried out by the Activity.
When two or more Activity implementations match an Intent, the system sends out an
ACTION_PICK_ACTIVITY Intent to allow the user (or some automated system) to select
which of the Activity implementations should be used to handle the Intent. The default
behavior is to ask the users which of the Activity implementations they wish to use.

Getting data back from an Intent
An Intent is not always a one-way structure, some Intent actions will provide feedback.
A great example of this is Intent.ACTION_PICK. The Intent.ACTION_PICK action is
a way to ask the user to "pick" or select some form of data (a common use would be to ask
the user to select a person or phone number from their contacts list).
When you need information back from an Intent, you use the startActivityForResult
method instead of the normal startActivity method. The startActivityForResult
method accepts two parameters, the Intent object to execute, and a useful int value
which will be passed back to you.
As mentioned earlier, when another Activity is visible instead of yours, your Activity
is paused, and may even be stopped and garbage-collected. For this reason, the
startActivityForResult method returns immediately and you can generally assume
your Activity will be paused directly after you return from your current event (passing
control back to the system).

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Leveraging Activities and Intents

In order to get information back out of the Intent you triggered, you will need to override
the onActivityResult method. The onActivityResult method is invoked every time
an Intent started with startActivityForResult returns some data to you. The first
parameter passed back into the onActivityResult method is the same integer value
that you passed into the startActivityForResult method (allowing you to pass simple
parameters back).
Passing information to another Activity
If you intend for an Activity implementation to pass information
back to its caller, you can make use of the Activity.setResult
method to pass both, a result-code and an Intent object with your
response data.

Pop quiz
1. When does onCreate get passed a valid Bundle object?
a. Every time the Activity is created
b. When the application stored information in the Bundle in a previous execution
c.

When the Activity is being restarted due to configuration changes, or a crash

2. When is the onSaveInstanceState method invoked?
a. After the onStop method
b. Before the onPause method
c.

When the Activity is being restarted

d. Before the onDestroy method
3. A Bundle object will be stored until:
a. The application is closed
b. The Activity is no longer visible
c.

The application is uninstalled

Time for action – viewing phone book contacts
In this example we will delve a little deeper into the workings of the Android system. We're
going to override the default "view contact" option, providing our own Activity to view
contacts from the phonebook on the device. When the user tries to open a contact to e-mail
or call them, they will be presented with an option to view the contact using our Activity
instead of the default one.
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Chapter 4

1.

Start by creating a new project from the command line:
android create project -n ContactViewer -p ContactViewer -k com.
packtpub.contactviewer -a ViewContactActivity -t 3

2.

Open the res/layout/main.xml layout resource in an editor or IDE.

3.

Remove the default content within the LinearLayout element.

4.

Add a new TextView object to contain the contact's display name:


5.

Then add a Button which will be used to "dial" the default phone number of the
displayed contact:


6.

Open the ViewContactActivity.java source file in your editor or IDE.

7.

Make ViewContactActivity implement OnClickListener:
public class ViewContactActivity
extends Activity implements OnClickListener {

8.

After the setContentView(R.layout.main) in the onCreate method, find the
TextView object you have created, to show the contact's name in:
TextView name = (TextView)findViewById(R.id.display_name);

9.

Then find the Button widget to display the phone number in:
Button number = (Button)findViewById(R.id.phone_number);

10.

Now use the Activity.managedQuery method to query the contact's database
for the data Uri specified in our Intent:
Cursor c = managedQuery(
getIntent().getData(),
new String[]{
People.NAME,
People.NUMBER
},
null,
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Leveraging Activities and Intents
null,
null);

11.

In a try {} finally{} block, tell the Cursor to moveToNext() and make sure it
does so (this works in exactly the same way as ResultSet.next()):
if(c.moveToNext()) {

12.

Fetch and display the contact display name from the Cursor:
name.setText(c.getString(0));

13.

Fetch and display the contact default phone number from the Cursor:
number.setText(c.getString(1));

14.

In the finally{} block, close the Cursor:
finally {
c.close();
}

15.

Now set the OnClickListener of the number Button to this:
number.setOnClickListener(this);

16.

Override the onClick method:
public void onClick(View clicked) {

17.

We know that the number Button is what was clicked (it's the only View with an
event-listener at this point). Cast the View parameter to a Button so that we can
use it:
Button btn = (Button)clicked;

18.

Create an Intent object to dial the selected phone number:
Intent intent = new Intent(
Intent.ACTION_DIAL,
Uri.parse("tel://" + btn.getText()));

19.

Use startActivity to open the dialler application:
startActivity(intent);

20.

Now open the AndroidManifest.xml file in your editor or IDE.

21.

Before the declaration of the  element, we need permission to
read the contacts list:

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Chapter 4

22.

Change the label of the ViewContactActivity to View Contact:


23.

Remove all of the default content inside the  element.

24.

Declare an  type of ACTION_VIEW for this :


25.

Set the  of this  to CATAGORY_DEFAULT:


26.

Add a  element to filter person entries (this is a MIME type):


27.

Add another  element to filter contact entries:


When installed on a device, the preceding code will become an option for opening
"Contacts" in the user's address book. As you can see, replacing part of the standard Android
framework is very simple, and allows more seamless integration of applications with the
base system than is possible with a more conventional application architecture.

What just happened
If you install this application on the emulator, you'll notice that in the launcher, there's no
icon to start it up. That's because this application doesn't have a main entry point like all of
the others we've written thus far. Instead, if you open the "Contacts" application, and then
click on one of the contacts in the address book, you'll be greeted by the following screen:

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If you select the second icon, your new ViewContactActivity will be started in order to
view the selected contact. The user (as you can see) also has the ability to use your application
in preference to the default (for as long as your application remains available on the device).
Overriding a default behavior is a very important decision when developing a new
application. Android makes it very easy to do, and as you can see, a third-party application
can slot in almost seamlessly between two of the default applications. In a normal operating
system environment, you would need to write an entire "contacts manager", while in
Android you need only write the bits that interest you.
This is a part of your user interface design since you can use it to extend the functionality of
various default parts of the system. For example, if you wrote a chat application, such as a
"Jabber" client, you could embed the client in the View contact Activity for each contact
in the user's address book that was linked with a Jabber ID. This would allow users to chat
with available contacts directly from their address book, instead of having to go to your
application. You application becomes a way for them to check a contact's status, and possibly
avoid a phone call entirely.

Summary
Implementing an Activity at the correct granularity is an important part of your user
interface design process. Although it's not a graphical part directly, it defines how the
system will interact with your application, and thus how the user will interact with it.
It's a good idea to keep implicit intents in mind when structuring how your Activity will be
started. Creating a generic Activity allows for other applications to integrate seamlessly with
your own, effectively turning your new application into a platform for other developers to work
with. An implicitly started Activity can be replaced or extended by another application, or
it can be re-used in other application. In both cases, the user becomes free to customize your
application in much the same way that they can customize the wallpaper image or theme.
Always try and provide a single Activity implementation for each action the user might
want to take, don't make an Activity do too many things in the same screen. A very good
example of granularity is the "Contacts" application—there's a contact list, contact viewer,
contact editor, and the dialler application.
When working with tabbed interfaces (as we did in the previous chapter), it's possible
to specify the tab content as an Intent, effectively embedding the Activity in your
application. I would strongly urge you to consider doing exactly this when building a tabbed
user interface, since it allows each tab to be re-used by your application far more easily, while
also allowing third-party developers to create extensions to your interface, one tab at a time.
So far we've only really worked with the LinearLayout class, and while it's a great base for
simple user interfaces, it's almost never enough. In the next chapter, we'll be looking at the
many other types of layouts that Android provides by default, exploring the way in which
each layout works, and how they can be used.
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5

Developing Non-linear Layouts
Non-linear layouts are normally a completely fundamental subject of user
interface design. However, on a device with a small screen (as many Android
devices are), it doesn't always make sense. That said, Android devices can
be turned to landscape mode, where suddenly you have an abundance of
horizontal space, and limited vertical space. In these situations (and as we'll
see, in many other situations as well), you will want to work with a layout other
than the plain old LinearLayout structure we've worked with so far.
The real power of Android layouts comes from the same place as the power
of the old Java AWT LayoutManagers—by combining the different layout
classes with each other. For example, combining a FrameLayout with other
ViewGroup implementations allows you to layer various parts of the user
interface on top of each other.

It's important to consider how your layout will act on screens of different sizes. While
Android does allow you to select different layouts based on the screen size of the device,
this means that you will have to maintain multiple layouts for the different screen sizes and
densities which your application will encounter in the wild. As far as possible you should
make use of the tools that Android provides, and work with layouts that will scale according
to the size of the various View objects.
In this chapter, we'll look into the various other layout styles that Android provides us with
by default, and investigate various alternative uses for each one of them. We'll also take a
closer look at how you specify parameters for different layouts, and how they can help in
usability, as opposed to simply putting your widgets in a particular order.

Developing Non-linear Layouts

Time for action – creating a layouts example project
Before we walk through each of the layouts, we need a common project inside which we will
showcase each of them.

1.

From a command prompt, create a new project named Layouts:
android create project -n Layouts -p Layouts -k com.packtpub.
layouts -a LayoutSelectorActivity -t 3

2.

Delete the standard res/layout/main.xml layout resource file.

3.

Open the res/values/strings.xml file in an editor or IDE.

4.

Add a new  by the name of layouts to the file:


5.

Add the following items to the new  element:
Frame Layout
Table Layout
Custom Layout
Relative Layout
Sliding Drawer

6.

Open the LayoutSelectorActivity source file in your editor or IDE.

7.

Have the class inherit from ListActivity instead of Activity:
public class LayoutSelectorActivity extends ListActivity {

8.

In the onCreate method, set the contents of your ListActivity you declared in
the strings.xml resource file to your layouts array:
setListAdapter(new ArrayAdapter(
this,
android.R.layout.simple_list_item_1, Have the class
inherit from"
getResources().getStringArray(R.array.layouts)));

9.

Override the onListItemClick method:
protected void onListItemClick(
ListView l,
View v,
int position,
long id) {

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Chapter 5

10.

Create a switch statement on the position parameter:
switch(position) {

11.

Add a default clause (the only one for now) to let yourself know that you haven't
implemented an example for the selected item yet:
default:
Toast.makeText(
this,
"Example not yet implemented.",
Toast.LENGTH_SHORT).show();

What just happened?
The new project will serve as a basis for each of the examples in this chapter. For each layout
we work through, we'll build a new Activity that will become part of this application.
Currently, the application consists of only a menu for accessing each of the layout examples.
The idea as of now is to fill each one with something interesting.
In this chapter, we will explore not just the basic layouts, but also how they can be made
to interact with each other.

FrameLayout
The FrameLayout class anchors each of its widgets at the top-left corner of itself. This
means each child widget is drawn on top of the previous one. This can be used to simulate
a CardLayout from AWT by using View.setVisible to show one of the children while
hiding all the others (this is effectively how TabHost works).
As FrameLayout actually paints all of its visible children, it can be used to layer the child
widgets on top of each other. It produces very strange effects in some cases, while in other
cases it can be amazingly useful. For example, darkening out all of the widgets except one
can be achieved by using a semi-transparent View object and a FrameLayout. The inactive
widgets are the first layer in the FrameLayout, a semi-transparent View object is the
second, and the active widgets are the third.

Common uses
The most common use of FrameLayout is probably in combination with TabHost—to hold
the content View objects for each tab. You can also use it to simulate a more desktop feel,
by layering widgets on top of each other. It can also be used very effectively in games, to
display the in-game menu, or draw an animated background behind the game's main menu.

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By combining a FrameLayout object with widgets that also take up the entire screen,
you can make use of the gravity attribute to place objects more precisely on top of
other widgets. For this, you'll generally want each of the FrameLayout children to be a
ViewGroup of some sort, since they generally don't paint in a background unless told to
(leaving the lower layers visible).
A FrameLayout is also capable of displaying a foreground. While all View objects have
a background attribute, FrameLayout includes a foreground (which is also an optional
Drawable). The foreground will be painted on top of all of the child widgets, allowing
a "frame" to be displayed.

Time for action – developing a FrameLayout example
To really understand what a FrameLayout does, and how it can be used, it's best to kick
it around a bit with an example. In this example, we'll use a FrameLayout to layer some
Button widgets on top of an ImageView, and show-and-hide a TextView message when
one of the buttons is clicked.
For this example to work, you're going to need an image to serve as a background image.
I'm going to use a photo of one of my friends. As always, place your image in the res/
drawable directory, and try to use a PNG file.

1.

Create a new layout resource file named res/layout/frame_layout.xml.

2.

Declare the root element as a FrameLayout consuming all available space:


3.

Inside the FrameLayout, create an ImageView to serve as the background image.
It should scale to fill all the available space:


4.

Now create a vertical LinearLayout where we will place two Button objects at
the bottom of the screen:


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5.

Create a Button that we will use to toggle one of the child layers of our
FrameLayout (creating a dialog-like effect):


6.

Create another Button to quit the demo and go back to the menu:


7.

After the , create a final TextView element that we will show
and hide when the first button is clicked. By default it's hidden:


8.

Create a new FrameLayoutActivity Java class in the root package of your
project, and open the source file in your editor or IDE. The new class needs to
extend from Activity and implement the OnClickListener class (for events
from those two Button widgets):
public class FrameLayoutActivity
extends Activity implements OnClickListener {

9.

Override the onCreate method:
protected void onCreate(Bundle savedInstanceState) {

10.

Invoke the super.onCreate method to get the Activity code working:
super.onCreate(savedInstanceState);

11.

Set the content layout to the frame_layout resource you just created:
setContentView(R.layout.frame_layout);

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12.

Find the overlay Button widget you declared in the frame_layout resource file
and create a reference to it:
Button overlay = (Button)findViewById(R.id.overlay_button);

13.

Set its OnClickListener to the new FrameLayoutActivity object:
overlay.setOnClickListener(this);

14.

Find the quit Button widget:
Button quit = (Button)findViewById(R.id.quit);

15.

Then set it's OnClickListener to the FrameLayoutActivity object:
quit.setOnClickListener(this);

16.

The OnClickListener interface requires us to implement an onClick method
with the following signature:
public void onClick(View view) {

17.

Create a switch statement on the ID of the View parameter:
switch(view.getId()) {

18.

If the View clicked by the user widget is the overlay_button Button, then use
the following:
case R.id.overlay_button:

19.

Fetch the overlay View object from the layout:
View display = findViewById(R.id.overlay);

20.

Toggle its visibility according to its current state, then break from the switch:
display.setVisibility(
display.getVisibility() != View.VISIBLE
? View.VISIBLE
: View.GONE);
break;

21.

If the View clicked by the user widget is the quit Button, then use the following:
case R.id.quit:

22.

Invoke the finish() method, and break from the switch statement:
finish();
break;

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23.

Open the LayoutSelectorActivity Java source in your editor or IDE.

24.

In the onListItemClick method, create a new case in the switch statement,
for position value 0:
case 0:

25.

Start the FrameLayoutActivity using an explicit Intent:
startActivity(new Intent(this, FrameLayoutActivity.class));
break;

26.

Open the AndroidManifest.xml file in an editor or IDE.

27.

Add the new FrameLayoutActivity to the manifest file:


What just happened?
The new FrameLayoutActivity makes use of a simple three layer FrameLayout. We
use an ImageView object to draw a nice background image, on top of which we placed our
two buttons. While the third layer (the TextView widget) is invisible until the top button
is clicked, it's important to note that not only is the background of the top TextView
transparent, it also delegates click events to widgets that are technically underneath it
(the TextView has a widget and height that consumes the entire FrameLayout). This
will continue to work, even if the background of the TextView is opaque. It has more
to do with the fact that the TextView is not "clickable". If you would have added an
OnClickListener to the overlay TextView object, the button underneath it would
have stopped working. This means you need to be careful how you layer widgets in a
FrameLayout (although so long as one widget doesn't take up the same space as another,
this won't become a problem for you).
In this example, we added a Quit button to the layout, and used the finish() method to
close the Activity when the Button was clicked. You'll find that you generally don't use
the finish() method directly since the user will mostly be moving forward through your
application. If a user wants to go back, they will most often use the hardware "Back" button,
or press the hardware "Home" button to exit your application entirely.
A final note on the above example—in the frame_layout.xml file, we declare the
overlay as a TextView widget. However, in the Java code we access it using the View
class instead of TextView. This is a simple case of decoupling. Unless you're working in
a performance-centric piece of code, it's a good idea to reference your layout widgets as
high up the class-tree as possible. This will allow you to modify your user interface much
more quickly later on. In this case, you could change the simple TextView to an entire
LinearLayout without the need to change the Java code at all.
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Following are two screenshots of the FrameLayout example, with and without the
overlay TextView enabled. This sort of layout is perfect for use in a game menu or
a similar structure where you need to layer different widgets on top of each other.

Table Layout
The Table Layout arranges its children in a HTML-style grid. It's a bit like the AWT Grid
Layout class, but with much more flexibility. Unlike most other layout classes in Android,
Table Layout uses its own specialized direct-child View class, named Table Row. The Table
Layout class also doesn't allow you to define the number of rows or columns (making it far
more like an HTML  element). Instead, the number of rows and columns is calculated
by the number of widgets in the Table Layout and its Table Row children.
A cell in a Table Layout may consume any number of rows and columns, although the
default for a View placed inside a Table Row is to take up exactly a single table cell. However,
if you place a View as a direct child of a Table Layout, it will consume an entire row.
Table Layout is also a relative layout structure, which is vitally important when working with

Android devices. Being able to align everything based on grid lines allows your user interface to
scale from the lowest resolution on a tiny phone, to a high-density screen on a 7-inch tablet.

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The android:gravity attribute comes into play far more in a Table Layout than in
many of the other layout classes. What looks great on a small screen may look completely
different on a large screen, not due to the size of the screen, but instead due to the scaling of
the fonts used. Be careful, especially with the vertical alignment of labels and widgets. The
easiest way to start with this is to vertically center all of your table widgets, and work from
there. Be sure to test any table based layout on a variety of screen resolutions and sizes.

Common uses
Most commonly you'll find yourself using a Table Layout to arrange input form. It's also
useful for laying out complex information, especially when making some View objects span
several rows and columns. The most important trait of Table Layout comes from the fact
that it aligns its cells in a very strict manner, and the fact that its a relative-size layout.
A Table Layout can also be used to achieve an effect similar to the AWT Border Layout
class. Generally, when sizing a Table Layout to fit the entire screen, it becomes a very
different tool to a simple grid, allowing you to fit a Scroll View in the middle of the
control widgets.
By using a Table Layout inside a FrameLayout, you can arrange a control View on top of
a content View (think of the controls in Google Maps). Also, try to bear in mind that unlike
an AWT GridLayout, the size of a View inside a TabelLayout is not attached to size of
the table-cell in which it is placed. By making use of the gravity attribute (and possibly a
layout margin), you can place a View object within the table-cell, leading to layouts that are
far more user-friendly.

Using TableLayout for a memory game
To demonstrate the TableLayout, I thought it would be fun to write a simple memory-card
game. You're presented with a grid (in the form of a TableLayout) of "cards" which you
can touch to effectively turn over. You can then attempt to match all these cards, with the
contents that are displayed on the cards (you're only allowed to turn over two at a time). For
this example, you'll need some images to place on the cards (I've re-used the fruit icons from
the delivery example). In this application, we'll also be creating a simple placeholder image,
in the form of an XML file.
To create the placeholder image, create a new XML resource in the res/drawable
directory, named line.xml. This will be a "shape" resource. Shape resources are very useful
for creating simple, scalable shapes. Also, shape resource files can make use of any color,
texture, or gradient which you can provide from your code.

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Copy the following code into the line.xml file in order to create the simple placeholder
image for our example:






Time for action – developing a simple memory game
Unlike almost all previous example, we'll generate the layout entirely in Java code in this
game. The main reason for this is that the content is highly repetitive, each cell containing
almost exactly the same widget. We use a TableLayout to create the grid, and display the
"cards" in ImageButton widgets. To encapsulate the individual card behavior, we create a
MemoryCard inner-class which holds a reference to the ImageButton it controls.

1.

Create a new Java class in the root package of your project, and name it
TableLayoutActivity.

2.

Make the new class extend Activity:
public class TableLayoutActivity extends Activity {
Declare and array of all the icon resources to use as card
images, there must be eight images resources declared in this
array:private static final int[] CARD_RESOURCES = new int[]{
R.drawable.apple,
R.drawable.banana,
R.drawable.blackberry,
// …
};

3.

You'll need a timer in order to flip cards back over, so declare a Handler:
private final Handler handler = new Handler();

4.

Declare an array of MemoryCard objects:
private MemoryCard[] cards;

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5.

We either have one or two cards turned over (that we want to keep track of).
Declare a placeholder for the first:
private MemoryCard visible = null;

6.

If there are two cards turned over, but they don't match, we disable touch with a
simple boolean switch (our event listeners will check this):
private boolean touchEnabled = true;

7.

Now declare an inner class named MemoryCard which implements the
OnClickListener interface:
private class MemoryCard implements OnClickListener {

8.

The MemoryCard class holds a reference to an ImageButton:
private ImageButton button;

9.

The MemoryCard class also has a value, which is a reference to the image resource
on its face:
private int faceImage;

10.

Finally, a MemoryCard uses a boolean value to remember its state (whether the
face image, or the placeholder image is visible):
private boolean faceVisible = false;

11.

Declare a constructor for the MemoryCard class, it only needs to take the resource
identifier for the face image:
MemoryCard(int faceImage) {

12.

Store the faceImage resource identifier for later use:
this.faceImage = faceImage;

13.

Create a new ImageButton object, using the TableLayoutActivity object as its
Context (which the ImageButton will use to load the images):
this.button = new ImageButton(TableLayoutActivity.this);

14.

Set the size of the ImageButton to a fixed 64 x 64 pixels:
this.button.setLayoutParams(new TableRow.LayoutParams(64, 64));

15.

Set the scale-type so that icons are made to fit into the ImageButton, then set the
image to the placeholder resource:
this.button.setScaleType(ScaleType.FIT_XY);
this.button.setImageResource(R.drawable.line);
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16.

Assign the MemoryCard object as the OnClickListener of the
ImageButton object:
this.button.setOnClickListener(this);

17.

For convenience later on, the MemoryCard needs a setFaceVisible method,
which will toggle between showing the placeholder and the faceImage resource:
void setFaceVisible(boolean faceVisible) {
this.faceVisible = faceVisible;
button.setImageResource(faceVisible
? faceImage
: R.drawable.line);
}

18.

Implement the onClick method in the MemoryCard class:
public void onClick(View view) {

19.

First, make sure that the face isn't currently visible (so we're turned down), and that
touch is enabled (and some other cards aren't about to be turned face-down again):
if(!faceVisible && touchEnabled) {

20.

If these conditions are met, we tell the TableLayoutActivity that we've been
touched and want to be turned face-up:
onMemoryCardUncovered(this);

21.

After the MemoryCell inner class, create a simple utility method in the
TableLayoutActivity to create an ordered array of MemoryCell objects with
a specific size:
private MemoryCard[] createMemoryCells(int count) {

22.

When we create each of the MemoryCell objects, we create them in pairs, and
in the same sequence as was specified in our array of icon resources:
MemoryCard[] array = new MemoryCard[count];
for(int i = 0; i < count; i++) {
array[i] = new MemoryCard(CARD_RESOURCES[i / 2]);
}

23.

When complete, return the new array of MemoryCell objects:
return array;

24.

Now override the onCreate method:
protected void onCreate(Bundle savedInstanceState) {

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25.

Invoke the Activity.onCreate method:
super.onCreate(savedInstanceState);

26.

Now create a new TableLayout object, passing it the TableLayoutActivity
as a Context for loading styles and resources:
TableLayout table = new TableLayout(this);

27.

By default, we create a four-by-four grid:
int size = 4;
cards = createMemoryCells(size * size);

28.

Then we shuffle it to randomize the order:
Collections.shuffle(Arrays.asList(cards));

29.

Create each of the required TableRow objects, and populate it with the
ImageButtons, created by the MemoryCard objects in the grid:
for(int y = 0; y < size; y++) {
TableRow row = new TableRow(this);
for(int x = 0; x < size; x++) {
row.addView(cards[(y * size) + x].button);
}
table.addView(row);
}

30.

Set the Activity content view to the TableLayout object:
setContentView(table);

31.

Now we write the onMemoryCardUncovered method, which is called by the
MemoryCard.onClick implementation:
private void onMemoryCardUncovered(final MemoryCard cell) {

32.

First, check to see if there is a currently visible MemoryCard, if not, the card
touched by the user is turned face-up and we remember it:
if(visible == null) {
visible = cell;
visible.setFaceVisible(true);
}

33.

If there is already a face-up card, check to see if they have the same image. If the
images are the same, disable the ImageButton widgets so we ignore the events:
else if(visible.faceImage == cell.faceImage) {
cell.setFaceVisible(true);
cell.button.setEnabled(false);
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Developing Non-linear Layouts
visible.button.setEnabled(false);
visible = null;
}

34.

Finally, if the face images don't match, we turn the card touched by the user face-up
and flip our touchEnabled switch so that the MemoryCard objects will ignore all
other touch events for a second:
else {
cell.setFaceVisible(true);
touchEnabled = false;

35.

Then we post a delayed message on our Handler, which will turn both cards
face-up again and re-enable touch events:
handler.postDelayed(new Runnable() {
public void run() {
cell.setFaceVisible(false);
visible.setFaceVisible(false);
visible = null;
touchEnabled = true;
}
}, 1000); // one second before we flip back over again

What just happened
In the previous example, it should be obvious to see why we wrote the layout code manually,
building it in an XML file would have been terribly repetitive. You'll notice that the code
creates a TableRow object as the direct children of the TableLayout, just as we would
have in an XML file.
The onClick method of MemoryCard uses the touchEnabled switch to determine
whether or not to call onMemoryCardUncovered. However, neither does this stop the user
from pressing the ImageButton objects, nor does it stop the objects from responding to the
user (although they won't turn over). For a more user-friendly experience, it would be better
to use the setClickable method on each of the enabled ImageButton objects, to stop
them completely from reacting to the user's touch.
When we create the ImageButton objects, we pre-size them at 64 x 64 pixels. While this
is fine for the big emulator screen, there are plenty of devices that wouldn't fit the 4 x 4
grid of buttons on the screen. I would recommend you use an XML resource to create the
ImageButton objects.

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The previous code uses setLayoutParams(new TableRow.LayoutParams(64,
64)); to set the size of the ImageButton objects. It's important to note that because
we are placing the ImageButton objects into a TableRow, their LayoutParams must
be of the type TableRow.LayoutParams. If you try changing to a generic ViewGroup.
LayoutParams, then the user interface won't layout (it'll just be blank). Following are two
screenshots of the working application:

Have a go hero
The TableLayout example works great, but the location of the grid isn't great (on the top
left of the screen), and having it against a black background is quite dull. Time to make it
look great!
Start by using a FrameLayout to add a background image to the game. This will enhance
the overall appeal of the game by adding more color. You should also take this opportunity
to center the grid on the screen. Having it in the top left makes it look lopsided somehow.
You should also try removing the touchEnabled switch, instead using setClickable on
each of the ImageButton objects. This will stop them from providing a visual "press and
release" feedback when you're about to turn cards face-down.

AbsoluteLayout/Custom Layouts
Do not use AbsoluteLayout! AbsoluteLayout is Deprecated! That said, there are times
when using the AbsoluteLayout class makes sense. So why shouldn't you use the
AbsoluteLayout class, and where should you use it? The answer to the first question is
very simple—all of the child widgets of an AbsoluteLayout have their locations specified
exactly, they don't change size or location on different screens. It also makes your layout
almost impossible to re-use (for example, importing it into another layout, or embedding
it into another application).

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If you're going to work with an AbsoluteLayout, you should approach it in either one of
the following two ways:
1. Carefully build a separate layout XML for each different screen size.
2. Write your layout data in Java code instead of XML.
The first is impractical unless you specify that the application only runs on specific devices,
and the layout cannot be used outside of your application. The second option however,
opens up the "right" way—write a custom layout manager. Since AbsoluteLayout requires
strict locations, and doesn't allow easy interaction with the measuring of child View objects,
the best way to define layouts that don't fit well into any of the over layout classes is to
define a custom layout in your own ViewGroup class.

Developing your own Layouts
Since AbsoluteLayout is deprecated, and yet many people seem to insist on using it, this
example will be to demonstrate not just how easy it is to write your own ViewGroup class
defining a new layout, but also how easy it is to then integrate that layout into a layout XML
resource. This will thus prove that there is no compelling reason to use an AbsoluteLayout
(unless it really makes sense).

Time for action – creating a custom layout
To really demonstrate the use of a custom layout, you need to try building something
unusual. In the following example, you'll put together a ViewGroup that arranges its
children in a nice circle. It's not a very brilliant layout, nor is it particularly useful, but
circles are nice to look at, and it would provide useful negative space in the screen center
(which could be filled using a FrameLayout).

1.

Create a new Java source file in the root package of the project named
CircleLayout.java, and open it in your editor or IDE.

2.

Declare the CircleLayout as extending the ViewGroup class:
public class CircleLayout extends ViewGroup

3.

Declare the three ViewGroup constructors and have them delegate directly to the
ViewGroup default constructors:
public CircleLayout(Context context) {
super(context);
}
// ...

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4.

We'll need to know the largest number of pixels taken up by a child View object's
width, and the largest number of pixels taken up by a child View object's height.
To avoid unnecessary overhead, we take this opportunity to measure the child
View objects as well. Declare a utility method named measureChildrenSizes to
perform these two operations:
private int[] measureChildrenSizes(int sw, int sh) {

5.

Declare an int to hold the maximum width and height we find:
int maxWidth = 0;
int maxHeight = 0;

6.

Create a for loop to iterate over each of the child View objects in this
CircleLayout object:
for(int i = 0; i < getChildCount(); i++) {

7.

Declare a reference to View at the current index:
View child = getChildAt(i);

8.

As a layout widget, your class will be responsible for setting the display size for all
of it's child widgets. In order to know a child widget's desired width and height, you
need to use the measureChild method in the ViewGroup class:
measureChild(child, sw, sh);

9.

Test the width and height of the child View object against the maximum width and
height variables you created earlier:
maxWidth = Math.max(maxWidth, child.getMeasuredWidth());
maxHeight = Math.max(maxHeight, child.getMeasuredHeight());

10.

At the end of the method, return an array containing the maximum width and
height found during the procedure:
return new int[]{maxWidth, maxHeight};

11.

Implement the onLayout method of ViewGroup:
protected void onLayout(boolean changed,
int l, int t, int r, int b) {

12.

Calculate the width and height of our available space:
int w = r – l;
int h = b - t;

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13.

Declare a variable to hold the number of child View objects:
int count = getChildCount();

14.

Perform the measurement of all child View objects against the amount of available
space:
int[] max = measureChildrenSizes(w, h);

15.

Subtract the maximum width and height from the available space so that all the
child View objects will fit on the screen:
w -= max[0];
h -= max[1];

16.

Calculate the center point in the CircleLayout:
int cx = w / 2;
int cy = h / 2;

17.

Create a for loop to iterate over each of the child View objects again:
for(int i = 0; i < count; i++) {

18.

Declare a variable to hold the current child View object:
View child = getChildAt(i);

19.

Calculate the x and y locations of the child View object:
double v = 2 * Math.PI * i / count;
int x = l + (cx + (int)(Math.cos(v) * cx));
int y = t + (cy + (int)(Math.sin(v) * cy));

20.

Invoke the layout method of the child View object with the calculated coordinates
in the circle:
child.layout(
x, y,
x + child.getMeasuredWidth(),
y + child.getMeasuredHeight());

What just happened?
The CircleLayout class is a very simple implementation of a ViewGroup. Except for the
requested width and height of its children, it has no special attributes that can be used in an
XML resource. However, it will take notice of the sizing that you declare for its children, and
so the layout_width and layout_height attributes will work normally.
It's important to note that in order to make use of a custom View or ViewGroup from
a layout XML resource, you need to have all three default constructors overridden.
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The LayoutInflater will make use of one of these constructors to create
instances of your class. If the one it wants to use isn't in place, you will get the
dreaded Force Close dialog when you try and inflate the layout XML file.

The CircleLayout has its own utility method to handle the measuring of its child View
objects. Generally, a ViewGroup would use the ViewGroup.measureChildren utility
method to see that all of its child View objects are measured before performing the actual
layout. However, we need to iterate over the list of child View objects in order to find
the largest used width and height, so instead of performing the iteration three times, we
perform the measurements ourselves.

Using the CircleLayout
To make use of your custom ViewGroup implementation, it's good to know that Android has
you covered as far as the XML layout resources are concerned. When you need to reference
a custom View or ViewGroup class from an XML layout resource, you simply use the full
class name instead of the simple class name. The following is a simple example of an XML
layout that uses the CircleLayout:






11.

In the generated root package, declare a new Java source file named
NewsFeedActivity.java.
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Animating Widgets and Layouts

12.

Register the NewsFeedActivity class in your AndroidManifest.xml file:


13.

The new class should extend the Activity class and implement Runnable:
public class NewsFeedActivity
extends Activity implements Runnable {

14.

Declare a Handler to be used as a timing structure for changing the headlines:
private final Handler handler = new Handler();

15.

We need a reference to the TextSwitcher object:
private TextSwitcher newsFeed;

16.

Declare a string-array to hold the mock headlines you added to the strings.xml
file:
private String[] headlines;

17.

You'll also need to keep track of which headline is currently being displayed:
private int headlineIndex;

18.

Override the onCreate method:
protected void onCreate(final Bundle savedInstanceState) {

19.

Invoke the onCreate method of Activity:
super.onCreate(savedInstanceState);

20.

Set the content view to the news layout resource:
setContentView(R.layout.news);

21.

Store a reference to the headline string-array from the strings.xml application
resource file:
headlines = getResources().getStringArray(R.array.headlines);

22.

Find the TextSwitcher widget and assign it to the field declared earlier:
newsFeed = (TextSwitcher)findViewById(R.id.news_feed);

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23.

Set the ViewFactory of the TextSwitcher to a new anonymous class that will
create TextView objects when asked:
newsFeed.setFactory(new ViewFactory() {
public View makeView() {
return new TextView(NewsFeedActivity.this);
}
});

24.

Override the onStart method:
protected void onStart() {

25.

Invoke the onStart method of the Activity class:
super.onStart();

26.

Reset the headlineIndex so that we start from the first headline:
headlineIndex = 0;

27.

Post the NewsFeedActivity as a delayed action using the Handler:
handler.postDelayed(this, 3000);

28.

Override the onStop method:
protected void onStop() {

29.

Invoke the onStop method of the Activity class:
super.onStop();

30.

Remove any pending calls to the NewsFeedActivity:
handler.removeCallbacks(this);

31.

Implement the run method which we'll use to swap to the next headline:
public void run() {

32.

Open a try block to swap the headline inside.

33.

Use the TextSwitcher.setText method to swap to the next headline:
newsFeed.setText(headlines[headlineIndex++]);

34.

If the headlineIndex is past the total number of headlines, reset the
headlineIndex to zero:
if(headlineIndex >= headlines.length) {
headlineIndex = 0;
}
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35.

Close the try block, and add a finally block. In the finally block, post the
NewsFeedActivity back onto the Handler queue:
finally {
handler.postDelayed(this, 3000);
}

36.

Open the auto generated AnimationSelector Java source in an editor or IDE.

37.

The AnimationSelector class needs to implement OnClickListener:
public class AnimationSelector
extends Activity implements OnClickListener {

38.

In the onCreate method, ensure that the content view is set to the main layout
resource created earlier:
setContentView(R.layout.main);

39.

Find the declared Button and set its OnClickListener to this:
((Button)findViewById(R.id.news_feed)).
setOnClickListener(this);

40.

Declare the onClick method:
public void onClick(final View view) {

41.

Use a switch to determine which View was clicked:
switch(view.getId()) {

42.

If it's the news feed Button, then use the following case:
case R.id.news_feed:

43.

Start the NewsFeedActivity using a new Intent:
startActivity(new Intent(this, NewsFeedActivity.class));

44.

Break from the switch statement, thus finishing the onClick method.

What just happened?
The TextSwitcher is an example of an animation utility View. In this case it's the perfect
structure to swap between the news headlines, displaying one headline at a time and
animating a transition between each of the texts. The TextSwitcher object creates two
TextView objects (using the anonymous ViewFactory class). When you use the setText
method, the TextSwitcher changes the text of the "off screen" TextView and animates a
transition between the "on screen" TextView and the "off screen" TextView (with the new
text content displayed).
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The TextSwitcher class requires that you specify two animation resources for it to work
with, in order to create its transition effect:
‹‹

Animate text onto the screen

‹‹

Animate text off the screen

In the previous case, we made use of the default slide_in_left and slide_out_right
animations. Both of these are examples of translation-based animations due to the fact that
they actually alter the "on screen" position of the TextView objects in order to create their
effect.

Using flipper and switcher widgets
The first example of this chapter made use of the TextSwitcher class, an animating View
class in the standard Android API. There are several other animation utility classes, some of
which you may have encountered before (such as ImageSwitcher). Both TextSwitcher
and ImageSwitcher are related classes, and both inherit from the more generic
ViewSwitcher class.
The ViewSwitcher class is a generic animation tool, and defines the ViewFactory
interface that we implemented anonymously in the previous example. A ViewSwitcher is
a ViewGroup with only two child View objects. One is displayed on the screen, while the
other is hidden. The getNext utility method allows you to find out which is the "off screen"
View object.
While you generally use a ViewFactory to populate a ViewSwitcher, you have the option
to populate it manually. You could have populated the TextSwitcher in the example by
using the addView method that is inherited from ViewGroup.

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Using the ImageSwitcher and TextSwitcher implementations
The ImageSwitcher and TextSwitcher classes are specialized implementations of the
ViewSwitcher that understand the type of View objects they contain. When you invoke
the setText method of the TextSwitcher object, it's much like invoking the following
code snippet on a ViewSwitcher containing two TextView children:
((TextView)switcher.getNext()).setText("Next text to display");
switcher.showNext();

TextSwitcher can be used to display content such as (as in the example) a news feed,

or as with the Android notification area, to display text content that doesn't fit into a
single line. Displaying multiple lines in a TextSwitcher is particularly effective when the
animation runs the text upwards, causing the text to appear to scroll upwards behind the
TextSwitcher object.
An ImageSwitcher is most commonly used in a gallery, slide show, or similar structure. You
could also use an ImageSwitcher to allow the user to select from a small list of images, for
example a short list of login avatars.

Have a go hero – populating the TextSwitcher
As an alternative to populating the TextSwitcher with a ViewFactory in the news feed
example, try populating it in the XML layout resource. Remember that it requires exactly two
TextView child widgets. If you get this right, try giving each of the two TextView objects
different font colors and styles.

Animating layout widgets
Using the animation utility widgets such as TextSwitcher and ImageSwitcher can allow
you to display much more information over time than you could fit on the screen at a time.
ViewGroup objects can also be animated without any serious modification through the
LayoutAnimationController class. However, in this case, animation needs to be added
in your Java code.
A LayoutAnimationController is best used to create "entry" or "exit" effects on a
ViewGroup as it appears or just before it disappears off the screen. The controller simply starts
a specified animation on each of the View children of a specified ViewGroup. However, it
doesn't have to do it all at the same time, or in a sequential order. You can easily configure a
LayoutAnimationController to leave a slight delay between the starting of an animation

on each child widget, creating a staggered effect.

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If applied correctly to a LinearLayout, you could achieve a result similar to the one
illustrated in the following diagram:

Time for action – animating a GridView
The GridView class has its own LayoutAnimationController specifically designed
to animate it in terms of rows and columns, allowing more complex effects than can be
achieved with a standard LayoutAnimationController. For this next part of the
"animations" example we're going to build a lovely color selector out of a GridView. When
the selector first appears on the screen, each color swatch will fade in, starting in the top left
corner and ending on the bottom right corner.

1.

Start by declaring a new Java source file in the root package of your project named
ColorAdapter.java, which will generate the color swatches for the GridView.

2.

The ColorAdapter needs to extend BaseAdapter to take care of the boiler plate
Adapter requirements:
public class ColorAdapter extends BaseAdapter {

3.

The ColorAdapter will be created with a specified number of rows and columns,
the same numbers which will be displayed on the GridView:
private final int rows;
private final int cols;
public ColorAdapter(int rows, int cols) {
this.rows = rows;
this.cols = cols;
}

4.

The number of items that the ColorAdapter will provide is the number of rows
multiplied by the number of columns:
public int getCount()
return rows * cols;
}
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Animating Widgets and Layouts

5.

The ID of a color is the position or index at which it's found:
public long getItemId(int pos) {
return pos;
}

6.

We use a utility method to compose the color from an index in the "list." For this
function we make use of the HSVtoRGB method in the Android Color class:
private int getColor(int pos) {
float h = (float)pos / (float)getCount();
return Color.HSVToColor(new float[]{h * 360f, 1f, 1f});
}

7.

The item at an index in the Adapter model is returned as it's color value:
public Object getItem(int pos) {
return getColor(pos);
}

8.

To create the color swatch View objects, we implement the getView method of
Adapter as usual:
public View getView(int pos, View reuse, ViewGroup parent) {

9.

The View we return is going to be an ImageView object, so we either re-use the
one given by the parent widget, or create a new one:
ImageView view = reuse instanceof ImageView
? (ImageView)reuse
: new ImageView(parent.getContext());

10.

We make use of the ColorDrawable class to fill the ImageView with the color
specified by our getColor utility method:
view.setImageDrawable(new ColorDrawable(getColor(pos)));

11.

The ImageView needs to have its android.widget.AbsListView.
LayoutParams set, and then it can be returned to the GridView for display:
view.setLayoutParams(new LayoutParams(16, 16));
return view;

12.

Create a new XML layout resource file named res/layout/colors.xml to hold
the declaration of the GridView that will act as the color selector.

13.

The contents of the colors.xml layout file are just a single GridView widget:


14.

Define another new Java source file in the root package of your
AnimationExamples project. Name this one ColorSelectorActivity.java.

15.

The new class declaration should extend Activity:
public class ColorSelectorActivity extends Activity {

16.

Override the onCreate method as normal, and set the content view to the colors
XML layout resource you just wrote:
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.colors);

17.

Now you can load the default Android "fade in" animation using the handy
AnimationUtils class from the android.view.animation package:
Animation animation = AnimationUtils.loadAnimation(
this, android.R.anim.fade_in);

18.

In order to animate the GridView correctly, you need to instantiate a new
GridLayoutAnimationController object, passing it the "fade in" animation:
GridLayoutAnimationController animationController =
new GridLayoutAnimationController(
animation, 0.2f, 0.2f);

19.

Now look for the GridView which you have declared in the colors.xml file:
GridView view = (GridView)findViewById(R.id.colors);

20.

Set the number of columns in the GridView to 10 (note that we didn't do this in
the XML layout resource as you normally would):
view.setNumColumns(10);

21.

When you set the adapter of the GridView to a ColorAdapter, you also need to
know the number of columns, and the easiest way to do this is to keep both in Java:
view.setAdapter(new ColorAdapter(10, 10));

22.

The view object is now ready for the GridLayoutAnimationController:
view.setLayoutAnimation(animationController);
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23.

In order to start the animation when the screen is displayed, we override the
onStart method. It is in here that we look up the GridView again and start the
animation:
protected void onStart() {
super.onStart();
((GridView)findViewById(R.id.colors)).
getLayoutAnimation().start();
}

24.

In order to integrate this new example with the other animation examples, you'll
need to open the res/layout/main.xml file in an editor or IDE.

25.

Add a new Button to the end of the LinearLayout, the one we'll use to start the
color selection example:


26.

Open the AnimationSelector source file in your editor or IDE.

27.

After setting the OnClickListener of the news_feed Button, find and set the
OnClickListener of the new colors Button in the same way:
((Button)findViewById(R.id.colors)).setOnClickListener(this);

28.

In the onClick method, after the switch case for the news_feed Button,
add another switch case for the new colors Button, and start the
ColorSelectorActivity:
case R.id.colors:
startActivity(new Intent(this, ColorSelectorActivity.class));
break;

29.

Open the AndroidManifest.xml file in your editor or IDE.

30.

At the bottom of the  section, register the new
ColorSelectorActivity:


What just happened?
The new example makes use of the GridLayoutAnimationController to start each "fade
in" animation a fraction of a second after the previous one started. This creates a fluid animation
effect of the color swatches appearing from the top-left to the bottom-right of the screen.
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When you instantiate a GridLayoutAnimationController, it requires you to provide
the animation and two parameters which indicate the amount of time between starting
animations for the next row, or the next column. The delay given is not specified in a "direct"
time format, but instead by how long the given animation takes to complete. In our case,
if the animation took one second to complete, the delay between each animation starting
would be 200 milliseconds, since the delay is specified as 0.2.
The fact that we animate the swatches just as this Activity becomes visible, effectively
makes this a transition animation, introducing the user to this new screen. For these types
of animations, it's imperative to take as little time as possible while still giving a pleasing
introduction. When you run the new example you should get an animation similar to the
ones illustrated in the following images:

Creating Custom Animations
So far we've explored using Android's stock animations with the normal widgets, but what
about applying a custom animation to a widget that isn't built for animations? Android
includes support for four basic animation types that can be applied to View objects:
‹‹

Translate/Move

‹‹

Rotate

‹‹

Scale

‹‹

Alpha/Transparency

These different animation structures can be applied by themselves, or merged together in an
animation set where any combination of the three can be run at the same time. By creating
an animation with a delay time before it starts, you can create complex animations by having
simple sets of animations follow each other.

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Animating Widgets and Layouts

Like so many things in Android, the easiest way to create your own custom animations is
to define it in a resource XML file. The elements in the animation format used by Android
correspond directly to the classes in the android.animation.view package. An animation
file can also reference animations in other animation resources, which makes it much easier
to compose complex animations and re-use simple animations.

Time for action – writing a custom animation
Writing a custom animation is very simple, but not entirely intuitive. For this section you'll
define a custom animation which will increase the size of its animated widget by a factor of
five, while at the same time fading it until its entirely transparent.

1.

Create a new XML resource file named res/anim/vanish.xml and open it in an
editor or IDE.

2.

The root element of the animation file will be an animation set element:


3.

In the  element, declare an element to define the scale up animation:


4.

The duration of the scale up animation needs to be set as 300 milliseconds:
android:duration="300"

5.

The animation starts to scale from the original size:
android:fromXScale="1.0"
android:fromYScale="1.0"

6.

The scale animation needs to increase the size by a factor of 5.0:
android:toXScale="5.0"
android:toYScale="5.0"

7.

We want the scale to expand from the center of the widget:
android:pivotX="50%"
android:pivotY="50%"

8.

The last part of the  element defines the acceleration curve of the
animation. Here we want the scale up to decelerate as it runs:
android:interpolator="@android:anim/decelerate_interpolator"

9.

Next, define a new element to handle the fade out part of the animation:

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10.

The duration of the fade out animation is also 300 milliseconds:
android:duration="300"

11.

We start with no transparency:
android:fromAlpha="1.0"

12.

The fade out ends with a completely invisible widget:
android:toAlpha="0.0"

13.

The fade out should accelerate as it runs, so we use an accelerate interpolator:
android:interpolator="@android:anim/accelerate_interpolator"

What just happened?
This is a relatively simple animation set, but its effect is visually pleasing. Keeping the
animation at 300 milliseconds is quick enough to not interfere with the user's interaction,
but just long enough to be seen in full by the user.
When defining animations in an animation  element, each non-set sub animation
needs to have its duration defined. The  element has no concept of its own
duration. However, you can define a single interpolator for the entire set to share.
The  animation will by default, scale the widget using the top-left corner as the
"pivot" point, causing the widget to grow to the right and downward, but not left and
upward. This causes a lopsided animation which is not very appealing. In the preceding
example, the scale animation runs with the pivot at the center of the animated widget.

Time for action – making a Button vanish
So how can we apply the nice shiny animation to a Button object? The Button object
doesn't have an animation attribute, and so you can't just reference it from the layout
resource file. What we want, is the animation to run when the Button widget is clicked.

1.

Create a new layout resource file named res/layout/vanish.xml and open in an
editor or IDE.

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Animating Widgets and Layouts

2.

At the root of the new layout, declare a RelativeLayout element:


3.

The Button needs to be nice and large, and centered on the screen. For this we give
it some inner padding:


4.

Create a new Java source file in the root package of the AnimationExamples
project named VanishingButtonActivity.java.

5.

The new class needs to extend Activity and implement the OnClickListener
interface:
public class VanishingButtonActivity extends Activity
implements OnClickListener {

6.

Override the onCreate method and invoke the Activity.onCreate method to
perform the required Android setup:
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);

7.

Set the content view to the new vanish layout resource:
setContentView(R.layout.vanish);

8.

Find the Button widget declared in the XML layout resource and set its
OnClickListener:
Button button = (Button)findViewById(R.id.vanish);
button.setOnClickListener(this);

9.

Implement the onClick method of OnClickListener:
public void onClick(View clicked) {

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10.

Load the Animation from the resource file:
Animation vanish = AnimationUtils.loadAnimation(
this, R.anim.vanish);

11.

Start the Animation on the Button object:
findViewById(R.id.vanish).startAnimation(vanish);

12.

Open the AndroidManifest.xml file in an editor or IDE.

13.

Declare the VanishingButtonActivity at the end of the 
section with a display label:


14.

Open the res/layout/main.xml layout resource in your editor or IDE.

15.

Add a new Button to the end of LinearLayout to activate the
VanishingButtonActivity:


16.

Open the AnimationSelector Java source file in your editor or IDE.

17.

At the end of the onCreate method, fetch the new vanish Button from the
layout and set its OnClickListener:
((Button)findViewById(R.id.vanish)).setOnClickListener(this);

18.

In the onClick method, add a new switch case to start the
VanishingButtonActivity:
case R.id.vanish:
startActivity(new Intent(
this, VanishingButtonActivity.class));
break;

What just happened?
The addition of the preceding example will display a single Button in the middle of the
screen. When clicked, the Button will be mutated by the vanish animation for 300
milliseconds. When it is complete, the animation won't have any effect on the Button
anymore. This is an important characteristic of animations—when they are complete, the
widget they have animated is returned to its original state.
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Animating Widgets and Layouts

It's also important to note that it's not the widget itself that is modified by an animation, but
rather the state of the Canvas it's painted on. This is much the same concept as modifying
the state of Graphics or Graphics2D object in Java AWT, or Swing before a widget uses
the Graphics object to paint itself.
In the following images you can see the effect that animation has on the Button when it's
clicked. The Button is actually re-painted for each frame in the animation, and remains
entirely active during that time.

Summary
In this chapter, we explored the various methods by which you can apply animations to
various parts of your user interface. We explored how some widgets are designed to animate
themselves. Layouts can be animated for transitions in and out of an Activity.
Several simple animations are available by default in the Android resource, but ultimately
creating your own animations and applying them to your user interface manually creates, by
far, the most visually appealing and user-friendly experience for your users.
Many applications on a mobile device need to present a large amount of information on the
screen, and present it in such a way that it can be easily absorbed. In the next chapter, we'll
explore user interface design with regards to presenting information to the user in a friendly
and useful manner. This allows users to access the information that they need as quickly as
possible, in a swift and easy manner, while not limiting the information they have access to.

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8

Designing Content-centric Activities
When you have a lot of data to display to the user, and you need a content
presentation Activity. Generally, such types of Activities turn out to be
content-centric. The main purpose of a content-centric Activity is to give the
user as much of the information as possible while not overwhelming them. This
is a common requirement of applications that perform some sort of search, or
present any type of specialized information.

Shopping and related e-commerce applications are an ideal example of a content-centric
application. Much of the effort in the design is dedicated to displaying information about
the products on sale. If the users can't find the information about a product they are looking
for, they will look somewhere else. For this reason, the product display must not only be
attractive and easy to use, but also provide as much information as possible, without being
cryptic or cluttered.
Another example of a content-centric layout is a user's profile page in a social-networking
application. People generally have a lot to say about themselves, and if they don't, other
people will often say a lot about them. These applications not only have a lot of information
to present to the user, but the information varies widely in terms of quality and relevance. Just
because one user thinks something is important, doesn't mean the next person will. In cases
like these, it's also very important to have an interface that can be customized to the user's
preferences (often just by re-organizing the order in which information is displayed), and is also
able to draw the user's attention to new information or areas they may find interesting.

Designing Content-centric Activities

A great example of a good way to draw a user's attention is seen in a chat application. If the
user has scrolled up, he/she is probably reading something that was said a few minutes ago.
If a new message arrives, it's very rude to just scroll them to the new message, since they
may well still be reading it. An audio tone to notify them of a new message is a common
option, but will also draw others' attention to the user (this is a mobile device after all). The
best option is a small animated icon at the bottom of the screen, possibly color-coded to
tell the user the relevance of the message (if that is available). Such an icon could also be an
interactive element, allowing the user to touch it in order to automatically scroll to the most
recently posted message. This type of thinking is important when designing any application,
but when building a content-centric Activity, putting some extra thought into your design
is even more critical.
In this chapter, we'll be exploring the different aspects to consider when displaying content
to the user, as well as different ways in which content screens can be developed. Specifically,
we'll be exploring:
‹‹

Thought process when designing content displays on Android

‹‹

How users use and view content screens

‹‹

Using the WebView class to display content

‹‹

Building native layouts for displaying content

‹‹

Formatting and styling text in Android

‹‹

Drawing attention to specific areas of the screen

Considering design options when displaying content on
an Android device
A content-centric Activity bears a strong resemblance to a web page, but has some
key design considerations that people don't have in mind when creating a web page. For
example, a touchscreen device generally doesn't have a software pointer, and so doesn't
have any concept of a "roll over". However, many web pages are built using cursor roll over
to drive everything from link highlighting to menus.
When designing a content-centric Activity, you'll want to consider carefully the aesthetics
of your design. The screen should avoid clutter since many elements may be interactive,
presenting the user with additional information when touched. At the same time, you should
attempt to minimize the need to scroll, especially horizontal scrolling. The need to keep
information concise is often the motivator to make more of the elements interactive. As
mentioned in previous chapters, it's a good idea to consider using icons instead of text where
you can, and to organize the information in order of importance to the user.

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Also bear in mind that screen-sizes change. Some devices have a large number of pixels
(such as the various Android Tablets), while others have tiny 3.5 inch screens. For this reason
it's important to consider that while some people will be able to see all of the presented
information on a single screen, others will be presented with three or four screens worth of
content for the same amount of information.
A web page is a great way to quickly and easily put together a content-centric layout when
working on an Android application. It has the advantage of having great HTML and CSS
support from WebKit, and easy integration with the rest of your application. It can also be
handled by an existing web-designer, or even just display a web page if your application is
connected to a web-based system.
A web page is however constrained (to some degree) to the layout structures dictated in HTML
and CSS. While these are extremely flexible at one level, HTML and CSS layout development
can also be a tedious and frustrating process even when only targeting a single rendering
engine (in Android's case: WebKit), if you are not used to building web-based systems. When it
comes to animations and similar structures, you are further constrained by the performance of
the HTML rendering engine, whether using JavaScript or CSS3 animations.

Considering user behavior
As with any type of user interface, it's important to understand your user behavior and how
they will interact with the screens you provide them with. In the context of large amount of
content information, it's important to understand both what information is important, and
how users will read and absorb that information.
While you may want to draw attention to a selected piece of information (such as price),
running a looping animation to change the color of that element will distract the user from
the other information on the screen. However, simply changing the font, placing the data
in a box, or changing the text color can also have the desired effect. It's also important to
consider how a user will interact with the screen. On a touchscreen device, users can and
will touch almost every part of the screen. They'll also drag items that look movable, and use
scroll gestures if the content appears to run over the screen length.

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Designing Content-centric Activities

Most people scan information in the same way. When a user is presented with a screen for
the first time, or with lots of information on it, their minds approach reading the information
in more-or-less the same way. The following are illustrations of the various movement
patterns a user's eyes will follow when scanning for important information on the screen.

You'll generally want to make sure that important information is in the areas where one
arrow meets another. The most important area is the corner in which your user normally
starts reading. For most Western users, this is the top-left corner of the screen, while Asian
and Arab users will often start at the top-right.
When designing a content screen, consider making the information in these
areas stand out a little more than normal. This will create a "linger" time where
the users' eyes will generally focus on that area a little longer than normal. This
is why we normally put a logo on the top-left of a web page.

Drawing user attention
Almost always, some information is more important than other information. You want your
user to be able to pick-out the important information as quickly as possible, and get on with
what they are doing. Once a person is familiar with your application, they may well stop
reading the fine print altogether. This is a good thing, you're helping your users make better
use of your application by letting them get on with their lives.
When you need to draw attention to specific information, such as a product's name or price,
it's a good idea to make use of the extensive options provided by the TextView class. Simply
changing an item's color can make it stand out for the user. If you need to go further, consider
adding a shadow, or placing the content in a "highlight box". As we've already discussed in
Chapter 7, Animating Widgets and Layouts, animations can also be used to draw attention
to specific areas of the user interface. A simple "blink" animation (consisting of a fade-out
followed by a fade-in animation) can be used to draw the users' attention to a change.

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A more specific example: money
If you are selling something to your user, and allowing them to choose between
different shipping methods and packaging options, the total price will change
based on their selections. Make sure that the total amount stands out by
rendering it in a bold font. When the price is updated, cycle through a series of
"intermediate" prices so that the total is graphically "counted up" or "counted
down" to its new value.

Think carefully about the widgets that you'll want to make use of in your user interface.
Instead of using the normal TextView, you may well want to place a piece of what would
normally be a single field in a TextSwitcher (or something similar) to allow you to animate
a single word or value.

Displaying content with the WebView class
The WebView class (in the android.webkit package) is often a logical choice for contentcentric designs and holds some very serious advantages over building the user interface
and a normal Android XML layout resource. The WebView class offers you a single point at
which you can place all of the content for a screen, and it handles all of its own history and
scrolling, making your code very simple to write.
When displaying content that requires complex layout and/or lots of text content (which
may require markup), the WebView class is a highly favorable option. Having built-in support
for HTML and CSS mark up, it reduces the number of widgets that you'll need on the screen.
Given that Android makes use of Web-Kit as a rendering engine, you also have many CSS3
structures available (such as CSS animations). Although the WebView is generally used for
browser like networked applications where hyperlinks are very important, you can just as
easily supply it with local content containing no links. You can also intercept link requests to
allow navigation to other parts of your application.
Generally when working with a WebView structure, you'll need some method by which you
can generate the content that you will be displaying. In contrast to building the user interface
in a layout resource, where you can simply ID the various View objects that you need to
inject dynamic content into. That said, a full template engine is often much easier to work
with than a hybrid of XML layout and Java code, although the ease of implementation is
strongly dependant on both the skills available to you and the type of information you need
to display on the screen.

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Designing Content-centric Activities

Using a WebView object
To work a bit with the WebView and give a more specific example on how it can be used to
present large amounts of content, we'll be building an Activity to display a food recipe on
the screen. For this example we'll be hard coding the actual recipe and the layout code to
generate the HTML. In practice, you would want to make use of a template engine such as
Velocity/FreeMarker or XSLT to generate the HTML code.

Time for action – creating a recipe viewer application
You'll notice that the following example doesn't use an XML layout resource, but rather
creates the entire Activity in Java. In this example we use a Recipe object to generate
HTML code into a StringBuilder for display. It's a simple but effective implementation.
However, it requires that the Java code be modified if a change to the look and feel of the
recipe is required.

1.

Create a new project to contain the recipe reader application:
android create project -n RecipeViewer -p RecipeViewer -k com.
packtpub.viewrecipe -a ViewRecipeActivity -t 3

2.

Create a new Ingredient.java source file in the root package of the new
application to hold information for a single required ingredient, and open this new
file in your editor or IDE.

3.

Declare fields for the name, amount, and unit required for a recipe:
private final String name;
private final double amount;
private final String unit;

4.

Create a constructor to take the parameters and assign them to the fields:
public Ingredient(
String name,
double amount,
String unit) {
this.name = name;
this.amount = amount;
this.unit = unit;
}

5.

Create a getter method for each of fields:
public double getAmount() {
return amount;
}
// . . .
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6.

In the root package of the project, create a new source file named Recipe.java to
contain a single recipe, and open it in your editor or IDE.

7.

Declare a field for the name of the Recipe object:
private final String name;

8.

Declare another field to contain the list of ingredients required for this Recipe. We
store these as an array of Ingredient objects:
private final Ingredient[] ingredients;

9.

Then declare an array of String objects that will contain the list of instructions that
need to be followed for the Recipe:
private final String[] instructions;

10.

Create a constructor to accept the field data and assign it for storage:
public Recipe(
String name,
Ingredient[] ingredients,
String[] instructions) {
this.name = name;
this.ingredients = ingredients;
this.instructions = instructions;
}

11.

Create a getter method for each of the three fields:
public Ingredient[] getIngredients() {
return ingredients;
}
// . . .

12.

In this example the Recipe class is responsible for generating the HTML. Declare a
new method named toHtml:
public String toHtml() {

13.

Create a DecimalFormat object to handle the formatting of the volumes:
DecimalFormat format = new DecimalFormat("0.##");

14.

Create a new StringBuilder object to build the HTML into:
StringBuilder s = new StringBuilder();

15.

Append the HTML headers:
s.append("").append("");
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16.

Append a first-level header element with the name of the recipe:
s.append("
").append(getName()).append("
");

17.

Append a second-level header element to open the ingredients section:
s.append("
You will need:
");

18.

Open an unordered list to list the ingredients required for the recipe:
s.append("
");

19.

For each Ingredient object open a list item for the new ingredient:
for(Ingredient i : getIngredients()) {
s.append("
");

20.

Append the amount of the ingredient to the StringBuilder after formatting it
with the DecimalFormat declared:
s.append(format.format(i.getAmount()));

21.

Then append the measurement unit for the ingredient:
s.append(i.getUnit());

22.

Now append the name of the ingredient to the StringBuilder, and close the
ingredient list item:
s.append(" - ").append(i.getName());
s.append("
");

23.

After closing the for loop, close the unordered list:
s.append("
");

24.

Create a second-lever header opening the Instructions section of the recipe:
s.append("
Instructions:
");

25.

Open another unordered list to render the recipe instructions into:
s.append("
");

26.

Use a for-each loop over the array of instructions to render them into the unordered
list structure in the StringBuilder:
for(String i : getInstructions()) {
s.append("
").append(i).append("
");
}

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27.

Close the unordered list, and HTML headers, returning the String contents of the
StringBuilder object:
s.append("
");
s.append("").append("");
return s.toString();

28.

Open the ViewRecipeActivity Java source code in your editor or IDE.

29.

In the onCreate method, directly after invoking super.onCreate, create a new
WebView object passing this to it as its Context:
WebView view = new WebView(this);

30.

Set the WebView LayoutParams to take up all available screen space, since the
WebView (much like a ListView) has built-in scrolling capabilities:
view.setLayoutParams(new LayoutParams(
LayoutParams.FILL_PARENT,
LayoutParams.FILL_PARENT));

31.

Create a Recipe object to display in the WebView, the full recipe is at the end of
this example section:
Recipe recipe = new Recipe(
"Microwave Fudge",
// . . .

32.

Load the HTML content generated by the Recipe object into the WebView:
view.loadData(recipe.toHtml(), "text/html", "UTF-8");

33.

Set the content view of the Activity to the WebView object we created:
setContentView(view);

What just happened?
The recipe viewer example shows a simple structure which can be extended in many
different ways to present large amounts of information to the user in an easy-to-use format.
Thanks to the fact that WebView works with HTML, it makes presenting non-interactive lists
of information more appealing than working with a ListView or similar structures.
The loadData method used previously is limited in that it doesn't allow for your page to
easily reference external structures such as style sheets or images. You can work around this
limitation by using the loadDataWithBaseURL method which works in much the same way,
but renders the page relative to a specified URL, which may be online or local on the device.

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Designing Content-centric Activities

The Recipe object is considered responsible for rendering its HTML, which works well in a
pure Java situation. You could also pass the Recipe to a template engine, or use something
like a visitor pattern to render the Recipe object as HTML code. The full code for the
Recipe object in the previous example is as follows:
Recipe recipe = new Recipe(
"Microwave Fudge",
new Ingredient[]{
new Ingredient("Condensed Milk", 385, "grams"),
new Ingredient("Sugar", 500, "grams"),
new Ingredient("Margarine", 125, "grams"),
new Ingredient("Vanilla Essence", 5, "ml")
},
new String[]{
"Combine the condensed milk, sugar and margarine "
+ "in a large microwave-proof bowl",
"Microwave for 2 minutes on full power",
"Remove from microwave and stir well",
"Microwave for additional 5 minutes on full power",
"Add the Vanilla essence and stir",
"Pour into a greased dish",
"Allow to cool",
"Cut into small squares"
});

An unfortunate side effect of using the WebView object is that it doesn't conform to the look
and feel of other widgets. It is for this reason it doesn't work well when you place it with
other widgets on the same screen. The end effect of the previous example is effectively a
non-interactive web page which looks as follows:

Have a go hero – improving the look of the recipe viewer
The previous example generates a very simple HTML page and doesn't include any styling.
Including an inline CSS is a very simple operation, and could even be done by reading the
styling content from an application resource. Create a CSS, include it inline in the HTML page,
with rules such as:
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‹‹

Color the background of first-level header and second-level header elements

‹‹

Change the font-color of the first and second-level headers to white

‹‹

Round the corners of the header elements by five pixels

‹‹

Change the list bullet to a square instead of a circle

Taking WebView further
The WebView class has significant functionality that can be very useful when dealing with
content screens, for example, using hyperlinks to provide a show/hide disclosure section
for less important content. This requires the use of JavaScript in the HTML page, at which
point it's strongly advisable that your application use a template engine to produce the
HTML pages instead of generating them in Java code (as the Java code will quickly become
difficult to maintain).
The WebView class also allows your application to interact with the JavaScript code on the
page using a very simple mechanism by which you can expose Java objects to the JavaScript
code. This is done with the addJavascriptInterface method. This allows the HTML
page to invoke actions on a Java object that you provide, effectively allowing the page to take
control of a part of your application. If your content screen needs to take a business action,
such as Buy or Cancel, the required functionality can be exposed in a JavaScript interface
object. When the Book HTML element is selected by the user, the JavaScript in the page can
invoke the appInterface.buy(); method which you defined.
Another important feature to consider with the WebView class is the "zoom" controls. When
presenting your user with lots of information, it may be useful for the user to be able to
zoom in or out in order to make some elements easier to read. To enable the built-in zoom
controls of the WebView, you'll need to access the WebSettings object:
webView.getWebSettings().setBuiltInZoomControls(true);

The WebSettings object can be used to enable and disable a large number of other
features that are available in the WebKit browser component, and it's well worth reading
through the available documentation.
The primary problem with the WebView class is its look and feel. Where an Android
application with the default theme is light grey on a black background, the WebView class is
black on a white background, which makes the screens driven by a WebView stand out to the
user as though they are a separate application.
The simplest way around the styling problem would appear to be to style the HTML pages
to look just like the rest of the application. The problem is that some device manufacturers
have their own Android application styling, so you can't really be sure what the rest of your
application is going to look like. Changing the background and foreground of the HTML page
to be in line with the standard Android theme could well make it stand out against the rest of
your application when run on manufacturer-themed devices.
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Pop quiz
1. What is the best way to render large object graphs to HTML for rendering
in a WebView?
a. Convert them to XML and run them through XSLT
b. Send them to an external web service to be rendered
c.

Hard code the HTML generation

d. With a simple template engine
2. How can you access external CSS and images with a WebView?
a. Use the loadDataWithBaseURL method
b. Specify the full URL path in the HTML page
c.

Generate HTML code that includes the in line data

3. What rendering engine does Android use for WebView?
a. Gecko
b. MSIE/Trident
c.

KHTML

d. WebKit

Creating relative layouts for content display
The WebView offers an easy way by which large amount of content can be displayed to the
user in an easy-to-read format. It also has many built-in features designed specifically for
viewing content. However, it doesn't always offer the easy way out and often doesn't allow
for functionality that other widgets provide out-of-the-box. The RelativeLayout class
provides much of the same layout functionality that the WebView class provides you with.
As we just discussed, the WebView stands out almost as though it were a separate
application. Working with a RelativeLayout, you'll be populating your screen with
standard Android widgets, which in turn means that there will be no change in the look
and feel from one screen to the next. While WebView requires some form of template
engine (whether it be in an API or simply a StringBuilder as in the example), a
RelativeLayout can be declared in an application resource as an XML file. Using a layout
file also means that the screen layout will be selected through the resource selection
process, allowing for sophisticated customizations that are difficult to achieve with the
WebView class and HTML code.

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Using a RelativeLayout in a way provides a form of template engine. By only giving IDs
to View objects that you will need to populate with data, you can populate the screen by
injecting the relevant content into these exposed objects. When we built the HTML-based
view, we needed to create header elements for the ingredients list and list of instructions,
with a coded layout structure those headers would be loaded from within the layout file, or
from a string bundle resource.
When dealing with lists of information, which is a common requirement of a content layout,
you can provide the data in several different ways. You could use a ListView object, or you
could use an embedded LinearLayout to act as a list. When working with either of them, it's
advisable to have a layout resource that can be reused for each of the items in the list. Making
use of a ListView means you have an Adapter through which you can convert your data
objects into View objects that can be displayed on the screen. However, ListView objects
have various other constraints (such as the size of the contained items) and are best used when
the items they display are interactive in some way. If you need a non-interactive list (or grid) of
items, it's a good idea to follow the Adapter mechanism by creating a separate class that is
responsible for creating View objects based on your data objects.

Taking full advantage of RelativeLayout
RelativeLayout structures have the major advantage that they offer direct integration
with the rest of your application. They can also be more easily localized than an HTML page.
The event structures provided by a direct ViewGroup structure are more versatile than
those provided by a WebView object via its specialized event listeners and JavaScript.

The XML layout structures also provide much the same effect as a template engine, avoiding
the need to import an external API such as an XSLT engine, a Java template engine, or hard
coding the HTML generation. Standard Android Activity classes also have the builtin Android animation structures to work with. While the WebView class allows for CSS
animations or could run JavaScript animations, this requires re-layout of the HTML structure
for each frame in the animation.
An Android Activity class implementing the entire content screen also has the advantage
that it can load its external resources from the application resource structure. This not only
allows you to do things such as localize your images more easily, but also means that all of
the resources are run through the resource compiler and as such can be optimized by the
Android tool chain. With a WebView you would need a base URL to load such resources
from, or be able to encode them inline in the HTML page.

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Considering Android layout constraints
There are some drawbacks to developing the entire content view as an Android layout. From
a skills point-of-view, only a developer can build and maintain the user interface. It also
means that any styling done to individual widgets must be managed by a developer. With a
WebView based layout, much of the creation work on the layout could be handled by a web
developer and graphic designer.
Adding more widgets to your screen comes with another problem—
performance. Not only can larger, more complex layouts lead to a very slow user
experience, it can cause your Activity to crash entirely.

Keeping fewer widgets on the screen means that the interface will have less information for
the user to absorb in one hit, and will be easier to work with.
Layouts with either too much length, or too much depth will cause an application to crash.
If you need to animate a single word in the middle of a sentence, you'll have to define two
additional TextView widgets that will display the non-animated text on either side of the
animated word. This increases the length of your layout. If you also needed a horizontal
LinearLayout in which to place these three TextView objects, you would be increasing
the depth of your layout structure. By factoring in both of these constraints, you can imagine
how quickly you can run out of memory or processing power when it comes to layout
rendering. Each of the widgets must be measured for layout before being rendered. Each
measurement, layout step, or rendering step makes use of the language stack (by recursively
invoking methods) in order to make sure all of the widgets are correctly rendered at the
correct point on the screen (or not rendered if they are off screen). The software stack size
in Android is limited, and each method call requires each of its parameters to be pushed
onto the stack for the invocation. On top of that, all of the measurement information needs
to be stored in the heap space, which is another seriously limited resource on the Android
platform (by default the Dalvik VM only allocates 8 MB of heap space to begin with).
The following diagram illustrates the difference between the length and depth of a layout
structure. The left screen illustrates a long layout, while the right screen illustrates
a deep layout:

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Styling TextView objects
At this point it's rather concerning to think about how you might make a single word in a
sentence bold, or give it a shadow. In the WebView it's as easy as adding a  element
with some special styling on it, but in a native layout, wouldn't you need to add separate
TextView objects for each section of the text? If this were so, you would be dramatically
limited in the amount of text you would be able to display to the user, since you would be
creating thousands of almost useless objects.
Fortunately, Android makes it very easy to mark up the text in all of its default widgets. Any
class that extends from TextView can handle text with style information or even images.
Generally the classes available in the android.text.style package can be used to style
sub-segments of the text strings you want to display.
In order to use these different styling structures, you will need to use a SpannableString
object. A SpannableString is a specialized type of Android string that keeps track of
styling information in relation to a normal CharSequence of text that needs to be displayed.
There are several other similar classes (such as SpannableStringBuilder) which
handle easy modification of the text, and are thus suited to text that will be edited. For
our current purposes, a SpannableString is perfect, and much simpler to work with. A
SpannableString has a method that it's required to implement, based on the Spannable
interface—setSpan. The setSpan method allows you to add markup structures to the
SpannableString, which affect how a specific part of the text is rendered.
If we simply wanted to write the text There is nothing to fear! on the screen, you would
normally just use a TextView object with the specified string. What if we wanted to strike
the nothing out of that string? The way forward now is to use a StrikethroughSpan
object for characters 9 through 16. In this case, the string can't just be defined in the layout
file anymore, a SpannableString needs to be created in the Java code. The following is a
simple example of how this can be done, and what the resulting TextView would look like:
TextView fear = new TextView(this);
SpannableString string = new SpannableString(
"There is nothing to fear!");
string.setSpan(new StrikethroughSpan(), 9, 16, 0);
fear.setText(string);

The result of this little snippet of Java code is a TextView widget displaying styled content
instead of a plain String, as shown in the following screenshot:

As you can see, using this type of markup is brilliantly effective, and really quite easy to work
with. This sample is also very quick to execute when compared to the WebView rendering,
since it doesn't include any form of parsing.
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There are a few problems with the mechanism though. The most important one being the
index handling. In order to know when to start or stop a Span of markup rendering, you
need to specify the first and last character that needs to be rendered with the given Span.
Not a problem unless you plan on changing your text, or even worse—internationalizing it.
Fortunately, once again Android already has a built-in solution, although it comes at the
expense of performance. You can convert almost any HTML text into a Spannable object
which can in turn be passed directly to any TextView object for rendering. The class to use
is the android.text.Html class, which includes utility methods for parsing HTML code
into a Spannable object, and also for converting a Spannable object into a HTML code.
If you need to internationalize strings that you plan on rendering with additional style
attributes, the Html class is probably the only sensible way to do it. It also has the added
advantage that the loading of images can be handled by your application (through use of
the Html.ImageGetter interface). Additionally, the TextView will still look and feel like a
normal Android widget, which enhances the users' experience.
Most HTML tags are handled by the Html class, but not quite all of them. For one thing—CSS
styles are ignored, so colors and borders are out of the question. However, great styling is
still possible, and at least you don't need to record the character index values somewhere in
an application resource so that all of the styling lines up.
If you wanted to format some text in a Button label as bold, it's really easy to do with the
Html class. It's much quicker to just pass the result of the fromHtml method directly to the
TextView object. For example, the following code snippet would yield a Button object with
the word Hello in italic script, while the word World would have a bold weight to it:
Button button = new Button(this);
button.setText(Html.fromHtml("Hello World!"));

You can also specify HTML content in a layout resource XML file, and it will be parsed with
the Html class before being passed into the TextView object's setText method.
The above Java snippet creates a Button widget that would look as follows:

HTML tags can also be used to render mini-documents into a TextView object, and while
they carry their own styling, they also adhere to the styling of the TextView object. This
means that if you're looking for a solution that's quicker to work with than a WebView for
carrying some static text (and no hyperlinks), then a TextView can actually serve as a good
alternative. For example, consider the following code snippet:

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TextView text = new TextView(this);
text.setTextColor(0xff000000);
text.setBackgroundColor(0xffffffff);
text.setText(Html.fromHtml(
"
Cows Love to Eat Grass
"
+ "
Do not fear the Cow
"));

This will render the TextView with a first-level header and a single-lined paragraph element.
Both of which will include some padding in order to space them apart from the other
elements on the screen. The resulting image should look quite familiar:

As you can see, a correctly styled TextView makes a great alternative to a WebView,
especially if you are fitting it inline with a series of native widgets. However, the black-onwhite styling does bring back the inconsistency problem. So unless you entire application
follows this model, it's a better idea to leave the styling as default.
If you are planning on using a TextView for longer content, it's important to consider some
additional factors:
‹‹

Make sure that the user will be able to scroll if the text runs longer than the size of
their screen. This is easily done by placing the TextView in a ScrollView object.

‹‹

If your text is very long, consider styling the content, either making the text brighter
white or working with a black on white background. While it is very inconsistent
with other Android applications, and other screens in your own application, it is
much easier on the eyes and your users will thank you for that.

‹‹

Consider allowing the user to change the font size with a long touch or a menu. If
their screen is low density, or they don't have perfect vision, you may be making
their lives a little easier.

Pop quiz
1. If you need to display a non-interactive bullet point list, which of these is preferable?
a. A WebView with an unordered list
b. A specially styled ListView object
c.

A TextView object with HTML content

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Designing Content-centric Activities

2. With regards to hyperlinks, you might use a WebView instead of a TextView
because:
a. TextView cannot handle hyper links
b. They look better in a WebView
c.

A WebView has built-in history management

3. A native interface works better for animation intensive applications because:
a. You can use Android animation resource files
b. The WebView class doesn't handle animations
c.

HTML animations are more expensive to run

Time for action – developing specialized content views
In many situations, you'll need a specific type of interactive logic that you would want to
reuse in many parts of your application. On a content screen, some parts of the display will
need to be updated, driven by changes that are made to other parts of the display. This
is often because while some area of the screen is giving the user information, the other
parts are capturing new data from them. Next, we'll build a simple widget responsible for
displaying an amount of money to the user. Its main reason for its existence is the fact that
it not only animates between changes, but also feeds back to the user whether the amount
has gone up or down by changing its color.

1.

Create a new Java source file named AmountBox.java for the new class, and open
the new file in an editor or IDE.

2.

The new class should extend the TextSwitcher class and implement the
ViewSwitcher.ViewFactory interface:
public class AmountBox extends TextSwitcher
implements ViewSwitcher.ViewFactory {

3.

Declare a field for the DecimalFormat to be used to render the amount:
private DecimalFormat format = new DecimalFormat("0.##");

Also declare a field to store the current numeric value displayed:
private double amount;

4.

Declare copies of the two constructors made available from the TextSwitcher
class in order to allow the LayoutInflator class to instantiate the AmountBox
class from resource files:
public AmountBox(Context context, AttributeSet attrs) {
super(context, attrs);
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Chapter 8
init();
}
public AmountBox(Context context) {
super(context);
init();
}

5.

Declare the init() method to take care of "common constructor" requirements:
private void init() {

6.

Set the "in" and "out" animations to the fade animations provided by Android:
setOutAnimation(getContext(), android.R.anim.fade_out);
setInAnimation(getContext(), android.R.anim.fade_in);

7.

Next, set the ViewFactory to the AmountBox:
setFactory(this);

8.

Finally, invoke setAmount(0) to ensure the displayed amount is specified:
setAmount(0);

9.

Declare a setter method to allow overriding of the default DecimalFormat:
public void setFormat(DecimalFormat format) {
this.format = format;
}

10.

Declare a getter method to allow easy access to the current numeric value:
public double getAmount() {
return amount;
}

11.

Override the makeView() method from ViewFactory:
public View makeView() {

12.

Create a new TextView object with the context given to this AmountBox:
TextView view = new TextView(getContext());

13.

Specify a large text size since the amount will represent money, and then return the
TextView object for display:
view.setTextSize(18);
return view;

14.

Now declare a setter method to allow the amount value to be changed:
public void setAmount(double value) {
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Designing Content-centric Activities

15.

This method will change the color of the text, so declare a variable for the new text
color that will be displayed:
int color;

16.

First check to see what color we should change the text to:
if(value < amount) {
color = 0xff00ff00;
} else if(value > amount) {
color = 0xffff0000;
} else {
return;
}

17.

Fetch the off screen TextView object:
TextView offscreen = (TextView)getNextView();

18.

Set the font color based on the change to the numeric value:
offscreen.setTextColor(color);

19.

Render a shadow around the text in order to create a "halo" effect:
offscreen.setShadowLayer(3, 0, 0, color);

20.

Set the text of the TextView to the new value:
offscreen.setText(format.format(value));

21.

Display the off screen TextView and remember the new value:
showNext();
amount = value;

What just happened?
The AmountBox class is a great example of a small unit of content that needs to be updated.
This class provides information to the user, but also provides a form of feedback. When the
user does something which affects the amount displayed, the AmountBox reacts by updating
the font color to reflect the direction of the change—green for the amount going down, and
red for an amount going up.
The example makes use of the standard Android fade through animations as discussed in
Chapter 7, Animating Widgets and Layouts. The speed of the animations provides a great
cross fade effect between the two amounts. Notice that in the setAmount method, the
updating of the text content and switching the View objects is handled manually.

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You could potentially replace the offscreen.setText and showNext method calls with
a call to setText, but it's nice to see how it works under the hood. This method is also not
subject to future implementation changes that may occur.

Developing an online music store
A great example of a content-centric layout is a music store built into a media player
application. The ability to buy music directly from the media player is a massively userfriendly feature, and also fits nicely with the way Android applications behave as "connected"
applications instead of purely offline systems. Android also makes it very easy to truly include
the shop as part of the application instead of simply providing a link to an appropriate website.
Generally, users are more inclined to feel a sense of trust if they pick the Buy Music button and
are not suddenly whisked off to their web browser. Having both online and offline parts of your
application properly integrated can also go a long way for your sales statistics.
Buying music online is very different to purchasing it in a store. The availability of additional
information about the songs, artists, or albums the user is looking at is part of the appeal.
For this reason, an online music store for a mobile device must be carefully designed to
provide as much information as possible without either cluttering the screen, or detracting
from the fact that the user is there to purchase music. The feeling of integration with the
application also helps build trust with the user, so the look and feel is very important.
Another advantage of buying music online is you pay only for what you want to purchase.
For this the user interface needs to allow the user to select which tracks from an album
the users would like to purchase, and which they either don't want or plan to buy later.
Also, how do they know which ones they like? They also need to be able to play a sample
(whether it be time limited, or just a lower quality) of each track.

Designing the music store
To really illustrate how a content-centric design fits together, you need to build one. For this
example we'll be working through the design process, and then the implementation of that
design. Since the design and its implementation are the important parts here, we won't go
into building a functional example. It'll really just be a pretty screen.

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To begin with, we need to have a basic user interface design. I find it best to start with a
whiteboard or a piece of paper and a pen. While there are plenty of tools for drawing mock
screens out there, none of them really approach the user interface of a paper and pen. To
start off, we draw a high level wireframe of the overall screen design. This is simply a series
of boxes that will tell us what type of information to show in what parts of the screen.

In the diagram, we've divided the user interface into three parts:
‹‹

‹‹

‹‹

Album and Artist Information area: This area displays the name and cover art of the
album the user is looking to buy
Track List area: In this area, users can listen to samples and select which tracks they
want to purchase
Purchasing Area: This area displays the total amount users will be paying, and also a
button to buy their selected tracks

In the previous diagram, I've stuck to the size of the screen, but depending on the screen size
and number of tracks available, the user interface may need a scrollbar to be fully accessible.
The next bit of work is to look at each of the sections of the user interface that we've defined
and decide what widgets will go into each of them. Firstly we'll need to look at the album
and artist information. The album information will be displayed as the album cover artwork
and the album name. We'll include an image area for an artist logo, and also include a text
block with the name of the recording label.

A simple block diagram like this lets you visually consider the various elements. It also allows
you to start thinking about things such as font size, borders, and spacing. In the previous
diagram, we want the three elements on the right to be roughly the same size as the cover
art on the left. Unfortunately the Android RelativeLayout class doesn't currently allow us
to directly stipulate this as a contract. The next element of the design we need to consider is
the track listing box. For this, instead of drawing everything in the box, we'll focus on what a
single line will look like and what information it will contain.
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Chapter 8

The preceding structure is a very simple one line structure for displaying the details of a
single track. The CheckBox on the left can be used to select those tracks that the user wants
to purchase, while the button on the right side can be used to play a sample of the given
track. The two button-like elements on either side create a kind of framing for the plain text
elements in the middle of the line.
Finally, we need to consider how we plan on asking the user to send their money. This is a
very important part of the user interface, it needs to be obvious—the amount of money they
will be expected to pay. We also need to make it really easy for the user to actually make the
transaction, so a single Purchase or Buy Selected Tracks button is needed.

The final section of the user interface simply has two widgets in it, one on the left for
purchasing, and the total amount the user is expected to pay on the right. For the left side
button, we'll work with a simple Android Button widget, while on the right side we'll make
use of the new AmountBox written in the previous part of this chapter.

Developing the music store
We'll start with the new example by building a new series of model classes, but first you'll
need to create a new project for our conceptual media player. To do this, run the following
command on a command line or console:
android create project -n PacktTunes -p PacktTunes -k com.packtpub.
packttunes -a ShopActivity -t 3

After creating the new project, copy the AmountBox source code into the root package
of the new project. You then need to create a class to contain the data for a single track.
This simply needs to have the name of the track and the duration of the track stored as the
number of seconds. We'll also include utility methods to calculate the minutes:seconds
values that we can use to display the duration data.
public class Track {
private final String name;
private final int length;
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Designing Content-centric Activities
public Track(final String name, final int length) {
this.name = name;
this.length = length;
}
public String getName() {
return name;
}
public int getLength() {
return length;
}
public int getMinutes() {
return length / 60;
}
public int getSeconds() {
return length % 60;
}
}

The Track class is a very simple structure which could easily be parsed from XML or
deserialized from a binary stream. We also need another class to hold the information about
a single artist. While the following class is really nothing more than a form of data store, it
could easily be extended to store things like biography information if needed:
public class Artist {
private final Drawable logo;
private final String description;
public Artist(
final Drawable logo,
final String description) {
this.logo = logo;
this.description = description;
}
public String getDescription() {
return description;
}
public Drawable getLogo() {
return logo;
}
}

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Finally, on the data class front, we'll need a class for linking the two previous classes to a
single album. This class will be used as a single point which can be handed to an Activity.
Copy the following code into a new file named Album.java in the root package of your
project:
public class Album {
private final Drawable cover;
private final String name;
private final Artist artist;
private final String label;
private final Track[] tracks;
public Album(
final
final
final
final
final

Drawable cover,
String name,
Artist artist,
String label,
Track... tracks) {

this.cover = cover;
this.name = name;
this.artist = artist;
this.label = label;
this.tracks = tracks;
}
public Drawable getCover() {
return cover;
}
public Artist getArtist() {
return artist;
}
public String getLabel() {
return label;
}
public String getName() {
return name;
}
public Track[] getTracks() {
return tracks;
}
}

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Time for action – building a track item
To get working on the new user interface, you'll need a few images. For this next section,
you'll need an image for the play buttons. The play image should be a simple "play" arrow,
the button we place it in will provide a background and border. The lines in the list structure
will be placed into a TableLayout in order to align all of the sub-structures.

1.

Create a new layout resource file in the res/layouts directory of the project, and
name the new file track.xml.

2.

Declare the root element of the new file as a TableRow element consuming all of
the available width and only the required height:


3.

As the first element of the TableRow, create a CheckBox the user can use to select
and unselect the tracks they want to buy:


4.

Declare a TextView element to display the name of the track with a larger-thanusual font, and a pure-white font color:


5.

Follow the track name TextView with another right-aligned TextView object to be
used to display the duration of the track:


6.

End the TableRow element with an ImageButton element, which can be used by
the user to sample the track before buying it:

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What just happened
The above layout resource file will handle the layout of the track list items for the second
part of the user interface. We need to be able to create several of these structures to handle
all of the tracks available in an album. We wrap them in a TableRow element which when
placed in a TableLayout object, will automatically align each of its sub-elements with
those in the other rows.
Later, in the Java code we'll use the LayoutInflator to load this resource, populate it
with the name and duration of a track, and then add it to a TableLayout object that we
will declare as part of the main user interface. Once this new item has been populated with
some data, it'll look something like the following screenshot:

Time for action – developing the main user interface layout
Having built the layout resource file that will become track items in a list later, we now need
to define the remaining elements of this user interface. While this structure is relatively
simple, it's also very easily extended and has a few minor details that keep it looking really
good. It also needs some Java code in order to be correctly populated, but we'll get into that
after we've finished with the resource file.

1.

Create or open the res/layout/main.xml file in the new project.

2.

The root element of the main layout needs to be a ScrollView in order to handle
the possibility that the interface runs longer than the available screen space. The
ScrollView should take up all available screen space:


3.

As the only element to the ScrollView, declare a RelativeLayout which
consumes the available width, but only the required height. The RelativeLayout
should include some padding at the top and bottom to provide a little "breathing
room" so that its contents don't look to cramped:
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Designing Content-centric Activities


4.

The first element of the RelativeLayout is the album art, a fixed size ImageView
object that will fit the album cover art in the available space:


5.

The second element after the album art is the artist's logo image, also an
ImageView. This element is required to center the logo in the available space:


6.

After the artist's logo, we need a plain TextView object with some font styling to
hold the name of the album we're trying to sell. We'll place this below the artist's
logo in the user interface as per the image we saw earlier:


7.

Below the TextView with the album name, we have a small non-styled TextView
to hold the name of the record label under which the album is released:

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8.

As promised, we follow these elements with a TableLayout which will hold the
available track information. We layout the TableLayout element against the album
art rather than the record label TextView:


9.

Underneath the track list, we start by placing the Buy Selected Tracks button
element on the left side of the screen:


10.

Finally, on the right side of the screen, we add our custom AmountBox widget
where we will tell the user how much they will be paying:
, after
which you will list your 


The name attribute in the above 

name="android:textSize">25dip
name="android:textColor">#ffffffff
name="android:textStyle">bold
name="android:gravity">center

Either of these will result in the exact same rendering of the new widget. The implementation
decision is really a matter of what you are trying to achieve. Styles are the best way to keep
commonality between different widgets that are used for the same purpose.
Using the above style on a TextView will result in a nice header widget that looks as follows:

Rendering ovals
The oval shape is exactly what the name implies—an ellipse. An oval is more limited in its
use than a rectangle, unless the widget drawing on top of it is best bordered by a circle or
ellipse, such as an analogue clock. That said, an oval, or rather a circle is a very useful shape
to use as an image in your user-interfaces. A perfect example is a symbol to inform the user
whether they are connected to the Internet or not, or whether a widget is valid or not. Using
an oval shape for such a purpose is exactly the same as using a bitmap. However, the oval
can be scaled according the users' preferences without any loss of quality, while you would
need several differently-sized bitmap images to achieve a similar effect (even then, some of
the bitmaps would require scaling).
If we wanted an oval shape to represent an invalid widget (for example, to show that two
password entries don't match when the user is selecting a password), then it would be best
to color the oval in red. In the following code-snippet, we declare an oval shape as XML with
a grey border and a red fill:





In the preceding case, we use the  element to fill the oval with a plain red color,
while using the  element to surround it with a thin grey outline. Also, notice the
lack of sizing on the shape elements. As previously stated, their dimensions are inherited
from the width they are placed in, either as a background, or in the case of an ImageView,
as the content of the widget. If you want to place this oval shape into an ImageView, you
would specify it in the src attribute, as follows:
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The preceding code is about the right size for a validation icon to sit next to a widget, while
scaling the icon up or down is as easy as changing the width and height of the ImageView. If
you use wrap_content as the size of the ImageView, it will be sized as zero-by-zero pixels,
and will effectively vanish off the screen.
Following is a screenshot of four different sizes of the same oval, each scaled to double the
size of the previous (starting off with the 8x8 dip on the left):

Time for action – applying a gradient to an oval shape
The previous screenshot shows that while the oval looks okay, it's not going to be very
visually appealing when surrounded by the gradient painted widgets which make up the
default Android toolkit. In order to get the little oval to fit in nicely, it needs to look more like
a ball, which requires a simple radial gradient to be applied.

1.
2.

Create a new shape resource XML file in the res/drawable directory named ball.
xml and open this file in an editor or IDE.

Declare the root element of the file as an oval:


3.

Instead of declaring a solid color as the fill, declare a gradient fill starting with a
light grey and ending in red:


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Styling Android Applications

4.

Define the thin light grey outline of the oval in a stroke element:


5.

Close the shape declaration:


What just happened?
Unfortunately, the affected radius of a radial gradient doesn't scale with the rest of the
image, leaving a very small gradient area when you scale the image to large sizes. The
effect in this case is that while the smallest version of the image looks great, the larger
versions look terrible. At the time of writing this book, there is no direct way to work around
this limitation. Instead, you will need to tie the size of your oval shape to the size of the
ImageView if you want to use a radial gradient.

Rendering rings
The ring shape is also circular in its rendering, but serves a very different purpose to the
oval shape. While the oval shape's content area is everything inside the outline space, a ring
shape's content area is a circle.
The following diagram illustrates the logical difference between the two shapes:

The ring shape also how two outlines, one on the outside and another on the inside (as
shown in the preceding diagram). Combine this with the ability to fill the ring's content area
with a gradient and you have the perfect shape to use for progress spinners (the default
Android indeterminate progress spinner it built with a ring).

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Time for action – rendering a spinner ring
By default, a shape will assume that it's being used as part of a LevelListDrawable, and
may not appear unless you disable this behavior. You do this by specifying the useLevel
attribute as false on the shape element. If you don't disable this functionality, the ring will
not render correctly, or at all.

1.

Create a new shape resource XML file in the res/drawable directory named
spinner.xml and open this file in an editor or IDE.

2.

Start the root element of the file as a ring shape:


5.

Declare a sweep gradient centered in the oval:


6.

Outline the ring with a thin white border:


7.

End the shape declaration:


What just happened
The sweep gradient is another form of radial gradient. Instead of extending out from the
center of the image, it sweeps in a circle like the hands of a clock.

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Styling Android Applications

The image on the left-hand side is a rectangle filled with the sweep gradient; while the
image on the right-hand side is the ring shape. As you can see, the two effects are quite
different. The image on the right-hand side is based on the image used by Android 1.6 for the
indeterminate spinner.

Defining layers
So far, we've only defined shapes as single-element images. It's possible to combine these
shapes into more complex images. These images are combined together in layers, which is
a commonly used graphics structure. In Android, this is done with a layer-list structure.
A layer-list is not a type of shape, but it is a Drawable structure which means it can be
used in place of a normal bitmap image.
Layered image resources are not confined to being used with vector Drawable structures
such as the shapes we've already talked about. A layered Drawable object may also include
some layers that are bitmap images, or any other Drawable type that can be defined.
For each layer in a layer-list, you need to define an  element. The item element
is used to declare optional meta-information such as an ID for the layer (which can be used
to retrieve the Drawable object for that layer in your Java code). You can also declare
location offsets or padding for the layer in the item element. While you can reference a layer
as an external Drawable resource, you are also able to inline the Drawable object inside
the  element, allowing you to compose various different Drawable structures in a
single file.
Sizing your layers
Only the first  of a layer-list will be sized according to the widget
it's placed in. All other layers will be sized to their "natural" size. For a bitmap
image, this is the size it is rendered in. For a  element, the natural
size is 0x0. In order to specify a natural size for a , you'll need to
give the  a  child-element with an android:width and
android:height attribute.

If you wanted a two-layer image to act as a large green button, you would probably declare
a layer for a grey rounded rectangle as a background, and another layer for a green oval to
look something like a light, or ball on top of the grey background. Such a layer-list could
look something similar to the following code-snippet:

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In the preceding snippet, there are only shape layers, but you could easily add in a bitmap
layer by referencing the bitmap resource on the  element, as in the following
code snippet:


Stretching using nine-patch images
There are times when you want a border that is more than a simple line, for example, if
you want to add a shadow. On a web-page, you'll commonly find various HTML tricks used
to insert eight or nine images into a box so that the content can be scaled while the border
remains intact. In Android, this technique is called a "nine-patch" image because it consists
of nine different parts. A nine-patch image in Android is handled specially when it's rendered
at sizes larger than its original size. In order to identify these images as special, they have a
.9.png extension (and must be valid PNG files).
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Styling Android Applications

A nine-patch image combines a border and a background in a single image. The background
area will grow when the content becomes too large for the image, and the border areas of
the image will be scaled up so that no "holes" are left.
Conceptually, you can start off by thinking about a nine-patch image as shown in the
following diagram:

The arrows in the diagram indicate the conceptual "border" areas that will grow in size
according to the size of the center "content" area. The corners of a nine-patch image will be
entirely unaffected by any scaling that takes place.

Creating nine-patch images
In order to create a nine-patch image, you'll need a decent image editing application. I
personally make use of the Gimp application (available for free at http://www.gimp.
org), although you may prefer to use another application. Whatever application you use, it
must be able to write out Portable Network Graphics (PNG) files, and should also be able
to zoom to fairly extreme levels. The entire data in a nine-patch image is actually encoded
into the image file, meaning there is no need for an XML file to tell Android what parts of the
image are border areas, and what parts must not be affected by scaling.
Unlike CSS boxes that appear on web pages, the size manipulation done on a nine-patch
image in Android is nearest-neighbor scaling. Nearest-neighbor scaling doesn't attempt to
improve the quality of the scaled image in any way, the pixels simply become larger solid
blocks of color. While this works excellently for gradient content backgrounds (provided they
aren't forced to grow too large), it may cause your image to have some strange artifacts to
it. Since currently there is no color interpolation performed during the scaling, some effects
may look rather strange when they are scaled. Scaling also takes longer than simple image
copying, so bear this in mind when sizing the image, it may need to get a lot larger than you
think. However, this also means that nine-patch images are far more flexible than those you
might know from the Web.
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The following two images are scaled-up versions of the same 32x32 pixel nine-patch image:

The image on the left-hand side is the raw PNG file that can be used as a nine-patch
image. The image on the right-hand side is the same image with a part of it highlighted to
show which areas will be scaled. The top, bottom-left, and right areas will be scaled only
horizontally or vertically, while the center area will be stretched to fit the size of the content.
The following image is the same image being used as the background of a TextView object:

So, the black lines on the left-hand side and top of the image tell Android what parts of
the image to scale, but what do the lines on the right and bottom signify? These two lines
determine where to place the content of the widget, much like the  element in a
 resource.
To get to grips with how your nine-patch image will be rendered and the possible ways it
can be scaled, Android provides you with a utility in the tools directory of an Android SDK
installation. The draw9patch utility renders your nine-patch scaled to various shapes and
sizes, and allows you to effectively debug the image before using it in your application.

Using bitmap images in Android
Images are a major part of styling your application. They are used for icons, borders,
backgrounds, logos, and many other purposes. Android does its best to make sure the
images you use as resources render as well as possible across the different types of screens
used on Android devices.
Android's automatic handling of images if far from perfect. However, there are times when
you will need several different variations of the same image for your application to look right
on all of the different devices.

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Styling Android Applications

Handling different screen sizes
When working with any bitmap image in Android, it's very important to consider that your
application will be run on a variety of different screens, both different sizes and densities.
When working on very large screens (such as those found on a laptop or tablet), you will want
to use larger images than you use on an extremely small screen. While nine-patch images go a
long way to keep things simple, they are still scaled with a nearest-neighbor algorithm, and this
may start to show on a large screen with a larger font-size than you anticipated.
You can provide images of different sizes in your resources directory. For each screen
size, you can provide a different drawable directory. The resource loading tools will
automatically pick files from the directory that most closely matches the current device
configuration. You don't need a copy of every resource in each of these directories, but only
the ones you want to provide a more suitable alternative for. The resource loader will fall
back on looser matching directories when it attempts to find a resource file to load.
Android recognizes five important parameters with regards to the size of a screen. While you
can specify parameters that relate to the exact number of pixels on the screen, this is not a
good idea as you won't easily be able to cater to all of the different screen sizes. Instead, it's
best to stick to the five parameters that Android provides:
‹‹

small

‹‹

medium

‹‹

large

‹‹

long

‹‹

notlong

The first three parameters are directly related to the size of the screen, while the last two are
related to whether the screen is "traditional" (such as VGA) or has a "wide" (such as WVGA)
format. These parameters can be mixed together in various combinations such as:
‹‹

/res/drawable-small/

‹‹

/res/drawable-medium-long/

‹‹

/res/drawable-large-notlong/

The preceding examples are all valid resource directories that can be used to override file in the
normal drawable directory. You can't combine parameters that contradict each other, such as:
‹‹

/res/drawable-small-large/

‹‹

/res/drawable-long-notlong/

In the preceding cases, you will receive an error from the resource packaging tool. Whenever
you work with bitmap images, it's important to consider these size parameters, since some
devices have screens very different from the one that the emulator shows by default.
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Handling different screen densities
Screen density generally refers to the number of pixels packed into a given physical space
(that is, dots-per-inch or DPI). It also has a relationship to the size of the pixels on the screen.
While most Android devices have medium or high-density screens, a large number of
cheaper devices make use of a relatively low-density screen.
Why does this affect nine-patch and bitmap images? The same reason it affects font
rendering—the lower the density, the worse anti-aliasing and shadows look. The best way to
explain this is with images. In the following images, the one on the left is a simple roundedrectangle as it would appear on a high-density screen. The image on the right is similar to
how the same image would render on a low-density screen:

Although both are the same source image rendered at the same physical size, a reduction in
the number of pixels available makes the image look blocky on a low-density screen.
The following two images are taken from the bottom-right corner, and enlarged to illustrate
in better detail what happens:

Again, these images are configured to take up the same amount of physical space. If an image's
size is specified in screen-pixels, it will take up much more physical space on a low-density
screen. This is one of the reasons it's recommended that you size images in Android using the
"density-independent-pixels" (dp or dip) unit instead of the normal pixels (px) unit.
As with screen sizes, Android provides a series of configuration parameters that can be used
to provide different resources for different screen densities. The parameters available for
selecting a screen-density can be mixed with those selecting based on the screen-size. The
following is a list of parameters Android makes available for resources to be provided based
on the screen-density of the current device:
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Styling Android Applications
‹‹

ldpi: Low-density screens (~120dpi)

‹‹

mdpi: Medium-density screens (~160dpi)

‹‹

hdpi: High-density screens (~260dpi)

‹‹

nodpi: Special case

The final "special case" can be used when you have a nine-patch image, or a bitmap image,
that you don't want scaled according to device density. Android, by default, will re-scale
an image in an attempt to keep the image's physical size as close to the intended size as
possible. An image in a nodpi directory will not be scaled automatically by Android, and will
be rendered on a pixel-for-pixel basis.
Different density icons
There are times when a large high-resolution icon does not scale down very well.
In these cases, it's often a good idea to design entirely different icons for lowdensity screens.

Handling configuration changes
When you provide Android with different resource directories relating to various possible
hardware configurations, the resource loader will attempt to match the best resource files
for the device that your application is running on. However, not all of the configuration
parameters relate directly to the hardware, but instead describe the device state or some
software configuration parameter. Examples of these types of parameters are the device
language, network IDs, and device orientation. These parameters may change while your
application is running. The most common example being the device orientation. Android has
a built-in mechanism to handle such changes for you, and for the most part you won't need
any special Java code to handle these changes. However, it is strongly desirable to at least
provide resource files for some of these parameters.
When a configuration parameter changes, Android will store any of your Activity state in
a Bundle object, and then shut down the Activity. It will then start up a new instance of
the Activity object with the new configuration parameters, and restore the state from the
Bundle object. All of the default Android widgets will store their current state before your
Activity is shut down by the system. This means you don't generally need to perform any
special handling for the configuration changes.

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Providing landscape layouts
So far through the book, we've only built portrait layouts. Unlike a desktop or web system,
a mobile application's orientation by default is portrait (hence the configuration parameters
long and notlong as opposed to wide and narrow). One of the great things about having
the Android platform is that an accelerometer is a required piece of hardware, which means
that your application can respond to the orientation of the device. Thanks to Android's
configuration handling (as mentioned previously), you, as a developer, don't need to do
anything except provide alternative landscape layout resources, assuming you don't build
major parts of your user interface in Java. In order to provide layouts which are specific to
either a portrait or a landscape orientation, you place the specific versions of your layout's XML
resources in directories configured with the following resource configuration parameters:
‹‹

port: Portrait-specific layouts

‹‹

land: Landscape-specific layouts

When the screen is longer vertically than horizontally (that is, portrait orientation), using a
simple vertically-oriented LinearLayout to layout an input form makes quite a lot of sense.
Any input widgets you make use of will be positioned below their labels and so have more
horizontal space to display their data. The additional horizontal space allows for labels to
include more information as well.
The following diagram illustrates the difference between these two layout concepts:

The layout method used on the right is very common in a web or desktop system, and will
work well on a mobile device if the size of the labels and input widgets are small enough.
When switching to a landscape orientation, the dramatic increase in horizontal space
coupled with the massive loss in vertical space makes the vertical LinearLayout a terrible
choice. If you are working with a simple input form, then a landscape layout should use
TableLayout or RelativeLayout to position the labels on the same lines as the input
widgets they relate to.

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Providing text input on a landscape layout
When building your landscape layouts, you need to carefully consider what parts of the
user interface are most important. If your screen is being used to compose an e-mail or a
document, your landscape layout could be almost identical to the portrait layout. However,
such a layout has a mostly hidden enemy: the software keyboard. On a portrait layout, the
software keyboard will confine itself to the bottom of the screen and consume a relatively
small amount of space (about a quarter to one-third of the available screen space). On a
landscape layout however, the software keyboard can consume as much as half of your
vertical screen space, making it very hard to build content-centric landscape layouts. If
your layout is strongly input-driven, it may make sense to either remove parts of your
user-interface when the orientation is landscape, re-working your user interface so that
the software keyboard won't get in the way.
Android does provide a series of configuration parameters which will tell you about the
keyboard on the device on which your application is running. It's a good idea to take all of
the possibilities into account when building your application. The following is a short list of
the possible keyboard situations that your application may be faced with:
‹‹
‹‹
‹‹

Software Keyboard only
Hardware Keyboard
Hardware Keyboard available; Software Keyboard in use

On top of these possibilities, devices with smaller screens will often make use of a 12-key
keyboard instead of a full QWERTY keyboard. If this is a software keyboard (which it often is),
the keyboard may take as much as 80 percent of your available screen space. This problem
if often handled by Android opening a "text input" screen when a text-input box is activated
by the user. You can determine the different states of keyboard availability, and the type of
keyboard used, with the following configuration parameters:
‹‹

nokeys: Software keyboard only

‹‹

qwerty: A full hardware keyboard is available

‹‹

12key: A 12-key hardware phone-keyboard is available

‹‹

keysexposed: The user has a keyboard visible, whether it's hardware or software

‹‹

keyshidden: There isn't any keyboard currently visible

‹‹

keyssoft: The user will use a software keyboard (though it may not be visible)

When designing your screens, consider that the software keyboard may take up to half
of your vertical space. Ensure that content areas will scroll, while vital widgets will always
remain visible on the screen. If a chat application is simply wrapped in ScrollView, the
input EditView object may become invisible when the software keyboard is visible. It's
important not just to consider how the screen will look, but how it will react to the changes
that your users will throw at it. Finally, it's vital to test how your screen will look and behave
with and without a software keyboard.
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Chapter 9

Altering screen content
One of the great advantages of the Android XML layout format is the decoupling it provides.
Portrait and landscape layouts are often quite different from each other, and users may
individually find a preferred orientation from which to use your application. A not-verycommon, but useful trick when designing the new layouts, is the ability to add or remove
"non-functional" elements from the two different layouts.
In a simple example, you may want to abbreviate the text in labels for the portrait layout and
include some icons as graphical hints, while for the landscape layout, you may want icons
double the size and two-line labels, all on the same line as your input field.
The following diagram illustrates this concept:

On the landscape layout in the preceding diagram, you could make use of an additional
TextView element for the sub-text on the label. Assuming your Java code doesn't look
for the additional TextView object, your application will run perfectly. The ability to
alter the actual structure of the user interface, and not just its layout, is a very important
consideration when designing alternative layouts for an Activity.

Summary
The look and feel of an application is vital. A single change to color or font can make or break
a screen's usability. At the same time, over-styling an application can make it feel out-ofplace on the user's device. An alien look and feel will drive users away from the application
towards those that look and feel more familiar and comfortable to them.
Android provides an extremely powerful set of capabilities with the style resource structure.
When combined with the ability to place your graphics in resource files and override
the defaults, you can effectively re-style any widget. Using styles also helps with the
maintenance of your application as you will only need to change styling in the style resources
and not on each widget declaration of a particular style.
Keeping most of your widget graphics as  resources will ensure the most consistent
look and feel possible for your application. However, this is not always practical. When you
need to provide bitmap resources, it's vital to provide different images for the various screen
sizes and densities the user may be working with.

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Styling Android Applications

Styling an application also includes the layout and the ability for the application to adapt
to the device it's running on. Having a great idea is only half of an application's appeal, its
styling and execution are critical to its survival in the "wild". Attention to detail is a powerful
tool that will draw users to your application. Applications that "just work" are always favored
over those that require time and effort to work with.
Make use of the various screen-sizes and densities provided to you by the Android emulator
to ensure that your application will look good on as many devices as possible. Don't forget
that many devices don't have hardware keyboards and that the software keyboard can take
as much as half of your screen space.
In the next chapter, we'll be extending this styling knowledge into the overall design and
theming of an application. We'll be building a styled application with many of the provided
layouts and will be performing fairly extensive styling.

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10

Building an Application Theme
Whether graphical styling or not, every application has a theme. The theme of
an application is what gives it a distinct appearance and logic.

When a person uses a mobile application (which accounts for most Android devices), there
are some fundamental differences in their behavior when compared to a desktop or laptop:
‹‹

They often have less time for the application, and therefore less patience

‹‹

They are often focused entirely on a single application at a time

‹‹

Touchscreen devices encourage an almost tactile response

Android devices are diverse and run on almost everything including common mobile phones,
tablets, laptops, and a few desktop machines. An Android application is expected to function
well in all of these environments, and the theme of the application should be carefully
constructed to allow the user the best possible access to each of these devices.
The device interface forms a part of your application theme. When using a mouse on a
desktop or laptop device, a user interface designed with only touchscreen in mind may
feel over-sized to a user (since all widgets need to be finger-sized). Contrary to this, an
application designed for a mouse-driven system will normally include rollover effects, which
won't work properly on a touchscreen device. The only way to make sure your application
works on all these different devices is to consider all of these environments when building
the screens of your application.
Android itself defines a theme of sorts and as far as possible, applications built for the
Android platform should attempt to conform or extend this theme, rather than redefine it.
This doesn't mean your application must look and behave exactly the same way as all other
Android applications, but rather that your application should be based on the underlying
principles that Android lays down.

Building an Application Theme

Keep in mind that many of the device manufacturers define additional parts to
the basic Android theme, and your application should do the same.

In this chapter, we will examine the building of an application, including the design of
the screens, their construction, and their styling. We'll also examine how this application
will interact with various different devices, making sure it looks right and functions as the
user would expect it to. The application we're going to build is a calculator, having both a
standard and a scientific calculator. The calculator will be styled to look more like a physical
calculator than a generic Android application, and will change its functionality according to
the capabilities of the device it's running on. Overall, we'll be defining an application with its
own, consistent theme.

Creating a basic calculator layout
The first thing we need in order to build this project is a basic portrait layout for a standard
calculator. This basic layout will serve as the screen that the user will look at when they
first start the application. Given the nature of a calculator application and how the user
perceives it, it's very important that the screen be simple and that the application starts as
quickly as possible.
It's important that the calculator screen takes up all available space with
functional components, in order to make itself as quick to use as possible (bigger
buttons equals easier usage).

Pop quiz
1. When do layout resources become Java classes?
a. When the resource-processor is run
b. When the application package is built
c.

When the layout resource is loaded

d. Never
2. How do you reference widgets that are not defined by default in Android?
a. By using the full class name as an element name
b. By defining an XML namespace for the Java package
c.

It's currently impossible

d. By specifying the Java package name in the the android:package attribute
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Chapter 10

3. What is the default width and height of a View object?
a. The size of it's content
b. Zero-by-zero pixels
c.

It depends on the ViewGroup it's placed in

d. The width of its parent and the height of its content
4. You write a layout resource as XML, what format is it stored in?
a. As raw XML text
b. Android binary XML
c.

Layout specific binary format

d. Java classes

Designing a standard calculator
Before starting to build the calculator application, it's a good idea to sketch out what it's
going to look like. This will also help you to decide how exactly to construct the screens.
Since a calculator is something that is both, a rather old invention as well as something
people are very familiar with, it's important to stick to the most common design. If you
introduce a calculator that is too foreign to people, they may well not have the patience to
"get to know" your application. New ideas are good (that is, slide keyboards), but the most
successful are those that are extensions of existing ideas. Also, make it obvious to the user
how they work. The following is a block diagram of the standard calculator screen that we
will start building:

It's important that we maximize the use of the screen space, so we'll do our best to make
the buttons as large as possible. Also, we want to space the buttons slightly apart in order
to avoid the undesired button from being pressed by the user. Since we only have a single
output area, we'll make sure that the display area is also sufficiently large.

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Building an Application Theme

The arrow in the display area will be an icon which will act as a Backspace button, allowing the
user to delete unwanted content. It's always important to give the user a way to undo what
they have done. We'll use an icon similar to the one used in the dialer application, which will
keep a feeling of overall consistency with the rest of the system. This also effectively gives
us space for an additional button. This user interface doesn't include the normal "memory"
functions associated with many calculators. The basic screen is designed to be as simple as
possible, and we'll introduce more functionality as we develop the application.

Time for action – building the standard calculator
The first layout for the calculator will consist of a normal series of 0 to 9 buttons with a
button for the various basic arithmetic operations—add, subtract, multiply, and divide. It will
also have buttons for equals and a button for the decimal point. While this would be a very
easy screen to build in Java code, we'll build this example entirely as an XML resource. Since
this application will have several different permutations of the same screen, using layout
resource files with no Java code will make your life much easier.

1.

Start by creating a new project for the calculator:
android create project -n Calculator -p Calculator -k com.
packtpub.calculator -a CalculatorActivity -t 3

2.

Open the standard main layout file /res/layout/main.xml.

3.

Remove the generated layout structure from the file.

4.

Start by declaring a vertical LinearLayout as a root element to consume all the
available space on the screen:


5.

Declare a RelativeLayout that will compose the display with the Delete or Cancel
button that the user can use to remove unwanted input:


6.

Use the standard Android input delete icon in an ImageView on the right side of the
RelativeLayout:


7.

On the left side of the RelativeLayout, create a TextView that will actually
display the numeric status of the calculator:


8.

Inside the LinearLayout, declare a TableLayout that will be used to contain the
button inputs for the simple calculator:


9.

The TableLayout will be made up of four TableRow objects. Declare the first of
these with no margin and a layout_weight of 1:


10.

The top-right Button object needs to be the plus sign, which we also use as the
name for the Button ID:


11.

The next three Button objects on the first row will be the numbers 1, 2, and 3.
These all need IDs as well:




12.

Continue to declare TableRow objects with buttons in the order defined in the
block-diagram.

13.

Open the CalculatorActivity.java source file in an editor or IDE.

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14.

In the onCreate method, ensure that the content view of the Activity is set to
the main layout you've just defined:
setContentView(R.layout.main);

What just happened?
You should now have a basic user interface for a calculator; although it still looks like a very
generic Android application, but it's a start at the basic level. The user interface will need
styling work, including colorization and some font changes, but the basic structure is now
complete. The use of the RelativeLayout is to ensure that we can correctly position the
delete icon to the right of the TextView, no matter what the size of the screen is.
In order for the buttons to consume as much of the available space as possible, we tell the
TableLayout to stretch all of its columns. If the TableLayout doesn't stretch its columns,

then it will only consume as much horizontal space as its children require (effectively the
same as wrap_content width). Although the TableLayout is told to consume all of the
vertical space as well, its children will be sized according to the amount of space they need,
which is why the buttons don't take up all of the available screen space. The following image
is a screenshot of the basic calculator running in the emulator:

Building the calculator styling
We really want this calculator to look more like a real calculator, and for that we need to
apply some styling. The current theme of the calculator is entirely the standard Android
theme, and while it looks exactly like the rest of the Android system, it doesn't really suit this
application. We want to style both the buttons and the display area of the application. We'll
define style values in a resource file and relate to these in the layout XML file.
To start with, we'll define a series of nine-patch images to create our own button designs.
We need three different images for this purpose. The first image is the "normal" state of the
button, the second will be the "pressed" state of the button, and finally, a "focused" state of
the button.
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Pop Quiz
1. What are the black lines around the border of a nine-patch image for?
a. Hints to the system as to what parts of the image to copy
b. To indicate what parts of the image to scale and where to put the widget content
c.

Defines what parts of the image contain meta-information

2. What formats may a nine-patch image be stored as?
a. JPEG, GIF, or PNG image file
b. An XML file with an embedded TIFF
c.

A portable-network-graphic image

3. What does the draw9patch application do?
a. Renders a nine-patch image in various shapes and sizes
b. It's an application for drawing nine-patch images
c.

Generates the meta-data for a nine-patch image as an XML file

Time for action – creating the button images
In order to build the button images in this section you will need to download "The GIMP"
(available at http://www.gimp.org). It's perfect for this sort of image creation or
manipulation and has the added advantage that it's open source.

1.

Open "The Gimp", and select File | New to create a new image.

2.

Change the width and height to 38x38 pixels.

3.

Open the Advanced Options and change the Fill With option to Transparency so
that there is no background color.

4.

To help with sizing, zoom in to about 800%.

5.

Select the Rectangle tool in the top-left of the toolbox (the default keyboard
shortcut key is R).

6.

Enable the Rounded Corners option and set it to 5.

7.

Enable the Fixed option and select Size in the drop-down list.

8.

Enter 36x36 as the fixed size of the rectangle selection.

9.

Place the selection box at the center of the image canvas and there should be a
one-pixel border between the selection box and the edge of the image.
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Download from Wow! eBook 

Building an Application Theme

10.

Double-click on the "Foreground color" (black by default) in the toolbox.

11.

Enter 444444 in the Hex Notation box of the color selector.

12.

Close the color selector dialog box.

13.

Select the Bucket Fill tool in the toolbox (the default keyboard shortcut is Shift-B).

14.

Click inside the selection box to fill it with the selected color.

15.

Use the Select menu and click the None option to remove the selection box.

16.

Select Filter | Decor | Add Bevel.

17.

Change the Thickness option to 3.

18.

Uncheck the Work on Copy option and select the Ok button.

19.

Select the Rectangle tool from the toolbox again.

20.

Uncheck the Rounded Corners and Fixed options.

21.

Use the selection tool to select a single pixel wide vertical box on the inside of the
"button" shape, being careful to only select part of the content area of the button,
avoiding the beveled border space:

22.

By placing the cursor in the middle of the selection box, drag the selection horizontally
to the very edge of the of the image canvas (inside the one-pixel border).

23.

Double-click on the "Foreground" rectangle again.

24.

Reset the color to pure black.
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25.

Select the Bucket Fill option.

26.

Click inside the selection box to create a single pixel wide, black vertical line down
the left-side of the image.

27.

Create a similar vertical line on the right side of the image.

28.

Create a horizontal single-pixel high black line at the top and bottom of the image.

29.

Save the image in your res/drawable directory as button.9.png, leaving the
PNG Options as their defaults.

30.

Repeat this exact process, changing the 444444 foreground color, as done in step
11, to c16400 and save the new image as button_focus.9.png.

By inverting the image with the Flip Tool (default keyboard shortcut Shift + F), you will create
the button_down.9.png image.

What just happened?
While there are many steps to building images, they are fundamentally very easy to create
with the right tool and a bit of experimentation. If all you need is a simple button or
something similar, then it's well worth finding a few tutorials on how to use "The GIMP" or a
similar tool. There are great tutorials online at the following links:
‹‹

http://www.gimp.org/tutorials/

‹‹

http://gimp-tutorials.net/

The images you have saved in the last section should look like the following images that I
have created for my calculator application:

Time for action – styling the calculator buttons
The next thing we need to do is use a selector-list and the nine-patch images you've just
created to style the calculator buttons. We'll also be defining the button styling in a resource
file so that we don't have to specify all of the styling for each of the buttons. In order to
replace the standard button with our image, we only need to replace its background with the
one we create for the purpose.

1.

In the res/drawable directory, create a new XML file named button.xml and
open it in an editor.
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Building an Application Theme

2.

Define the root element of the file as a fixed-size selector:


3.

Create the pressed button state, as the first child of the selector:


4.

The second child of the selector should be the focused state:


5.

The final selector child is generic, and is the normal state:


6.

Create a new file named styles.xml in the res/values directory and open it in
an editor.

7.

The root element of the styles.xml file should be a resources element with no
namespace declaration (it's not needed in this file):


8.

Define the first style in the file as CalculatorButton with a parent style of the
default Android Button widget style:




The first style resource in the preceding snippet is used for the display area of the calculator.
As with the standard calculator, we use the display_background shape written earlier in
this chapter. We also define a new style for the scientific buttons. The scientific buttons will
be exactly the same as the standard calculator buttons, except with a much smaller font.
Since there are many more scientific buttons than standard buttons, the smaller font allows
us to comfortably fit more of them on the screen.

Building the scientific layout
The scientific layout is comprised of the standard calculator buttons on the right side of the
screen, with twenty additional buttons on the left side of the screen. The additional buttons
represent mathematical functions and constants, most of which can be found in the java.
lang.Math and java.lang.StrictMath classes. The following figure illustrates how we
want to layout the scientific calculator:

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The effect of the new styles for the landscape layout on the calculator display will "remove"
the margin between the display and the buttons. Since the landscape layout has less vertical
space, such padding is nothing more than a waste of space that should be used for the
buttons in order to maintain a reasonable size.

Time for action – coding the scientific layout
The landscape layout is broken into various sub-layouts in order to maintain IDs for the two
individual functional areas: Scientific functions and standard functions. Maintaining these
with their own ID values allows much easier detection of the available functionality from
the Java code. Instead of the Java code deciding on the available functionality based on the
configuration, it can use findViewById and test for null to check if the scientific functionality
is available. This is not unlike "capability testing" in JavaScript (as apposed to inspection).

1.
2.
3.

Create a new resource directory named res/layout-land.
Create a new layout resource XML file in the layout-land directory named main.
xml and open this file in an editor or IDE.

Declare the root element of the new layout as a vertical LinearLayout consuming
all of the available screen space:


4.

The first element of the new layout is a RelativeLayout element to wrap the
TextView and ImageView that are used as the calculator display:


5.

Copy the TextView and ImageView elements from the standard calculator layout
(res/layout/main.xml) as the two child elements of the RelativeLayout
declared previously:



Chapter 10
android:layout_alignParentTop="true"
android:layout_toLeftOf="@id/delete"
android:layout_width="fill_parent"
android:layout_height="wrap_content"/>

6.

The second child element of the root LinearLayout is a horizontally-oriented
LinearLayout consuming the remainder of the screen space:


7.

Inside the new LinearLayout child, declare a new TableLayout to fill with the
scientific buttons:


8.

Create a TableRow element inside the scientific_functions TableLayout,
to contain the first row of scientific Button elements:


9.

Declare the first five scientific functions as Button elements inside the new
TableRow. The Button ID should be the same as the name of the resource string to
be used as the Button label:


10.

The first row of scientific Button widgets contains inverse, square, cube, pow,
and percent.

11.

Create a TableRow with the second row of scientific Button widgets containing
cos, sin, tan, log2, and log10.

12.

The third scientific Button widgets in the third TableRow should be acos, asin,
atan, log, and log1p.

13.

The fourth and final TableRow of Button widgets should be e, pi, random, sqrt,
and hyp.

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Building an Application Theme

14.

That is all of the scientific functions, now create another TableLayout in the
LinearLayout child element for the standard functions:


15.

Copy the contents of the standard_functions TableLayout in res/layout/
main.xml into the new TableLayout element.

What just happened?
In the preceding layout, we reuse much of the basics that we created in the standard
calculator layout, with the addition of a new TableLayout structure to contain the scientific
functions. The new TableLayout is sized to a width of wrap_content and will only
consume the amount of horizontal space needed to fit all of the Button widgets. The other
main difference between the two TableLayout elements is that the scientific table doesn't
stretch its columns, since this would effectively be the same as sizing it as fill_parent and
leaving no space for the standard functions.
You'll also notice that among the string resources used to create the scientific Button labels,
those that use HTML markup, do so without using XML escape entities (such as < and >).
This is the main indicator to the resource compiler that a string resource contains markup and
should be consumed differently. This usage requires that all HTML markup that is placed into a
string resource must both conform to the HTML 3.2 specification, and remain valid XML content.
In order to test the new landscape layout, you'll either need to define an emulator device
with a landscape screen size, or run the application on a physical device. Creating a virtual
device in the emulator can be done with the android application in the tools directory of
your Android SDK installation, the same tool used to create skeleton projects. The following
is a screenshot of the new layout running on a physical Android device:

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Have a go hero – using include in existing layouts
The preceding layout has several elements of the standard layout that it reuses. This is a
good time to extract these elements into their own layout files and then make use of the
include element to place them into the two specific layout resources. Information on
layout includes can be found in Chapter 5, Developing Non-Linear Layouts.
1. Create a display.xml layout resource to contain the RelativeLayout with the
calculator display, and include this at the appropriate point in the main.xml layout
resource files.
2. Create a standard_buttons.xml layout resource to contain the TableLayout
named standard_functions, and include this at the appropriate point in the
main.xml layout resource files.

Handling the Activity restart
When the device changes orientation, the CalculatorActivity object on the screen is
restarted with the new orientation. In this application, the restart leads to a serious problem:
the state of the calculator is lost. As discussed in Chapter 4, Leveraging Activities and Intents,
there are times when you need to take control of your application state in Android—saving it
before shutdown and restoring it when the Activity is started again.
You'll need to override the Activity.onSaveInstanceState method to store the
current state of your calculator in the provided Bundle. This Bundle object will be provided
to you in the onCreate method when being restarted due to a configuration change. In your
onCreate method, check to make sure that the provided Bundle object is non-null before
restoring the save parameters from it.

Have a go hero – implementing the scientific calculation logic
The calculator should currently be able to function from the standard calculation buttons.
However, the new scientific functions don't have any backing structures. Further, if you reorientate your device to change between scientific and standard layouts, any "in-progress"
calculation will be lost.
The steps that need to be completed for the scientific calculations to function as expected,
are as follows:
1. Implement the onSaveInstanceState to save the calculation state to the
provided Bundle object.
2. Implement the onCreate method to restore the saved state from its provided
Bundle object (assuming one is given).
3. Add the functionality required to make the scientific Button widgets function as
expected, to the Calculator class you wrote earlier.
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Supporting hardware keyboards
The calculator we developed here is now a great on-screen Android calculator application,
with both the simple and scientific functionality you'd expect. However, if a device has
a hardware keyboard, the user will probably expect to be able to use it, which currently
they can't. Further, if the device lacks a touchscreen, clicking on-screen buttons will quickly
become frustrating. We need to implement hardware keyboard support for the application.
Implementing the hardware keyboard handling code is only useful to you if you've done
the "Have a go Hero" sections and built a Calculator class to perform the required
functions. In order to handle hardware keyboard events, you'll use the methods declared
in the KeyEvent.Callback interface. The Activity class implements the KeyEvent.
Callback interface already, and provides default handling for all of the methods. For our
handling of these key-events, we only need to override the onKeyDown method.
For this onKeyDown implementation, it's a good idea to make sure that the key events are
coming from a hardware keyboard by checking the flags of the KeyEvent. It's also a good
idea to pass it to your parent class before processing it yourself. Finally, if you're working on
Android 2.0 (API-Level 5) or higher, you should check that the KeyEvent is not cancelled
before processing it (again this is one of the KeyEvent flags). The following is a code snippet
from my implementation of the onKeyDown method:
@Override
public boolean onKeyDown(
final int keyCode,
final KeyEvent event) {
super.onKeyDown(keyCode, event);
boolean handled = false;
if((event.getFlags() & KeyEvent.FLAG_SOFT_KEYBOARD) == 0) {
switch(keyCode) {
case KeyEvent.KEYCODE_0:
calculator.zero();
handled = true;
break;
case KeyEvent.KEYCODE_1:
calculator.one();
handled = true;
break;
// Cases for each of the handles keys
}
display.setText(calculator.getCurrentDisplay());
}
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return handled;
}

The preceding code snippet invokes a method for each one of the different keys that can be
pressed on a hardware keyboard.
If your Android device doesn't have a hardware keyboard, you can test this code
using the emulator—your PC's keyboard, and the on-screen keyboard to the right
of the emulator display, are both classified as hardware keyboards by the emulator.

Adding in display animations
Currently, the application has all the makings of a great calculator application. However, the
display is currently just a simple TextView object. In order to improve the user experience,
we should make use of a ViewSwitcher object to swap the TextView out when the
calculator operation is changed, or when the "equals" Button is pressed.

Time for action – animating the display
In order to build a nice slide-out-slide-in animation for the calculator display, we'll need to
define our own animations and bind them to a ViewSwitcher object. This will also require
us to make changes to the Java code in order to handle the new mechanism. Since we don't
want the view to animate each time a new digit is typed, we will make direct changes to the
TextView currently on the screen.

1.

Create a new XML resource file in the res/anim directory named slide_out_
top.xml, and open this in an editor or IDE.

2.

Declare a y-translate animation from 0% to 100% as the only element in the
animation resource:


3.

Create a new XML resource file in the res/anim directory named slide_in_
bottom.xml, and open this file in an editor or IDE.

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4.

Declare a y-translate animation from 100% to 0% as the only element in the
animation resource:


5.

Open either your display.xml file, or both of the main.xml files in your editor
of IDE, and which among them you should open will depend on whether you have
completed the "Have a go Hero – Layout Includes".

6.

In the RelativeLayout used for the display, replace the TextView named
display with a ViewSwitcher element using the two new animation resources:


7.

As child elements to the ViewSwitcher, declare two TextView elements with the
CalculatorTextDisplay style:


8.

Both of the TextView elements will be identical to each other.

What just happened?
The use of the ViewSwitcher for the display will cause any existing Java code to crash,
since the Java code will be expecting the object to be a TextView of some sort. What you
need to do instead is update the display using the ViewSwitcher.getCurrentView,
instead of the ViewSwitcher itself.
When an operation Button is used, for example, the multiply or equals Button, you'll
want to place the next display content on the ViewSwitcher.getNextView widget, and
then invoke the ViewSwitcher.showNext() method. The animation of the number
disappearing upwards with the new content appearing from the bottom of the display is a
simple, but explanatory animation. It's also quite commonly used in calculator applications,
meaning the user will usually be comfortable with it.
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Chapter 10

In this application's case, the animation is more eye-candy than useful. However, if you
implemented a history-stack in the calculator, the animation could be reversed when the
user presses the "back" Button. A history-stack in a calculator is a very useful structure
because it allows slight variations of the same calculations to be run over and over again.

Have a go hero – rounding off
This calculator application is quite complete at this point. It's been styled, and has some nice
eye-candy and functions as expected. It does have a few caveats, however—the scientific
calculation layout doesn't work very well on small-screen devices. The following screenshot
is the application running in scientific layout on a small-screen phone:

The preceding image also demonstrates how some devices theme applications. In order to
make sure the application works well on all devices:
1. Define a new values directory for small-screen devices.
2. Create a new styles.xml file in the directory with styles that have less margin and
padding than the defaults.
3. Reduce the size of the display font when on a small-screen device that has a
landscape orientation.
This sort of rounding-off process will follow most successful Android application projects.
It's a matter of trying the application out on various different emulator configurations and
devices, and then leveraging the resource-loaders to ensure the application works well on as
many devices as possible.

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Building an Application Theme

Summary
Creating an application theme is a key part of the success of a new application, whether
running on Android, the desktop, or on the Web. We've explored how to make use of the
various tools that Android provides in order to keep an application consistent in order to
keep it user-friendly.
An application's theme, and its look and feel go far beyond the simple styling. The more you
personally use you application, the more you will see places where a slightly different color,
or a transition animation will make a difference. Each of those small differences is what
makes an application truly user-friendly, because it makes the application feel polished.
While running on hundreds of wildly different devices, Android makes it easy for developers
to keep their applications running as though they were built specifically for that hardware.
The resource-loader system is one of the most key structures in Android, and not to leverage
it, can be suicidal to the application.
I strongly recommend familiarizing yourself with existing Android applications, as well as
applications on other mobile devices. Knowing how to drive a decent image-manipulation
application also goes a long way. Draw a diagram of each of your screens before your start
building them, and pencil and paper is often the best way to get an idea about the user
interface, before you start coding.
Think carefully about where you can use the existing Android icons and styles, and where
you will want to replace or extend them. You always want to keep your application
consistent, but adding some flashy eye-candy is often what makes an application stand out
from the crowd.
With the combination of XML resources and the Java language, Android is a highly
compelling platform to design and code for. It's widely deployed and has excellent developer
support. There are dozens of hardware manufacturers producing Android devices, in all
shapes and sizes, and thousands of developers making applications.
In this book, we've worked on leveraging the Android platform to build applications that are
user-focused, easy to use, and good looking. The Android platform and the Android Markets
allow for a captive audience and great exposure for new ideas. From here on, you should
be able to add your own unique ideas and work to the Android ecosystem. Anything that
has been done can always be done better, and anything that hasn't been done, has people
waiting for it. Whether you're part of a team, or hacking away on the-next-big-thing in your
attic at night, the key to a successful application is a great user-interface.

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Pop quiz answers
Chapter 1
Layouts as XML fles
Question number

Answers

1

b

2

d

3

c

Populating an activity
Question number

Answers

1

b

2

c

3

c

Download from Wow! eBook 

Pop quiz answers

Chapter 2
List views and adapters
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1

c

2

a

3

c

Chapter 3
Gallery objects and ImageViews
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1

c

2

b

3

a

Chapter 4
Intents & Activities
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1

c

2

b

3

a

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Appendix

Chapter 5.
Custom layouts
Question number

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1

d

2

b

3

c

Chapter 6
Text input
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1

c

2

c

3

a

Chapter 8
The WebView widget
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1

d

2

b

3

d

WebView versus native layouts
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1

a

2

c

3

c
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Pop quiz answers

Chapter 10
Layout resources
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1

d Hint:(they are loaded as objects, not compiled to classes)

2

d

3

c

Nine-Patch Images
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1

b

2

c

3

a

Android resources
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1

c

2

a

3

c

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Index
Symbols
-a option 13
-k option 13
-n option 13
 element 241
-p option 13
 animation 189
 element 189
 element 207
 element 120
 resource 41