Android User Interface Development Beginner S Guide
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- Cover
- Copyright
- Credits
- About the Author
- About the Reviewer
- www.PacktPub.com
- 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
- Time for action – running the example project
- The screen layout
- 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
- 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
- Time for action – creating a fast food menu
- Time for action – improving the restaurant list
- Time for action – creating a Burger item layout
- Time for action – presenting Burger objects
- Time for action – implementing TheBurgerPlaceActivity
- Using the ExpandableListView class
- Using the GridView class
- Time for action – creating the fruit icon
- 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
- Implementing the ReviewActivity
- Time for action – writing the ReviewActivity class
- Time for action – creating the Review layout
- Time for action – turning on the TextSwitcher
- Creating a simple photo gallery
- Time for action – building the Photos tab
- 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
- Time for action – listening to the SeekBar
- Time for action – selecting date and time
- Creating complex layouts with Include, Merge, and ViewStubs
- Summary
- Chapter 4: Leveraging Activities and Intents
- Chapter 5:
Developing Non-linear Layouts
- Time for action – creating a layouts example project
- FrameLayout
- Time for action – developing a FrameLayout example
- Table Layout
- Time for action – developing a simple memory game
- AbsoluteLayout/Custom Layouts
- Time for action – creating a custom layout
- Time for action – finishing the CircleLayout example
- RelativeLayout
- Time for action – creating a contact editor
- Time for action – integration with the layout example
- SlidingDrawer
- Time for action – creating a SlidingDrawer
- Time for action – sliding drawer integration
- Summary
- Chapter 6:
Validating and Handling Input Data
- Dealing with undesirable input
- Avoiding invalid input entirely
- 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
- Summary
- Chapter 7: Animating Widgets and Layouts
- Chapter 8:
Designing Content-centric Activities
- Considering design options when displaying content on an Android device
- Displaying content with the WebView class
- Time for action – creating a recipe viewer application
- Creating relative layouts for content display
- Time for action – developing specialized content views
- Developing an online 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
- Time for action – drawing a broken line
- Time for action – creating a rounded border
- Time for action – applying a gradient to an oval shape
- Time for action – rendering a spinner ring
- Stretching using nine-patch images
- Using bitmap images in Android
- Handling configuration changes
- Summary
- Chapter 10:
Building an Application Theme
- Creating a basic calculator layout
- 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
- Time for action – coding the scientific layout
- Supporting hardware keyboards
- Adding in display animations
- Time for action – animating the display
- Summary
- Appendix: Pop quiz answers
- Index
Android User Interface Development
Beginner's Guide
Copyright © 2011 Packt Publishing
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First published: February 2011
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Published by Packt Publishing Ltd.
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ISBN 978-1-849514-48-4
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Credits
Author
Jason Morris
Reviewers
David J. Groom
Marn Skans
Acquision Editor
Chaitanya Apte
Development Editor
Reshma Sundaresan
Technical Editor
Harshit Shah
Copy Editor
Neha Shey
Indexer
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Editorial Team Leader
Akshara Aware
Project Team Leader
Priya Mukherji
Project Coordinator
Shubhanjan Chaerjee
Proofreader
Joel T. Johnson
Graphics
Nilesh R. Mohite
Producon Coordinators
Kruthika Bangera
Aparna Bhagat
Cover Work
Kruthika Bangera
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About the Author
Jason Morris has worked on soware as diverse as fruit tracking systems, insurance
systems, and travel search and booking engines. He has been wring soware for as long
as he can remember. He is currently working as a Soware Architect for Travelstart in South
Africa. He works on mulple 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 wring
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 Marn Skans for his invaluable input.
www.allitebooks.com
About the Reviewer
Marn Skans graduated from Lund University in Sweden, with a Master's degree in
Computer Science. Aer a couple of years in the online markeng 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 plaorm which has been recently launched
for the mobile market.
www.allitebooks.com

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Table of Contents
Preface 1
Chapter 1: Developing a Simple Acvity 11
Developing our rst example 11
Creang the project structure 12
Time for acon – seng up the Android SDK 12
Time for acon – starng a new project 13
Examining the Android project layout 14
Time for acon – running the example project 14
The screen layout 15
The layout XML le 16
Resource selecon qualiers 16
Time for acon – seng up the queson acvity 18
Populang a View and a ViewGroup 19
Time for acon – asking a queson 19
Time for acon – adding a space for answers 21
Time for acon – adding more buons 23
Dening common dimensions 25
Limitaons of the layout XML format 27
Populang the QuesonAcvity 29
Time for acon – wring more Java code 30
Dynamically creang widgets 32
Time for acon – pung the quesons on the screen 32
Handling events in Android 34
Summary 36
Chapter 2: Presenng Data for Views 37
Lisng and selecng data 38
ListView choice modes 38
No selecon mode – CHOICE_MODE_NONE 38
Single selecon mode – CHOICE_MODE_SINGLE 39
Mulple selecon mode – CHOICE_MODE_MULTIPLE 40
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Table of Contents
[ ii ]
Adding header and footer widgets 40
Creang a simple ListView 41
Time for acon – creang a fast food menu 41
Styling the standard ListAdapters 43
Dening standard dimensions 43
Time for acon – improving the restaurant list 44
Creang custom adapters 47
Creang a menu for The Burger Place 47
Time for acon – creang a Burger item layout 48
Time for acon – presenng Burger objects 50
Creang TheBurgerPlaceAcvity class 52
Time for acon – implemenng TheBurgerPlaceAcvity 53
Registering and starng TheBurgerPlaceAcvity 54
Using the ExpandableListView class 56
Creang ExpandableListAdapter implementaons 57
Using the GridView class 58
Time for acon – creang the fruit icon 59
Displaying icons in a GridView 60
Time for acon – building the fruit menu 61
Time for acon – creang the FourBucketsAcvity 62
Summary 64
Chapter 3: Developing with Specialized Android Widgets 67
Creang a restaurant review applicaon 68
Time for acon – creang the roboc review project structure 68
Building a TabAcvity 70
Creang tab icons 70
Android tabs and icons 71
Implemenng the ReviewAcvity 72
Time for acon – wring the ReviewAcvity class 72
Time for acon – creang the Review layout 74
Working with switcher classes 75
Time for acon – turning on the TextSwitcher 76
Creang a simple photo gallery 78
Time for acon – building the Photos tab 79
Creang a thumbnail widget 80
Implemenng a GalleryAdapter 80
Time for acon – the GalleryAdapter 81
Time for acon – making the gallery work 83
Building the reservaon tab 86
Time for acon – implemenng the reservaon layout 86
Time for acon – inializing the reservaon tab 89
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Table of Contents
[ iii ]
Time for acon – listening to the SeekBar 92
Time for acon – selecng date and me 93
Creang complex layouts with Include, Merge, and ViewStubs 96
Using Include tags 97
Merging layouts 97
Using the ViewStub class 99
Summary 100
Chapter 4: Leveraging Acvies and Intents 103
Exploring the Acvity class 104
Using Bundle objects 105
Time for acon – building an example game: "guess my number" 106
Creang and consuming intents 110
Dening Intent acons 111
Passing data in an Intent 112
Adding extra data to an Intent 112
Using advanced Intent features 113
Geng data back from an Intent 113
Time for acon – viewing phone book contacts 114
Summary 118
Chapter 5: Developing Non-linear Layouts 119
Time for acon – creang a layouts example project 120
FrameLayout 121
Common uses 121
Time for acon – developing a FrameLayout example 122
Table Layout 126
Common uses 127
Using TableLayout for a memory game 127
Time for acon – developing a simple memory game 128
AbsoluteLayout/Custom Layouts 133
Developing your own Layouts 134
Time for acon – creang a custom layout 134
Using the CircleLayout 137
Time for acon – nishing the CircleLayout example 137
RelaveLayout 140
Common uses 140
Integrang the RelaveLayout 141
Time for acon – creang a contact editor 141
Time for acon – integraon with the layout example 144
SlidingDrawer 146
Common uses 146
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[ iv ]
Creang a SlidingDrawer example 147
Time for acon – creang a SlidingDrawer 147
Time for acon – sliding drawer integraon 148
Summary 150
Chapter 6: Validang and Handling Input Data 153
Dealing with undesirable input 153
Correctly labeling input 154
Signaling undesirable input 154
Recovering from undesirable input 155
Giving users direct feedback 155
Avoiding invalid input enrely 156
Capturing date and me 156
Using spinners and ListView for selecon 159
Changing the data set 159
Disabling selecons 159
Capturing text input 160
Autocompleng text input 160
Building acvies for results 162
Generic ltering search Acvity 162
Time for acon – creang the ListItemSeleconAcvity 163
Time for acon – creang an ArrayAdapter 164
Time for acon – creang the CursorAdapter 165
Time for acon – seng up the ListView 169
Time for acon – ltering the list 170
Time for acon – returning the selecon 171
Using the ListItemSeleconAcvity 172
Summary 174
Chapter 7: Animang Widgets and Layouts 175
Using standard Android animaons 176
Time for acon – animang a news feed 176
Using ipper and switcher widgets 181
Using the ImageSwitcher and TextSwitcher implementaons 182
Animang layout widgets 182
Time for acon – animang a GridView 183
Creang Custom Animaons 187
Time for acon – wring a custom animaon 188
Time for acon – making a Buon vanish 189
Summary 192
Chapter 8: Designing Content-centric Acvies 193
Considering design opons when displaying content on an Android device 194

Table of Contents
[ v ]
Considering user behavior 195
Drawing user aenon 196
Displaying content with the WebView class 197
Using a WebView object 198
Time for acon – creang a recipe viewer applicaon 198
Taking WebView further 203
Creang relave layouts for content display 204
Taking full advantage of RelaveLayout 205
Considering Android layout constraints 206
Styling TextView objects 207
Time for acon – developing specialized content views 210
Developing an online music store 213
Designing the music store 213
Developing the music store 215
Time for acon – building a track item 218
Time for acon – developing the main user interface layout 219
Time for acon – developing the main user interface Java code 222
Summary 225
Chapter 9: Styling Android Applicaons 227
Working with style resources 228
Using shape resources 230
How shapes behave 231
Rendering lines 231
Time for acon – drawing a broken line 231
Rendering rectangles 232
Time for acon – creang a rounded border 232
Rendering ovals 234
Time for acon – applying a gradient to an oval shape 235
Rendering rings 236
Time for acon – rendering a spinner ring 237
Dening layers 238
Stretching using nine-patch images 239
Creang nine-patch images 240
Using bitmap images in Android 241
Handling dierent screen sizes 242
Handling dierent screen densies 243
Handling conguraon changes 244
Providing landscape layouts 245
Providing text input on a landscape layout 246
Altering screen content 247
Summary 247
Downloa d f r o m W o w ! e B o o k < w w w.woweb o o k . c o m >

Table of Contents
[ vi ]
Chapter 10: Building an Applicaon Theme 249
Creang a basic calculator layout 250
Designing a standard calculator 251
Time for acon – building the standard calculator 252
Building the calculator styling 254
Time for acon – creang the buon images 255
Time for acon – styling the calculator buons 257
Time for acon – styling the display 260
Scienc landscape layout 263
Dening string resources for the scienc layout 263
Styling the scienc layout 265
Building the scienc layout 265
Time for acon – coding the scienc layout 266
Handling the Acvity restart 269
Supporng hardware keyboards 270
Adding in display animaons 271
Time for acon – animang the display 271
Summary 274
Appendix: Pop quiz answers 275
Chapter 1 275
Layouts as XML es 275
Populang an acvity 275
Chapter 2 276
List views and adapters 276
Chapter 3 276
Gallery objects and ImageViews 276
Chapter 4 276
Intents & Acvies 276
Chapter 5. 277
Custom layouts 277
Chapter 6 277
Text input 277
Chapter 8 277
The WebView widget 277
WebView versus nave layouts 277
Chapter 10 278
Layout resources 278
Nine-Patch Images 278
Android resources 278
Index 279
Preface
On 9th January, 2007, Apple ocially launched the iPhone, and the world of user interface
design shied. While tablet PCs had been around for a while, the iPhone was the rst 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
competor to iPhone.
What is it about touchscreen phones that we love? The answer is simple—feedback.
Touchscreens oer a way to directly manipulate on-screen objects, which in the past had to
be driven through a keyboard, mouse, joysck, or other input device. The touchscreen model
of direct manipulaon has a large impact on the way we think about our user interfaces as
developers, and changes the expectaons a user has for the applicaon. 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 operang system for a rapidly expanding range of consumer
electronics, including:
Smartphones
Netbooks
Tablets
Some desktop systems
While all of these devices have dierent purposes and specicaons, all of them run
Android. This is unlike many other operang environments which are almost always have a
special purpose. The services and the APIs they provide to developers generally reect their
target hardware. Android on the other hand makes the assumpon that a single applicaon
may be required to run on many dierent types of devices, with very dierent hardware
capabilies and specicaons, and makes it as easy as possible for developers to handle the
dierences between these devices simply and elegantly.

Preface
[ 2 ]
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 poinng device
You oen don't have a keyboard
Any keyboard that does exist may be a soware keyboard
A soware keyboard may consume some of your applicaon's screenspace
The soware keyboard reduces the amount of screen space available to your applicaon,
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 dierent Android devices dierent, but they
may also appear to change features while your applicaon is running.
The rule of nger
Most Android devices have touchscreens (although this is not a requirement). The rst
restricon placed on any touchscreen user interface is the size of the human forenger,
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 noce 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 applicaon and all the
widgets that it uses must be enrely self-explanatory (even more than usual). Far too oen,
we substute good user interface planning and design with a roll-over or toolp to indicate
a widget's funcon. On a touchscreen device, there is no mouse or poinng device. The rst
interacon 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 capabilies will also vary from one device to the next, depending on the
intended use of the device and oen its intended market segment.

Preface
[ 3 ]
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 informaon onto the
screen as possible. So don't give your users informaon that they don't want, and also avoid
asking them for informaon 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 applicaon is successful. Throughout
the rest of the book, we'll be walking through these guidelines with praccal examples of
improvements that can be made to a user interface.
Consistency
This is the cornerstone of good user interface design. A buon should look like a buon.
Make sure that the layout of each screen has a relaonship with every other screen in your
applicaon. People oen mistake this principle for "sck to the plaorm look and feel". Look
and feel is important, consistency mostly applies to the layout and overall experience of the
applicaon, 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 rst glance, this suggeson looks merely like a "good object-oriented" pracce. 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 applicaon. Oen, when a user encounters a
new applicaon, it's because they are looking for something. They may not have the me
(or more oen paence) to learn a new user interface. Make sure that your applicaon
asks for as lile as possible, and guides the user to the exact informaon they want in as
few steps as possible.

Preface
[ 4 ]
The Zen approach
Generally, when you are using a mobile device, your me is limited. You may also be using
an applicaon in less-than-ideal circumstances (perhaps, in a train). The lesser informaon
a user needs to give an applicaon, and the lesser they need to absorb from it, the beer.
Stripping away opons and informaon 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" buon, 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 unl they are needed.
Feedback
Feedback is what makes a touchscreen device excing. When you drag an object, it scks to
your nger across the screen unl you let go of it. When the users puts their nger on your
applicaon, they expect some reacon. However, you don't want to get in their way—instead
of showing an error message when they touch a buon, disable the buon unl 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 soware. Just because the applicaon makes sense to you,
the developer, it doesn't mean it seems logical to your user. Adding transion animaons,
breadcrumbs, and progress gauges help the user to idenfy where in the applicaon they
are, and what's happening.
The road to recovery
A common way to tell users that something is wrong on a desktop applicaon, or on the web
is to open an error dialog. On a mobile device, people want smoother use of an applicaon.
While in a normal applicaon you may inform the user that they selected an invalid opon,
in a mobile applicaon, you generally want to make sure they can't select that opon in the
rst place. Also, don't make them scroll through huge lists of opons. Instead, allow them to
lter 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 nd any
ights for your search". Instead tell them, "There were no available ights for your search,
but if you're prepared to leave a day earlier, here is a list of the available ights". Always
make sure your user can take another step forward without having to go "Back" (although
the opon to go backwards should always exist).

Preface
[ 5 ]
The Android way
The Android plaorm is in many ways similar to developing applicaons for the web.
There are many devices, made by many manufactures, with dierent capabilies and
specicaons. 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 dierences, 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 applicaon is assembled
Dierent types of Android layouts
Presenng various types of data to the user
Customising of exisng Android widgets
Tricks and tools to keep user interfaces looking great
Integraon between applicaons
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 Acvity introduces the basics of building an Android
applicaon, starng with a simple user interface. It also covers the various opons available
to you when implemenng 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 plaorm provides, and how they relate to the mundane widgets.
This chapter covers widgets such as the gallery and rang-bar, and how they can be used and
styled.
Chapter 4, Acvies and Intents discusses more about how Android runs your applicaon,
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 applicaon will behave the way users expect it to, with minimal
eort on your part.

Preface
[ 6 ]
Chapter 5, Non-Linear Layouts takes a look at some of the advanced layout techniques which
Android oers. It talks about the best way to present dierent screens to the user while
taking into account the wide discrepancy in the screens on Android devices.
Chapter 6, Input and Validaon provides ps regarding taking input from a user, and how
to keep this experience as painless as possible. This chapter invesgates the dierent input
widgets Android provides and how to congure them best, depending on the situaon. Also,
when everything else fails, how best to inform your users that what they are doing is wrong.
Chapter 7, Animang 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
animaons are provided by default, how to compose them together, and how to build your
own. This chapter looks at the importance of animaons in a mobile user interface and
demonstrates how complex animaons are made easy by Android.
Chapter 8, Content-centric Design details how to go about designing the screen layout, when
presenng the user with informaon on the screen. This chapter looks at the pros and cons
of some of the dierent display techniques which Android oers.
Chapter 9, Styling Android Applicaons shows us how to keep the look of our enre
applicaon consistent, in order to make our applicaon easier to use.
Chapter 10, Building an Applicaon Theme looks at the design process, and how applicaon-
wide themes can be applied to help your applicaon stand out.
What you need for this book
Please have a look at "System Requirements" menoned 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
applicaons on the Android plaorm. It will also be of use to people who have developed
applicaons on the Android plaorm and would like to gain addional knowledge about
its user interface design. It will also be a helpful reference for the numerous widgets and
resource structures that the Android plaorm provides.

Preface
[ 7 ]
This book will also be helpful to:
Java developers learning Android development
MIDP developers looking to broaden their skill-set
iPhone developers wanng to port applicaons
Entrepreneurial Android developers wanng to widen their user base
Conventions
In this book, you will nd several headings appearing frequently.
To give clear instrucons 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.
Instrucons oen need some extra explanaon so that they make sense, so they are
followed with:
What just happened?
This heading explains the working of tasks or instrucons that you have just completed.
You will also nd some other learning aids in the book, including:
Pop quiz – heading
These are short mulple choice quesons intended to help you test your own understanding.
Have a go hero – heading
These set praccal challenges and give you ideas for experimenng with what you have learned.
You will also nd a number of styles of text that disnguish between dierent kinds of
informaon. Here are some examples of these styles, and an explanaon of their meaning.
Code words in text are shown as follows: "We'll start o by creang a selector Activity,
and a simple NewsFeedActivity".
www.allitebooks.com

Preface
[ 8 ]
A block of code is set as follows:
<activity
android:name=".AskQuestionActivity"
android:label="Ask Question">
<intent-filter>
<action android:name="questions.askQuestion"/>
<category android:name="android.intent.category.DEFAULT"/>
</intent-filter>
</activity>
When we wish to draw your aenon to a parcular part of a code block, the relevant lines
or items are set in bold:
<?xml version="1.0" encoding="UTF-8"?>
<FrameLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="fill_parent">
<ViewStub android:id="@+id/review"
android:inflatedId="@+id/inflated_review"
android:layout="@layout/review"/>
<ViewStub android:id="@+id/photos"
android:inflatedId="@+id/inflated_photos"
android:layout="@layout/photos"/>
<ViewStub android:id="@+id/reservations"
android:inflatedId="@+id/inflated_reservations"
android:layout="@layout/reservations"/>
</FrameLayout>
Any command-line input or output is wrien 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 buon and are not suddenly
whisked o to their web browser".
Warnings or important notes appear in a box like this.

Preface
[ 9 ]
Tips and tricks appear like this.
Reader feedback
Feedback from our readers is always welcome. Let us know what you think about this
book—what you liked or may have disliked. Reader feedback is important for us to
develop tles that you really get the most out of.
To send us general feedback, simply send an e-mail to feedback@packtpub.com, and
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Customer support
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to get the most from your purchase.
Downloading the example code for this book
You can download the example code les for all Packt books you have purchased
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http://www.packtpub.com/support.

Preface
[ 10 ]
Piracy
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we take the protecon of our copyright and licenses very seriously. If you come across any
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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.

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 informaon to the
user. You can construct your Activity screens by using either: XML layout les
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 applicaon resource le
Tie the resource le to an Activity class
Dynamically populate the Activity with a series of mulple-choice quesons
Developing our rst example
For our rst example, we're going to write a mulple-choice queson and answer Activity.
We could use it for applicaons such as "Who wants to be a millionaire?", or "What type of
a monkey are you?". This example will pose quesons in order to answer a very important
queson: "What should I have to eat?" As the user answers the quesons, this applicaon
will lter a database of food ideas. The user can exit the process at any me to view a list of
suggested meals, or just wait unl the applicaon runs out of quesons to ask them.
Since it's a user interface example, we'll skip building lters and recipe databases. We'll just
ask our user food preference-related quesons. For each queson, we have a list of preset
answers which the user can select from (that is, mulple-choice quesons). Each answer
they give will allow us to narrow the list of suitable recipes.

Developing a Simple Acvity
[ 12 ]
Creating the project structure
Before we can start wring code, we need a project structure. An Android project
is made up of far more than just its Java code—there are also manifest les, 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 operang 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 installaon instrucons on the
website at http://developer.android.com/sdk/installing.html to install the latest
SDK "starter package" and one or more plaorm 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 operang system, you'll need
to install it and then download at least one Android Plaorm 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 buon. 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 buon.
What just happened?
The above command tells the new Android SDK installaon to look for available packages
and install them. This includes installing a Plaorm Package. Each Plaorm 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 conguraon
and data. These are virtual machines that the Android emulator will run your soware on
when you wish to test.

Chapter 1
[ 13 ]
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 nd it under the tools directory of your
Android SDK. It's capable of creang a basic directory structure and a build.xml le (for
Apache Ant) to help get you started with your Android applicaon 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
opons to specify the structure of the new project:
Opon Descripon
-n Gives the project a name, in our case, KitchenDroid. This is really just an internal
idener for the project.
-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 Species the root Java package for the applicaon. This is a fairly important concept
since it denes 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 applicaon from.
The skeleton project will be pre-congured to load this Activity when it's started.
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 Plaorm packages. You
can generally run the android tool by itself and use its graphical interface to download and
install Android Plaorm packages. The previous example uses API Level 3 which corresponds
to Android Plaorm version 1.5.
Downloa d f r o m W o w ! e B o o k < w w w.woweb o o k . c o m >

Developing a Simple Acvity
[ 14 ]
Examining the Android project layout
A typical Android project has almost as many directories and les as an enterprise Java
project. Android is as much of a framework as it is an operang environment. In some ways,
you can think of Android as an applicaon container designed for running on phones and
other limited devices.
As part of the new project structure, you will have the following important les and directories:
Folder name Descripon
bin Your binary les will be placed in this directory by the compiler.
gen Source code generated by various Android tools.
res Applicaon resources go here, to be compiled and packaged with
your applicaon.
src The default Java source code directory, where the build script
will look for source code to compile.
AndroidManifest.xml Your applicaon descriptor, similar to a web.xml le.
Resource Types and Files
Most types of applicaon resources (placed in the res directory) receive special
handling by the Android applicaon packager. This means these les consume
less space than they usually would (since XML is compiled into a binary format
instead of being le 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 dierent le format. Therefore, you cannot
put les 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 le in its
raw state, put it in the res/raw directory. Files in the raw directory are copied
byte-for-byte into your applicaon 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" applicaon.
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

Chapter 1
[ 15 ]
4. Now install your applicaon in the emulator:
ant install
5. In the emulator, open the Android menu and, you should see an icon named
QuesonAcvity in the menu. Click on this icon.
What just happened?
The Android emulator is a full hardware emulator including the ARM CPU, hosng the enre
Android operang system stack. This means soware 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 applicaons, you will need to use the install
Ant target. The install Ant target looks for a running emulator and then installs the
applicaon 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.
Applicaon Signatures
Every Android applicaon package is digitally signed. The signature is used to
idenfy you as a developer of the applicaon, and establish permissions for the
applicaon. It's also used to establish permissions between applicaons.
You will generally use a self-signed cercate, since Android doesn't require that
you use a cercate authority. However, all applicaons 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 le, we will declare the screen layout in an XML le. This is an important
decision for several reasons. The rst is that, using the Android widgets in Java code requires
several lines of code for each widget (a declaraon/construcon line, several lines invoking
seers, and nally adding the widget to its parent), while a widget declared in XML takes up
only one XML tag.

Developing a Simple Acvity
[ 16 ]
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 le. Therefore your applicaon uses less space on
the device, takes less me 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 compilaon, 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
selecon process as all the other external resources. This means that a layout can be varied
based on any of the dened properes, such as language, screen orientaon and size, and
even the me of day. This means that you can add new variaons on the same layout in
the future, simply by adding new XML les, and without the need to change any of your
Java code.
The layout XML le
All XML layout les must be placed in the /res/layout directory of your Android
project in order for the Android packaging tools to nd them. Each XML le will result
in a resource variable of the same name. For example, if we name our le /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 le, it will be loaded by the applicaon
resource loader (having been compiled by the resource compiler). A resource is subject to a
selecon process, so while there is only one resource that the applicaon loads, there may
be mulple possible versions of the same resource available in the applicaon package. This
selecon process is also what Android internaonalizaon is built on top of.
If we wanted to build a dierent version of the user interface layout for several dierent
types of touchscreens, Android denes three dierent types of touchscreen properes for
us: notouch, stylus, and finger. This roughly translates to: no touchscreen, resisve
touchscreen, and capacive touchscreen. If we wanted to dene a more keyboard-driven
user interface for devices without a touchscreen (notouch), we write a new layout XML le
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 qualiers
Here is a list of commonly used qualiers (property names) that will be taken into account
when Android selects a resource le to load. This table is ordered by precedence, with the
most important properes at the top.

Chapter 1
[ 17 ]
Name Descripon Examples API
Level
MCC and MNC The mobile-country-code (MCC) and mobile-network-
code (MNC). These can be used to determine which
mobile operator and country the SIM card in the device is
ed to.
The mobile-network-code oponally follows the mobile-
country-code, but cannot be used on its own (you must
always specify country-code rst).
mcc505
mcc505-mnc03
mcc238
mcc238-mnc02
mcc238-mnc20
1
Language and
region codes
Language and region codes are probably the most
commonly used resource properes. This is generally
how you localize your applicaon to the user language
preferences.
These values are standard ISO language and region
codes, and are not case-sensive. You cannot specify a
region without a country code (similar to java.util.
Locale).
en
en-rUS
es
es-rCL
es-rMX
1
Screen size There are only three variaons 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 medium-
density type screens
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 orientaon of the device.
long
notlong
4
Screen
orientaon
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 orientaon.
land
port
1
Night mode This value simply changes with the me of day. night
notnight
8
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Developing a Simple Acvity
[ 18 ]
Name Descripon Examples API
Level
Screen density
(DPI)
The DPI of the device screen. There are four possible
values for this property:
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
ldpi
mdpi
hdpi
nodpi
4
Keyboard
status
What sort of keyboard is available on this device? This
aribute shouldn't be used to determine whether the
device has a hardware keyboard, but instead whether a
keyboard (or soware keyboard) is currently visible to the
user.
keysexposed
keyshidden
keyssoft
1
Time for action – setting up the question activity
To kick things o we're going to be working with Android's simplest layout called:
LinearLayout. Unlike Java AWT or Swing, Android layout managers are dened as
specic 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-boom structure (which LinearLayout is perfect for).
1. Open the le 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 le:
<?xml version="1.0" encoding="UTF-8"?>
<LinearLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="fill_parent"
android:layout_height="wrap_content">
</LinearLayout>
What just happened?
We just removed the "Hello World" example, and put in an enrely empty layout structure
which will serve as the plaorm for us to build the rest of the user interface upon. As you can
see, Android has a special XML namespace for its resources.

Chapter 1
[ 19 ]
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 aribute prexed with
the Android namespace corresponds to an aribute that is interpreted by the Android
resource compiler.
The AAPT (Android Asset Packaging Tool) will generate an R.java le into your root (or
primary) package. This le contains all of the Java variables used to reference your various
applicaon resources. In our case, we have the main.xml package in the /res/layout
directory. This le becomes an R.layout.main variable with a constant value assigned
as its idencaon.
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 populang 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 le may have
only one root View.
Time for action – asking a question
In order to ask our user a queson, 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 vercal space for our queson to t. We populate the TextView
with Please wait... as its default text. Later, on we will replace this with a dynamically
selected queson.
1. Go back to your main.xml le.
2. Between the <LinearLayout...> and </LinearLayout> create a <TextView
/> element, ending it with the empty element /> syntax since elements
represenng View objects are not allowed to have child elements.
3. Give the TextView element an ID aribute:
android:id="@+id/question"

Developing a Simple Acvity
[ 20 ]
4. Change the layout width and height aributes to fill_parent and wrap_
content respecvely (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 applicaon using Apache Ant from your project root folder:
ant install
7. Run the applicaon again in the emulator and it should look like the following
screenshot:
The code for the TextView should end up looking something like this:
<TextView android:id="@+id/question"
android:text="Please wait..."
android:layout_width="fill_parent"
android:layout_height="wrap_content"/>
What just happened
In the preceding example, we used fill_parent and wrap_content as values for the layout
width and height aributes. 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 aribute (as in our
example), then it's the width of the parent view. If it's used in the android:layout_height
aribute, 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 aribute values is in the android:layout_width and
android:layout_height aributes. Anywhere else will result in a compiler error.

Chapter 1
[ 21 ]
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 aribute 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 idener for each resource of id as part of your R class. The ID aribute
is also needed for accessing resources from another resource le.
Time for action – adding a space for answers
While posing a queson to the user is all very ne and well, we need to give them some
way to answer that queson. We have several possibilies 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 interacon, 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 le.
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 queson, you will need to add a
<LinearLayout /> 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 vercally below each other, so
you'll need to set the orientation aribute 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
buons it will be populated with:
android:layout_height="wrap_content"

Developing a Simple Acvity
[ 22 ]
The resulng code should look like this:
<LinearLayout android:id="@+id/answers"
android:orientation="vertical"
android:layout_width="fill_parent"
android:layout_height="wrap_content"/>
What just happened?
You may have noced that for this example, we have no content in our new LinearLayout.
This may seem a lile unusual, but in this case, we want to populate it with a variable
number of buons—one for each possible answer to our mulple-choice quesons.
However, for the next part of the example we need some simple content Button widgets
in this LinearLayout so that we can see the enre screen layout in acon. Use the
following code in your layout resource le to add Yes!, No!, and Maybe? Button widgets
to the LinearLayout:
<LinearLayout android:id="@+id/answers"
android:orientation="vertical"
android:layout_width="fill_parent"
android:layout_height="wrap_content">
<Button android:id="@+id/yes"
android:text="Yes!"
android:layout_width="fill_parent"
android:layout_height="wrap_content" />
<Button android:id="@+id/no"
android:text="No!"
android:layout_width="fill_parent"
android:layout_height="wrap_content" />
<Button android:id="@+id/maybe"
android:text="Maybe?"
android:layout_width="fill_parent"
android:layout_height="wrap_content" />
</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 nesng those View
elements inside the element of your ViewGroup (as opposed to closing it with no child
XML elements).
Downloa d f r o m W o w ! e B o o k < w w w.woweb o o k . c o m >

Chapter 1
[ 23 ]
The following is a screenshot of the preceding Yes!, No!, Maybe? opons:
Time for action – adding more buttons
We have two addional buons to add to the screen layout. One will allow the user to skip
the current queson; the other will allow them to look at the short list of meals that we have
ltered through so far (based on the quesons they have already answered).
1. Start by creang an empty <Button /> element below our answers ViewGroup
<LinearLayout /> (but sll 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 buon by using a margin:
android:layout_marginTop="12sp"
3. Give it the display label Skip Queson:
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 <Button /> element below the Skip Queson buon
6. The ID for the new buon should be view:
android:id="@+id/view"
7. We want this buon to display the text: Feed Me!:
android:text="Feed Me!"

Developing a Simple Acvity
[ 24 ]
8. Again, put a lile space between the Skip Queson buon, and the new
Feed Me! buon:
android:layout_marginTop="12sp"
9. Finally, set the width and height of the Feed Me! buon 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 buons, your layout XML le should now end with:
<Button android:id="@+id/skip"
android:text="Skip Question"
android:layout_marginTop="12sp"
android:layout_width="fill_parent"
android:layout_height="wrap_content"/>
<Button android:id="@+id/view"
android:text="Feed Me!"
android:layout_marginTop="12sp"
android:layout_width="fill_parent"
android:layout_height="wrap_content"/>
</LinearLayout>
What just happened
Separaon 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 buons. 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 aribute name is prexed with layout_. Because padding is the
responsibility of a View object, the padding aribute has no such prex:
<Button android:id="@+id/view"
android:text="Feed Me!"
android:padding="25sp"
android:layout_marginTop="12sp"
android:layout_width="fill_parent"
android:layout_height="wrap_content"/>
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 buon.

Chapter 1
[ 25 ]
All of the measurements in the preceding example are specied in the sp unit, which is short
for "scale independent pixels". Much like CSS, you sux your measurement numbers
with the unit of size that you are specifying the measurement in. Android recognizes the
following measurements:
Unit sux Full name Descripon and uses
px Pixel Exactly one pixel of the device screen. This unit is the most
common when wring desktop applicaons, but with the wide
variety of phone screen sizes, it becomes much harder to use.
in Inch One inch (or the closest approximaon). 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 variaons in
the size of a device screen, it is not always very useful.
mm Millimeters Another real world measurement, made to the closest
approximaon. 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 relave to font sizes.
dp or dip Density-
independent-
pixels
A single DP is the same size as a single pixel is for a 160 dpi screen.
This size is not always a direct rao, not always precise, but is a
best approximaon for the current screen.
sp Scale-
independent
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 nd the screen hard to read. Using an
sp unit ensures that your user interface scales with it.
Dening common dimensions
Android also allows you to dene 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 dene a common font size. Files containing dimension declaraons
are placed in the /res/values directory in your project. While the actual le name isn't
signicant, 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 runme.

Developing a Simple Acvity
[ 26 ]
One advantage of having your dimensions in their own le as opposed to being declared
inline) is that you can then localize them based on the size of the screen. This makes screen-
resoluon-signicant scales (such as pixels) much more useful. For example, you can place a
dimens.xml le with dierent values into /res/values-320x240 and another version of
the same dimensions into /res/values-640x480.
A dimensions resource le is a simple values le (much like strings.xml), but dimensions
are dened with the <dimen> tag:
<resources>
<dimen name="half_width">160px</dimen>
</resources>
To access this as a size in a layout XML le, you use a resource reference (much the same way
as you access a resource string):
<TextView layout_width="@dimen/half_width" />
Building a list of common dimensions comes in handy when you want to build complex
layouts that will look good on many dierent screens since it avoids the need to build several
dierent layout XML les.
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 buons, and the Skip Queson and Feed
Me! buons, you can't really tell them apart. We need to let the user know that these buons
all do dierent things. We also need to draw more aenon to the queson, especially if they
don't have a lot of me to squint at their screen. You may need the Android documentaon,
which can be found online at http://developer.android.com/reference/.
We have a queson 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 rst me they use the applicaon).
Try making the following styling changes to the queson TextView at the top of our screen.
These will only require you to add some aributes 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 boom of the queson and the answer buons

Chapter 1
[ 27 ]
The Feed Me! buon is also very important. This is the buon that gives the user access to
the list of suggested recipes that the applicaon has ltered based on their answers, so it
should look good.
The following styling should help the Feed Me! buon 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 nished styling the screen, it should look like the following screenshot:
Limitations of the layout XML format
One of the most obvious limitaons 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 le.
In our example, this limitaon shows itself in the form of our empty LinearLayout.
Because each queson has any number of possible answers, we need a varying number of
buons in the group. For our purposes, we will create the Button objects and put them into
the LinearLayout as part of the Java code.
www.allitebooks.com

Developing a Simple Acvity
[ 28 ]
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 aribute on the TextView element—question. We could have
referenced an external string using the following syntax:
<TextView android:id="@+id/question"
android:text="@string/question"
android:gravity="center"
android:textStyle="bold"
android:layout_width="fill_parent"
android:layout_height="wrap_content"/>
This will eecvely reference a stac variable from the strings.xml le. It's not suitable
for a dynamically selected queson, which will change each me we inialize the Activity.
Pop quiz
1. What reason do you have for wring your layouts in XML instead of in pure Java
code?
a. Android can read the layout le externally for opmizaon.
b. The layout becomes part of the resource selecon 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 Queson buon bold?
a. Use the android:typeface aribute.
b. Create a custom Button implementaon.
c. Add a CSS aribute: style="font-weight: bold".
d. Use the android:textStyle aribute.
3. What would happen if we changed the LinearLayout from vertical orientaon,
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 queson TextView would be visible on the screen.
d. The queson, and possibly some other View objects may be visible on the
screen depending on the number of pixels available.
e. The layout would overow, causing the widgets to appear next to each other,
over several lines.

Chapter 1
[ 29 ]
Populating the QuestionActivity
We have a basic user interface, but right now, it's stac. We may want to ask our user many
dierent quesons, each of which have dierent answers. We may also want to vary which
quesons we ask in some way or another. In short, we need some Java code to populate the
layout with a queson and some possible answers. Our quesons are made up of two parts:
The queson
A list of possible answers
In this example, we will make use of string array resources to store all of the queson and
answer data. We will use one string array to list the queson ideners, and then one string
array for each queson and its answers. The advantages of this approach are very similar
to the advantages of using a layout XML le instead of hard-coding it. The res/values
directory of your project will have an auto-generated strings.xml le. This le contains
string and string-array resources that you want your applicaon to use. Here is the start
of our strings.xml le, with two quesons to ask the user:
<?xml version="1.0" encoding="UTF-8"?>
<resources>
<string name="app_name">Kitchen Droid</string>
<string-array name="questions">
<item>vegetarian</item>
<item>size</item>
</string-array>
<string-array name="vegetarian">
<item>Are you a Vegetarian?</item>
<item>Yes</item>
<item>No</item>
<item>I\'m a vegan</item>
</string-array>
<string-array name="size">
<item>How much do you feel like eating?</item>
<item>A large meal</item>
<item>Just a nice single serving of food</item>
<item>Some finger foods</item>
<item>Just a snack</item>
</string-array>
</resources>
The rst item of each question array (vegetarian and size) is the queson itself, while
each following item is an answer.

Developing a Simple Acvity
[ 30 ]
Time for action – writing more Java code
1. Open the QuestionActivity.java le in an editor or IDE.
2. Import the Android Resources class below the package declaraon:
import android.content.res.Resources;
3. In order to start asking the quesons from your strings.xml le, you'll need a
method to look in the questions <string-array> and nd the name of the
array that contains the current queson. This is not normally something you need to
do with applicaon resources—their ideners are generally known to you through
the R class. In this case however, we want to work in the order dened in the
questions <string-array>, making things a lile bit more dicult:
private int getQuestionID(Resources res, int index) {
4. We can now look at the questions string-array, which contains the idenfying
name of each queson (our index string-array):
String[] questions = res.getStringArray(R.array.questions);
5. We have the array of quesons, and we need to nd the idener value. This is
much like using R.array.vegetarian for the vegetarian queson, 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 quesons to the user. We want
the applicaon to "play nice" with the phone and its environment. For that reason,
each queson 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 queson: How do we know the index of the queson 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" informaon (similar to request
aributes 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);
}

Chapter 1
[ 31 ]
These two methods form the basis for populang our queson screen and navigang our
way through a dened list of quesons. 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 prey straight forward. In our code we use R.array.
questions to access the <string-array> which idenes all of the quesons we
are going to ask the user. Each queson has a name in the form of a String, and a
corresponding resource idencaon number in the form of an int.
In the getQuestionID method, we make use of the Resources.getIdentifier
method, which looks for the resource idener (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 <string-array>
we currently are, and thus, which queson to ask the user. This is based on the "extra"
informaon 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 "primive" or "serializable" type. Here
we fetch the KitchenDroid.Question extra integer value from our Intent, substung
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 specied that value, so we start from the rst queson.

Developing a Simple Acvity
[ 32 ]
Dynamically creating widgets
Up to this point we've only used the layout XML le 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 buons that
they can touch to answer the quesons posed to them. We could pre-create some buons
and name them button1, button2, and so on, but that means liming the number of
possible answers.
In order to create buons from our <string-array> 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 buons.
Time for action – putting the questions on the screen
Your applicaon now knows where to nd the quesons to ask, and knows which queson
it should be asking. Now it needs to put the queson on the screen, and allow the user to
select an answer.
1. Open the main.xml le in your editor or IDE.
2. Remove the Yes!, No!, and Maybe? Button elements from the layout resource.
3. Open the QuestionActivity.java le in an editor or IDE.
4. We will need a new class eld 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 quesons on
the screen: initQuestionScreen:
private void initQuestionScreen() {
6. In this method, we assume that the layout XML le has already been loaded into the
Activity screen (that is, it will be invoked aer 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 queson and its possible
answers. For that we need the <string-array> (from our strings.xml le)
which contains that data, so we need to know the resource idener for the current
queson. Then we can fetch the actual array of data:

Chapter 1
[ 33 ]
int questionID = getQuestionID(resources, getQuestionIndex());
String[] quesionData = resources.getStringArray(questionID);
8. The rst element of a question string array is the queson to pose to the user.
The following setText call is exactly the same as specifying an android:text
aribute in your layout XML le:
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 queson 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 aer 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 specic
idenfying integer value. By default, any resource declared with an android:id aribute
in its resource le 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 dierent 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 rst construct the object, and then use setText to
dene its label.

Developing a Simple Acvity
[ 34 ]
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 applicaon
resources and services that they require in order to funcon correctly.
You can now try running the applicaon, in which case you'll be greeted with the following
screen. You may have noced that there is addional styling in this screenshot. If you don't
have this, you may want to backtrack a lile to the previous Have a go hero secon.
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 re 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 idenfy an event-listener interface
by the fact that their class names are prexed with On (much like HTML event aributes). 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 briey to give the user
some informaon:

Chapter 1
[ 35 ]
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 dierent widgets. However, most of the me
you'll want to be listening for events on widgets you've declared in an XML layout resource.
In these cases it's beer to either have your Activity class implement the required
interfaces, or create specialized classes for dierent event-driven acons. While Android
devices are very powerful, they are sll limited when compared to a desktop computer
or laptop. Therefore, you should avoid creang 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 le, 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 eld in the Activity class.
2. What is the "best" way of listening for events in an Android applicaon?
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 denes 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 operang environment.
Downloa d f r o m W o w ! e B o o k < w w w.woweb o o k . c o m >

Developing a Simple Acvity
[ 36 ]
Summary
Android comes with some great tools to create and test applicaons, even if you don't
have an Android device handy. That said, there's no replacement for actually touching your
applicaon. It's part of what makes Android such a compelling plaorm, 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 selecon
system. With it you can build highly dynamic applicaons that respond to changes in
the devices, and thus, the user environment. Changing the screen layout based on the
orientaon 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 applicaon.
When building user interfaces in Android, it's strongly recommended to build at least the
layout structure in an XML le. The XML layout les are not only considered as applicaon
resources, but Android also strongly favors building XML user interfaces over wring Java
code. Somemes, however, a layout XML le isn't enough, and you need to build parts of the
user interface in Java. In this case it's a good idea to dene 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 quesons. We could have used
RadioButton objects instead, but then the user would have needed to select an opon,
and then touch a Next Queson buon, 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 "acon" 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 sck 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
selecon process to build dierent screen designs for small, medium, or large screens.
You could also dene your own measurement unit and base it on the screen size.
Always think about how your user will interact with your applicaon, and how much
(or lile) me 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 soluons of Android user interface design.
In the next chapter, we will focus on working with data-driven widgets. Android has several
widgets designed specically for displaying and selecng from more complex data structures.
These widgets form the basis of data-driven applicaons such as an address book or a
calendar applicaon.

2
Presenting Data for Views
In the rst chapter we covered the basic creaon of a project, and how to put
together a simple user interface. We backed our rst Activity with enough
code to dynamically generate some buons that the user can use to answer our
mulple-choice quesons.
So now we can capture some data, but what about displaying data? One large
advantage of soware is its ability to present and lter 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 presenng 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 soware needs to display to the user. This paern allows the soware 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 oen 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.
www.allitebooks.com

Presenng Data for Views
[ 38 ]
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 selecon, as dened 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 aribute in your
layout XML le, 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 dened 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 navigang by touch. The default mode of a ListView allows the
user to tap on one of the elements, and trigger an acon. As a result of this behavior, there's
no need for a "Next" buon, 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
dierent strings from a String array Java object, taken from one of the default ApiDemos
examples in Android SDK.

Chapter 2
[ 39 ]
Single selection mode – CHOICE_MODE_SINGLE
In this mode, the ListView acts more like a desktop List widget. It has the noon of
the current selecon, and tapping on a list item does nothing more than selecng it.
This behavior is nice for things like conguraon or sengs, where the user expects the
applicaon to remember his or her current selecon. Another place a single selecon list
becomes useful is when there are other interacve widgets on the screen. However, be
careful not to put too much informaon in a single Activity. It's quite common for a
ListView to occupy almost an enre screen.
Single-choice selecon: It doesn't directly change the way your list items
appear. The look and feel of your list items is dened enrely by the
ListAdapter object.
Android does, however, provide a collecon of sensible defaults in the system resources.
In the android package you will nd an R class. It's a programmac way to access the
system's default resources. If you wanted to create a single-choice ListView with a
<string-array> 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 dened 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.

Presenng Data for Views
[ 40 ]
Multiple selection mode – CHOICE_MODE_MULTIPLE
In mul-selecon mode, the ListView replaces the radio buons of single-selecon mode
with normal checkboxes. This design structure is oen used on desktops and web-based
systems as well. Checkboxes are easily recognized by users, and make it easy to go back and
turn opons o 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 addional widgets at the top and
boom 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);
Oen you don't want your headers and footers to be items in the ListView, but instead a
label or group of labels idenfying parts of the ListView, or providing other informaon. 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 implementaon 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 ghtly 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 oset 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).

Chapter 2
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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 aached to the top and boom 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 secons of this chapter. The rst Activity we will create will use a
simple ListView populated from a <string-array> resource.
Time for action – creating a fast food menu
To connue with the food and eang theme, let's build a simple applicaon that allows us
to order various types of fast food, and get it delivered! The user will rst select where they
want to order from, and then select the various foodstus 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 le in your favorite editor or IDE.
3. Create a string-array structure lisng the various fast-food restaurants our users can
order from:
<string-array name="restaurants">
<item>The Burger Place</item>
<item>Mick's Pizza</item>
<item>Four Buckets \'o Fruit</item>
<item>Sam\'s Sushi</item>
</string-array>
4. Open the /res/layout/main.xml le in your favorite editor or IDE.
5. Remove any widget that is inside the default LinearLayout.
6. Add a new <ListView> element.
7. Assign the <ListView> element an ID of restaurant:
<ListView android:id="@+id/restaurant"/>
8. Assign the width and height of the ListView to fill_parent:
android:layout_width="fill_parent"
android:layout_height="fill_parent"

Presenng Data for Views
[ 42 ]
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 le:
android:entries="@array/restaurants"
10. When you've completed the specied steps, you should have a main.xml layout le
that looks like the following:
<?xml version="1.0" encoding="UTF-8"?>
<LinearLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="fill_parent"
android:layout_height="fill_parent">
<ListView android:id="@+id/restaurant"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:entries="@array/restaurants"/>
</LinearLayout>
What just happened
If you install your applicaon into the emulator, and run it, you'll be presented with a screen
where you can select from the list of restaurants specied in your string-array resource.
Noce that the choiceMode on the ListView is le 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 aribute in the layout XML le 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.

Chapter 2
[ 43 ]
Using the android:entries aribute 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-specied defaults, and so cannot be easily themed.
You cannot dene data objects that will be represented in the ListView. Since
string-arrays are easily localized, your applicaon will rely on the index locaons of
items to determine what they indicate.
You may noce that at the top of the screenshot, the label Where should we order
from? is not the applicaon default. The label for an Activity is dened in the
AndroidManifest.xml le as follows:
<activity
android:name=".SelectRestaurantActivity"
android:label="Where should we order from?">
Styling the standard ListAdapters
The standard ListAdapter implementaons require each item be represented in a
TextView item. The default single-choice and mulple-choice items are built using a
CheckedTextView, and while there are plenty of other TextView implementaons
in Android, it does limit our opons a bit. However, the standard ListAdapter
implementaons are very convenient and provide solid implementaons for the most
common lisng 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 implementaons
are more suitable than others (for example, a ToggleButtonView won't maintain the
specied text-value when the user touches it).
Dening standard dimensions
In this example we'll be creang various menus for the applicaon. In order to maintain a
consistent look and feel, we should dene a set of standard dimensions which will be used
in each of our layout les. This way we can redene the sizes for dierent types of screens.
There's nothing more frustrang for a user than only being able to see a paral item because
it's been sized bigger than their screen.
Create a new resource le to contain the dimensions. The le should be named res/
values/dimens.xml. Copy the following code into the new XML le:

Presenng Data for Views
[ 44 ]
<?xml version="1.0" encoding="UTF-8"?>
<resources>
<dimen name="item_outer_height">48sp</dimen>
<dimen name="menu_item_height">52sp</dimen>
<dimen name="item_inner_height">45sp</dimen>
<dimen name="item_text_size">24sp</dimen>
<dimen name="padding">15dp</dimen>
</resources>
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 le is used to dene a standard amount of
whitespace between two visual elements. This is dened 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 interacve items
In this styling, you'll noce 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 crical. If your users have large ngers, they will struggle to tap on their
target list item if you make them too small.
This is a general "good pracce" 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 implementaon, you will need to specify the ListAdapter object
in your Java code.
1. Create a new le in the res/layout directory named menu_item.xml.
2. Create the root XML element as a TextView:
<?xml version="1.0" encoding="UTF-8"?>
<TextView />
3. Import the Android resource XML namespace:
xmlns:android="http://schemas.android.com/apk/res/android"

Chapter 2
[ 45 ]
4. Center the text in the TextView widget by seng 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 le.
9. In the onCreate method, aer 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 string-
array of restaurants we created in our values.xml le:
restaurants.setAdapter(new ArrayAdapter<String>(
this,
R.layout.menu_item,
getResources().getStringArray(R.array.restaurants)));
What just happened
We rst 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 le you
wrote should contain the following code:
<?xml version="1.0" encoding="UTF-8"?>
<TextView xmlns:android="http://schemas.android.com/apk/res/android"
android:gravity="center|center_vertical"
android:textSize="@dimen/item_text_size"
android:textColor="#ffffff"
android:layout_width="fill_parent"
android:layout_height="@dimen/menu_item_height" />

Presenng Data for Views
[ 46 ]
Unlike our previous layout resources, menu_item.xml contained no ViewGroup (such as
LinearLayout). This is due to the fact that the ArrayAdapter will aempt to cast the
root View of the menu_item.xml le 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 acon eliminates
the use of the android:entries aribute on the ListView in the main.xml layout
XML resource. If you want, you can remove that aribute. 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<String>(
this,
R.layout.menu_item,
getResources().getStringArray(R.array.restaurants)));
}
Try re-installing the applicaon 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 mulple-choice queson applicaon we wrote in Chapter 1, Developing a
Simple Acvity. It uses LinearLayout and Button objects to display the possible answers to
the quesons, but it also uses string-arrays for the answers. Try modifying the applicaon to:
Use a ListView instead of a LinearLayout
Style the ListView with Button objects, as we styled our restaurant menu with
TextView objects

Chapter 2
[ 47 ]
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 oen want to order more than one of the same item. The
ListView implementaon, and the standard ListAdapter implementaons 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 dierent foods that the user can order in mulple quanes, we need
a customized ListAdapter implementaon.
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 invesgate dierent ways of organizing and
displaying the menu data. Our rst 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 me they
tap a list item, they order another burger. We will display the number of burgers they are
ordering in bold to the le 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 le 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 noce that there are no geers and seers in the preceding code, and that both the
name and count elds are declared as package-protected. In versions of Android prior to
2.2, methods incurred a heavy expense when compared to a straight eld lookup. Since this
class will be a small part of the rendering procedure (we will be extracng data from it for
display), we should make sure we incur as lile expense as possible.
www.allitebooks.com

Presenng Data for Views
[ 48 ]
Time for action – creating a Burger item layout
The rst 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 me, we are not forced to use a single TextView, but can instead build a more
complex layout.
1. Create a new XML le in the res/layout directory named burger_item.xml.
This le 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):
<LinearLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="horizontal"
android:layout_width="fill_parent"
android:layout_height="@dimen/item_outer_height">
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:
<TextView android:id="@+id/counter" />
4. The counter text size is exactly the same as all of the other list items in the
applicaon. However, it should be bold, so it can be easily idened 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:
<TextView android:id="@+id/text" />
8. The text size is standard:
android:textSize="@dimen/item_text_size"

Chapter 2
[ 49 ]
9. We want a lile bit of space between the counter and the text label:
android:layout_marginLeft="@dimen/padding"
10. The label's width should ll 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 vercally, to match the locaon of the
counter. However, the label should be le-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 exible going forward if we want to apply addional styles to them independently.
Your complete burger_item.xml le should now appear as follows:
<?xml version="1.0" encoding="UTF-8"?>
<LinearLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="horizontal"
android:layout_width="fill_parent"
android:layout_height="@dimen/item_outer_height">
<TextView android:id="@+id/counter"
android:textSize="@dimen/item_text_size"
android:textStyle="bold"
android:layout_width="@dimen/item_inner_height"
android:layout_height="@dimen/item_inner_height"
android:gravity="center|center_vertical" />
<TextView android:id="@+id/text"
android:textSize="@dimen/item_text_size"
android:layout_marginLeft="@dimen/padding"
android:layout_width="fill_parent"
android:layout_height="@dimen/item_inner_height"
android:gravity="left|center_vertical" />
</LinearLayout>

Presenng Data for Views
[ 50 ]
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 implementaons 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 presentaon 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 ideners 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 exisng
View object that could be reused. We will implement a simple method to handle
this case, and if required, inate the burger_item.xml le 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;
}

Chapter 2
[ 51 ]
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 rst 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 populang 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 aempt 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 rst me around. It's important to try and honor this behavior, since
it has a direct impact on the responsiveness of your applicaon. In our preceding example, we
implemented the getViewGroup method so that it would take this requirement into account.

Presenng Data for Views
[ 52 ]
The getViewGroup method is also used to inate the burger_item.xml le 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) denes
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 sll take up its space in the
layout, which will keep the text labels le-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 automacally handle the highlighng of a selected item. This includes
when the user holds their nger on the item, and when they use a track-pad or direconal
buons to navigate the ListView. When an item is highlighted, its background generally
changes color, according to standard UI convenons.
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 selecon
highlighng 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 specied items in the ListView. A common
use of this is to turn certain items into logical separators. For example, a
secon separator in an alphabecally sorted list, containing the rst leer of
all items in the next "secon".
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
specic event occurs (in this case the user touches a specic 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 ulity methods for this scenario.

Chapter 2
[ 53 ]
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 implementaon 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
reect 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();
}
Downloa d f r o m W o w ! e B o o k < w w w.woweb o o k . c o m >

Presenng Data for Views
[ 54 ]
What just happened?
This implementaon 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 eecvely stac View objects. This means
that the Adapter must be allowed to update or recreate that View when the data model
is updated. A common alternave is to fetch the View represenng 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 le. First, open the AndroidManifest.xml
le in an editor or IDE, and copy the following <activity> code into the
<application>...</application> block:
<activity android:name=".TheBurgerPlaceActivity"
android:label="The Burger Place\'s Menu">
<intent-filter>
<action android:name=
"com.packtpub.deliverydroid.TheBurgerPlaceActivity"/>
</intent-filter>
</activity>
To start the Activity, you'll need to go back to SelectRestaurantActivity
and implement the OnItemClickListener interface. Aer seng 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);

Chapter 2
[ 55 ]
ListView restaurants = (ListView)
findViewById(R.id.restaurant);
restaurants.setAdapter(new ArrayAdapter<String>(
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 applicaon 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" buon in
The Burger Place menu will take you back to the restaurant menu.

Presenng Data for Views
[ 56 ]
Pop quiz
1. Seng 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 denes 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 posioned:
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 mes when it will ood your user with far too much informaon. Think about an email
applicaon. 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 rst few, seeing the scrollbar shrink to a few pixels in size doesn't have a good
psychological eect on your user.
In desktop mail clients, you will oen group the email list by me: 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 mes 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 rst few items in the group and adding a special View
More item at the end. Alternavely, use a ListView for the groups, and a
separate Activity for the nested items.

Chapter 2
[ 57 ]
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 implemenng a custom ExpandableListAdapter,
it's generally easiest to have your ExpandableListAdapter implementaon inherit from
the BaseExpandableListAdapter, as it provides implementaons for event registraon
and triggering.
The ExpandableListAdapter will place an arrow pointer on the le side of
each group item to indicate whether the group is open or closed (much like a drop-
down/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 pung an item on the
le-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 'o' 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 le 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 quanty.
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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Presenng Data for Views
[ 58 ]
When the user taps on a pizza topping, it changes its status between the three values: O,
On, and Extra.
Using the ListView.getAdapter() method will not return your
ExpandableListAdapter implementaon, 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 xed number of columns, arranged le-to-right,
top-to-boom. The standard (un-themed) Android applicaon 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 xed column count, a GridView is very well suited
for lists of icons.
Using GridViews eecvely
A GridView can display signicantly more informaon on a single screen
than a ListView, at the expense of not being able to show as much text
informaon. From a usability point of view, icons are oen easier to work
with than text. Icons can be recognized more quickly than text, thanks to their
colors. When you have informaon 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 enre applicaon.

Chapter 2
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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 applicaon menu.
This next example will focus less on the implementaon 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-p. If the user is touching the object, it's oen obscured by their nger
and/or hand, making the icon and tool-p 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
le. 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 le in the res/layout directory named fruit_item.xml.
2. Declare the root element of the icon as a vercal LinearLayout:
<LinearLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="fill_parent"
android:layout_height="fill_parent">
3. Create the ImageView element that will serve as our icon:
<ImageView android:id="@+id/icon"
android:layout_width="fill_parent"
android:layout_height="wrap_content"/>
4. Next, create the TextView element that will serve as the label:
<TextView android:id="@+id/text"
android:textSize="@dimen/item_description_size"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:gravity="center|center_vertical" />

Presenng Data for Views
[ 60 ]
What just happened?
The fruit_item.xml le 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, aempt
to scale their content to their size. In our previous example, the root LinearLayout has the
width and height dened as fill_parent. When placed in a GridView as a single item,
using fill_parent as a size will cause the LinearLayout to ll the space provided for
that grid item (not the enre 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 straighorward at this point. It's a normal ListAdapter
implementaon built on top of an item class and the layout XML we dened 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 applicaon resources and IDs in Android, it's always with an integer. For this
example we're assuming that all of the dierent types of fruit each have an icon as an
applicaon resource. Another opon 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 potenally 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 pracce in Android to store bitmap images
as PNG les. Since you will be accessing these les from your code, make
sure they have Java-friendly lenames. The PNG format (unlike JPG) is
lossless, can have various dierent color depths, and correctly handles
transparency. This generally makes it a great image format on the whole.

Chapter 2
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Time for action – building the fruit menu
For the Four Buckets 'o Fruit menu, we're going to need a ListAdapter implementaon 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 dened 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:
<GridView
xmlns:android="http://schemas.android.com/apk/res/android"
android:numColumns="3"
android:horizontalSpacing="5dip"
android:verticalSpacing="5dip"
android:layout_width="fill_parent"
android:layout_height="fill_parent"/>
What just happened?
The new four_buckets.xml layout resource has nothing but a GridView. This is unlike
the other layout resources we've wrien 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 specied horizontal and
vercal 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.

Presenng Data for Views
[ 62 ]
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 creaon of the Activity step-by-step. Unlike previous
Activity implementaons, we will need a direct reference to the GridView dened in
four_buckets.xml, and this means loading it manually.
1. Start by creang a new class in your project's root package:
public class FourBucketsActivity extends Activity {
2. Override the onCreate method, and invoke the super implementaon:
protected void onCreate(final Bundle istate) {
super.onCreate(istate);
3. Get the LayoutInflator instance for your Activity object:
LayoutInflater inflater = getLayoutInflater();
4. Inate 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 applicaon 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 aacks you).

Chapter 2
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If you look through the Activity documentaon, you'll noce that while there's a
setContentView method, there's no corresponding getContentView method. Take a
closer look and you will noce the addContentView method. An Activity object may
have any number of View objects aached to it as "content". This precludes any useful
implementaon of a getContentView method.
In order to get around this limitaon, we inated 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 le contains (much the
same way the ArrayAdapter class works with TextView objects). If we wanted to make
things a lile more abstract, we could have cast it to an AdapterView<ListAdapter>, in
which case we could have swapped in implementaon in the le with a ListView. However,
this wouldn't have been very useful for this example.
If you now re-install and run the applicaon, 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 opons for dierent types of sushi:
Sashimi
Maki Roll
Nigiri
Oshi

Presenng Data for Views
[ 64 ]
California Roll
Fashion Sandwich
Hand Roll
Below the Spinner, use a GridView to display icons for each dierent type of sh that the
user can order. Here are some suggesons:
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 addional 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 aribute on the Spinner XML element.
Summary
Data display is one of the most common requirements of a mobile applicaon, and Android
has many dierent opons 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 mul-line to-do notes.
The GridView is eecvely a tabular version of ListView, and is well suited for presenng
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 signicantly less space, and
in a GridView, you could easily t four to six icons in a portrait screen without making the
user interface cluered or more dicult to work it. This also frees up precious screen space
for other items to be displayed.

Chapter 2
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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 unl the web service responds with actual
data. Take a good look at the default Adapter implementaons, 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 specic,
but they can be customized in any number of ways to t some very unusual purposes.

3
Developing with Specialized
Android Widgets
Along with the many generic widgets such as buons, text elds, and
checkboxes, Android also includes a variety of more specialized widgets. While
a buon is fairly generic, and has use in many situaons, 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 menoned
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 preest
date-selector in the world. It's not a calendar widget, so it's somemes quite dicult 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 implementaon.
This chapter will work with Android's more specialized View and layout classes:
Tab layouts
TextSwitcher
Gallery
DatePicker
TimePicker
RatingBar
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Developing with Specialized Android Widgets
[ 68 ]
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 implementaon details. We'll also discuss how
best to incorporate these elements into an applicaon, and into a layout.
Creating a restaurant review application
In the previous chapter, we built an ordering-in applicaon. In this chapter, we're going
to take a look at reviewing restaurants. The applicaon will allow the user to view other
people's opinions on the restaurant, a gallery of photos of the restaurant, and nally
a secon for making an online reservaon. We will divide the applicaon into three secons:
Review: Review and rangs informaon for this restaurant
Photos: A photo gallery of the restaurant
Reservaon: Request a reservaon with the restaurant
When building an applicaon where all three of these secons need to be quickly available
to the user, the most sensible opon available is to place each of the secons in a tab on the
screen. This allows the user to switch between the three secons without having all of them
on the screen at the same me. This also saves screen real estate giving us more space for
each secon.
The Review tab will include a cycling list of comments that people have made about the
restaurant being viewed, and an average "star" rang 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 Reservaon tab, we will want to capture the user's name and when they would like
the reservaon to be (date and me). Finally we also need to know for how many people
the reservaon 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 lile dierent 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 o with a skeleton that will later become our tab
layout structure. This can be lled with the three content areas.
Downloa d f r o m W o w ! e B o o k < w w w.woweb o o k . c o m >

Chapter 3
[ 69 ]
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 le in an editor or IDE.
3. Clear out the default code (leaving in the XML header).
4. Create a root FrameLayout element:
<FrameLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="fill_parent">
5. Inside the new FrameLayout element, add a vertical LinearLayout:
<LinearLayout android:id="@+id/review"
android:orientation="vertical"
android:layout_width="fill_parent"
android:layout_height="wrap_content">
</LinearLayout>
6. Aer the LinearLayout, add another empty LinearLayout element:
<LinearLayout android:id="@+id/photos"
android:orientation="vertical"
android:layout_width="fill_parent"
android:layout_height="wrap_content">
</LinearLayout>
7. Then, aer the second LinearLayout element, add an empty ScrollView:
<ScrollView android:id="@+id/reservation"
android:layout_width="fill_parent"
android:layout_height="fill_parent">
</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 secons, and a ScrollView for
the Reservaon tab.

Developing with Specialized Android Widgets
[ 70 ]
What just happened?
We've just started the "restaurant review" applicaon, building a skeleton for the user
interface. There are several key parts of this main.xml le which we should walk through
before connuing the example.
First, our root element is a FrameLayout. The FrameLayout anchors all of its children to its
own top-le corner. In eect, 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
relave tab is acve.
Second, each of the LinearLayout and the ScrollView have an ID. In order to idenfy
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 connue, we need our Activity class to set up the three tab content
elements we declared in our main.xml le 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 applicaons have a specic look and feel dened by the default widgets provided
by the system. In order to keep all applicaons consistent for users, there are a set of user
interface guidelines that applicaon developers should follow. While it's important to have
your applicaon stand out, users will oen get frustrated with applicaons that are not
familiar or look out of place (this is one of the reasons why automacally ported applicaons
are oen very unpopular).

Chapter 3
[ 71 ]
Android tabs and icons
When selecng tab icons for your applicaon, it's considered a good pracce to include
several dierent versions of the icon for dierent screen sizes and densies. The an-aliased
corners that look so good on a high-density screen, look terrible on low-density screens.
You can also provide enrely dierent 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 eect 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 les:
The selected icon image
The unselected icon image
An XML le 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). Dierent variaons of the image should be used for screens of
dierent pixel densies (see Chapter 1, Developing a Simple Acvity for "Resource Selecon"
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 applicaon 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 le res/drawable/ic_tab_unselstar.png. In order to switch
states between these two images automacally, we dene a special StateListDrawable
as an XML le. Hence the Review icon is actually in a le named res/drawable/review.
xml, and it looks like this:
<selector xmlns:android="http://schemas.android.com/apk/res/android"
android:constantSize="true">
<item
android:drawable="@drawable/ic_tab_selstar"
android:state_selected="false"/>
<item
android:drawable="@drawable/ic_tab_unselstar"
android:state_selected="true"/>
</selector>

Developing with Specialized Android Widgets
[ 72 ]
Note the android:constantSize="true" of the <selector> element. By default,
Android will assume that each state in the resulng StateListDrawable object will cause
the image to be of a dierent size, which in turn may cause the user interface to re-run its
layout calculaons. 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 les: A PNG for the selected
icon, a PNG for the unselected icon, and an XML le dening the state-selector. From our
applicaon, we will only make direct use of the state-selector XML les, leaving the Android
APIs to pickup the actual PNG les.
Implementing the ReviewActivity
As usual, we will want to have localized text in our strings.xml le. Open the res/
values/strings.xml le and copy the following code into it:
<resources>
<string name="app_name">Robotic Review</string>
<string name="review">Review</string>
<string name="gallery">Photos</string>
<string name="reservation">Reservations</string>
</resources>
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
liing for us, providing us with a ready-made TabHost object with which we can construct
the Activity tab structure.
1. Open the ReviewActivity.java le 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 ulity) completely.
4. Now start by fetching the TabHost object from your parent class:
TabHost tabs = getTabHost();

Chapter 3
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5. Next, we inate 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 applicaon resources:
Resources resources = getResources();
7. Now we dene 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. Dene two more TabSpec variables for the Photos and Reservaon tabs using the
preceding paern.
9. Add each of the TabSpec objects to our TabHost:
tabs.addTab(details);
tabs.addTab(gallery);
tabs.addTab(reservation);
This concludes the creaon 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 inated
the layout XML le 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 aached to the tab on the screen
(assigned using the setContent method). In this example, we opted for the simplest opon
and dened the tab content in our main.xml le. 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 opon.
You'll noce that the TabSpec (much like the StringBuilder class) supports chaining
of method calls, making it easy and exible 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).

Developing with Specialized Android Widgets
[ 74 ]
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 customizaon of the tab's look and feel.
To keep the example simple (and compable 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 rst tab is tled Review,
and this is where we are going to start. We've just nished enough Java code to load up the
tabs and put them on the screen. Now we go back to the main.xml layout le and populate
this tab with some widgets that supply the user with review informaon.
1. Open res/layout/main.xml in an editor or IDE.
2. Inside the <LayoutElement> that we named review, add a new TextView that
will contain the name of the restaurant:
<TextView android:id="@+id/name"
android:textStyle="bold"
android:textSize="25sp"
android:textColor="#ffffffff"
android:gravity="center|center_vertical"
android:layout_width="fill_parent"
android:layout_height="wrap_content"/>
3. Below the new TextView, add a new RatingBar, where we will display how other
people have rated the restaurant:
<RatingBar android:id="@+id/stars"
android:numStars="5"
android:layout_width="wrap_content"
android:layout_height="wrap_content"/>
4. Keeping this rst tab simple, we add a TextSwitcher where we can display other
people's comments about the restaurant:
<TextSwitcher android:id="@+id/reviews"
android:inAnimation="@android:anim/fade_in"
android:outAnimation="@android:anim/fade_out"
android:layout_width="fill_parent"
android:layout_height="fill_parent"/>
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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What just happened
We just put together the layout for our rst 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 t on the screen. If you
want control over how many stars appear on your RatingBar, sck 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 animaons, so we specify the "in" animaon as
the default fade_in as provided by the android package, while the "out" animaon
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 dierent 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 animaon. TextSwitcher needs two TextView objects (which it will ask us to
create dynamically), for our example. We want these to be in a resource le.
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
applicaon resources. Open the res/values/strings.xml le and add <string-array
name="comments"> with a few likely comments in it:
<string-array name="comments">
<item>Just Fantastic</item>
<item>Amazing Food</item>
<item>What rubbish, the food was too hairy</item>
<item>Messy kitchen; call the health inspector.</item>
</string-array>

Developing with Specialized Android Widgets
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Time for action – turning on the TextSwitcher
We want the TextSwitcher to display the next listed comment every ve seconds. For
this we'll need to employ new resources, and a Handler object. A Handler is a way for
Android applicaons 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 le in your res/layout directory named review_comment.xml.
2. Copy the following code into the new review_comment.xml le:
<TextView
xmlns:android="http://schemas.android.com/apk/res/android"
android:gravity="left|top"
android:textStyle="italic"
android:textSize="16sp"
android:padding="5dip"
android:layout_width="fill_parent"
android:layout_height="wrap_content"/>
3. Open the ReviewActivity.java le 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, aer 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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10. Next, nd the TextSwitcher and assign it to the switcher eld:
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 specicaon, we need to write a
makeView method. In our case it's really simple—inate 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 rst med 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 ve 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 creang Thread objects means all of the med tasks
can share the main user interface thread instead of each allocang a separate thread. This
reduces the amount of memory and CPU load your applicaon places on the device, and has
a direct impact on the applicaon performance and baery life.
www.allitebooks.com

Developing with Specialized Android Widgets
[ 78 ]
What just happened?
We just built a simple mer structure to update the TextSwitcher with a rotang array of
comments. The Handler class is a convenient way to post messages and acons between
two applicaon threads. In Android, as with Swing, the user interface is not thread-safe, so
inter-thread communicaon becomes very important. A Handler object aempts 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 inhering 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 calculaon),
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 interacon 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 lile misleading, it's really a horizontal row of items with
a "single item" selecon 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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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 sck to a fairly tradional 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:
<Gallery android:id="@+id/gallery"
android:layout_width="fill_parent"
android:layout_height="wrap_content"/>
3. Gallery objects, by default, squash their contents together, which really doesn't
look great in our case. You can add a lile padding between the items by using the
spacing aribute 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:
<ImageView android:id="@+id/photo"
android:layout_width="fill_parent"
android:layout_height="fill_parent"/>
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.

Developing with Specialized Android Widgets
[ 80 ]
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 selecon. 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 aribute 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 le gallery_thn.xml and
copy the following code into it:
<ImageView xmlns:android="http://schemas.android.com/apk/res/android"
android:scaleType="fitXY"/>
That's right, it has just two lines of XML, but to reiterate, this allows us to customize this
widget for many dierent conguraons without eding the Java code. While eding 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 sck to using applicaon resources to keep things simple. We'll
have two arrays of resource IDs, thumbnails, and the full-size images. An Adapter
implementaon is expected to provide an idener for each of the items. In this next
example, we're going to provide an idener as the resource idener of the full-size
image, which gives us easy access to the full-size image in classes outside of the Adapter
implementaon. While this is an unusual contract, it provides a convenient way for us to
pass the image resource around within an already dened 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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Time for action – the GalleryAdapter
Creang the GalleryAdapter is much like the ListAdapter classes we created in Chapter
2, Presenng 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 menoned 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];
}

Developing with Specialized Android Widgets
[ 82 ]
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 funcons 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 aempt
to reuse any View object specied in its getView method. A primary dierence however,
is that this GalleryAdapter is enrely 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 modicaon to the gallery_thn.xml
resource le in order to specify the required size of each thumbnail.
<ImageView xmlns:android="http://schemas.android.com/apk/res/android"
android:maxWidth="128dip"
android:adjustViewBounds="true"
android:scaleType="centerInside"/>
The adjustViewBounds aribute tells the ImageView to adjust its own size in
a way such that it maintains the aspect rao of the image it contains. We also change the
scaleType aribute to centerInside, which will also retain the aspect rao of the
image when it scales. Finally, we set a maximum width for the ImageView. Using the
standard layout_width or layout_height aributes is ignored by the Gallery class,
so we instead specify the desired thumbnail size to the ImageView (the layout_width
and layout_height aributes are handled by the Gallery, while the maxWidth and
maxHeight are handled by the ImageView).

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This would be a standard speed/size trade-o. Having the thumbnail images takes up more
applicaon space, but having the ImageView perform the scaling makes the applicaon
slower. The scaling algorithm in ImageView will also not be as high-quality as the scaling
performed in an image-manipulaon applicaon such as Adobe Photoshop. In most cases
this won't be a problem, but if you have high detail images, you oen get "scaling arfacts"
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 declaraon 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, nd 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);
}

Developing with Specialized Android Widgets
[ 84 ]
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 idener 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 full-
size 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
aempt to select the rst item returned from its new Adapter. This in turn noes 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
red immediately, but rather some me aer we return from the onCreate method. When
we call setAdapter(new GalleryAdapter()) on the Gallery object, it schedules a
selecon change event, which we then receive. The event causes the ReviewActivity
class to display the rst photo in the GalleryAdapter object.
If you now reinstall the applicaon 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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Pop quiz
1. What would happen in the previous example if you substuted
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 unl a thumbnail is clicked.
2. What is the primary dierence between the ScaleType values fitXY
and centerInside?
a. The fitXY type will anchor the picture to the top-le, 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 rao.
c. centerInside will cause the larger axis to be cropped in order to t 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 aribute?
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 aributes).
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 applicaon resources.
1. Change the ImageView object of full-size images to an ImageSwitcher, use the
standard Android fade-in/fade-out animaons.
2. Remove the thumbnail images from the project, and use the ImageView declared
in the gallery_thn.xml le to scale the images.
3. Change from a list of applicaon resource IDs to a list of Uri objects so that the
images are downloaded from an external website.
Downloa d f r o m W o w ! e B o o k < w w w.woweb o o k . c o m >

Developing with Specialized Android Widgets
[ 86 ]
Building the reservation tab
While the Review and Photos tabs of this example have been concerned with displaying
informaon, the Reservaon tab will be concerned with capturing the details of a
reservaon. We really only need three pieces of informaon:
The name under which the reservaon needs to be made
The date and me of the reservaon
How many people the reservaon is for
In this part of the example we'll create several widgets which have formaed 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 specied in the
layout le) will contain the format to use for display. As part of the inializaon 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 inial 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
Reservaon tab. Currently it consists only of an empty ScrollView, which enables
vercally-long layouts to be scrolled by the user if the enre user interface doesn't t
on the screen.
1. Open the main.xml le in your editor or IDE.
2. Inside the <ScrollView> we had created for the Reservation tab earlier. Declare
a new vercal LinearLayout element:
<LinearLayout android:orientation="vertical"
android:layout_width="fill_parent"
android:layout_height="wrap_content">
3. Inside the new LinearLayout element, create a TextView to ask the user under
what name the reservaon should be made:
<TextView android:text="Under What Name:"
android:layout_width="fill_parent"
android:layout_height="wrap_content"/>
4. Aer the TextView label, create an EditText to allow the user to input the name
under which reservaon is to be made:
<EditText android:id="@+id/name"
android:layout_width="fill_parent"
android:layout_height="wrap_content"/>

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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:
<TextView android:id="@+id/people_label"
android:text="How Many People: %d"
android:layout_width="fill_parent"
android:layout_height="wrap_content"/>
6. Add a SeekBar with which the user can tell us about how many people are going:
<SeekBar android:id="@+id/people"
android:max="20"
android:progress="1"
android:layout_width="fill_parent"
android:layout_height="wrap_content"/>
7. Use another TextView to ask the user what date the reservaon will be on:
<TextView android:text="For What Date:"
android:layout_width="fill_parent"
android:layout_height="wrap_content"/>
8. Add a Button to display the date for which the reservaon is made. When the user
taps this Button, we will ask him to select a new date:
<Button android:id="@+id/date"
android:text="dd - MMMM – yyyy"
android:layout_width="fill_parent"
android:layout_height="wrap_content"/>
9. Create another TextView label to ask the me of reservaon:
<TextView android:text="For What Time:"
android:layout_width="fill_parent"
android:layout_height="wrap_content"/>
10. Add another Button to display the me, and allow the user to change it:
<Button android:id="@+id/time"
android:text="HH:mm"
android:layout_width="fill_parent"
android:layout_height="wrap_content"/>
11. Finally add a Button to make the reservaon, and add some margin to separate it
from the rest of the inputs in the form:
<Button android:id="@+id/reserve"
android:text="Make Reservation"
android:layout_marginTop="15dip"
android:layout_width="fill_parent"
android:layout_height="wrap_content"/>

Developing with Specialized Android Widgets
[ 88 ]
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, me, or the number of people expected for
the reservaon.
What just happened?
In the Reservaon tab, we ask the user how many people the reservaon 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 selecng the number
of people for the reservaon, as long as that number is within a range that we dene.
SeekBar in Android is actually built on top of the ProgressBar class, and so inherits all of
its XML aributes, which will seem a lile strange at mes. Unfortunately, unlike a JSlider
or JProgressBar, the SeekBar class has no minimum value, and since you can't make a
reservaon 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 (seng the displayed value
to 2 people).
Most people would make a restaurant reservaon
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 reservaon 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 "me" Button labels, we want to display the currently
selected date and me for the reservaon. We set the label in the XML le 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 internaonalizaon in our previous example? Shouldn't we have put the labels
in the strings.xml le so that the layout doesn't need to change? The answer is: Yes, if
you want to internaonalize your user interface. Later, make sure you have all of your display
text in an applicaon resource le. However, I strongly recommend fetching the format
strings directly from the layout, since it allows you to decouple the format data
one addional level.
In the preceding layout, you created Button widgets to display the date and me. Why not
use a DatePicker and TimePicker object directly? The answer is: They unfortunately
don't t well into normal layouts. They take up a large amount of vercal 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 le, while the actual user interface is the
screenshot on the right.

Chapter 3
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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
situaon. 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 dramacally improves the look and feel of the applicaon. User interfaces that are not
properly aligned will irritate users, even if they can't tell why it's irritang. Screens that users
nd annoying or irritang are screens that they will avoid, or worse—simply uninstall.
Time for action – initializing the reservation tab
In the Reservaon tab we made use of formaed 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 funconality to remember the
format, and populate the label.
1. Open the ReviewActivity Java source in your editor or IDE.
2. Below of all the elds you've declared so far, we need to add some more for the
Reservaons tab. Declare a String to remember the formang of the How Many
People: label:
private String peopleLabelFormat;
3. Then declare a reference to the How Many People: label:
private TextView peopleLabel;

Developing with Specialized Android Widgets
[ 90 ]
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 inialize the Reservaons 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 me, 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 reservaon 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 me of day for a reservaon 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 buon 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 applicaon 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
reservaon parameters.
What just happened?
The Reservaons tab will now be populated with the default data for a reservaon, and
all the formang in the labels has disappeared. You will probably have noced 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 tradional 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 interacve 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. Open the ReviewActivity Java source in your editor or IDE.
2. Add SeekBar.OnSeekBarChangeListener to the interfaces that
ReviewActivity implements.
3. In onCreate, aer fetching the SeekBar with findViewById, set its
OnSeekBarChangeListener to this:
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 dierent to the normal java.text.MessageFormat
class, it's the preferred method in Android (although MessageFormat is sll supported).
What just happened?
When you reinstall the applicaon in the emulator, you'll now be able to use SeekBar
to select the number of people that the reservaon 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 reservaon 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 dierent date
or me for their reservaon. 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 declaraon 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 ulity method to parse a CharSequence into a Calendar object with
a specied SimpleDateFormat:
private Calendar parseCalendar(
CharSequence text, SimpleDateFormat format) {
4. Open a try block to allow handling of parse errors if the CharSequence is not
formaed 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 me on the Calendar object to the me 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, aer seng 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 me, 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 formaed Calendar:
date.setText(dateFormat.format(calendar.getTime()));
22. Implement the onTimeSet method to listen for when the user accepts the
TimePickerDialog aer selecng a new me:
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 elds 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 formang 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
me we update the Button labels to reect the new selecons.

Developing with Specialized Android Widgets
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What just happened
If you install and run the applicaon 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 applicaon. You should avoid using them for displaying status messages as they
eecvely render the rest of the applicaon useless during that me. If you do display a
modal Dialog, ensure that there is some way for the user to dismiss the Dialog without
any other interacon (that is, a Cancel buon or something similar).
The rst advantage to using a DatePickerDialog and TimePickerDialog comes
from the fact that both include Set and Cancel buons. 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 buon, but this would
take up addional screen space, and generally would seem out-of-place (unl 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 tle 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 eld that is noceably missing from the DatePicker widget,
which makes it surprisingly dicult 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 beer choice for date selecon than an inline DatePicker.
Creating complex layouts with Include, Merge, and
ViewStubs
In this chapter we've built a single layout resource with three dierent tabs in it. As a result
of this, the main.xml le has become quite large and hence, more dicult to manage.
Android provides several ways in which you can break up large layout les (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
le into another. For our previous example, we could separate each tab out into its own
layout resource le, and then include each one in the main.xml. The include tag has
only one mandatory aribute: layout. This aribute points to the layout resource to be
included. This tag is not a stac or compile-me tag, and so the included layout le will be
selected through the standard resource selecon process. This allows you to have a single
main.xml le, but then add a special reviews.xml le (perhaps for Spanish).
The layout aribute on the include tag is not prexed with the android XML namespace.
If you aempt to use the layout aribute as android:layout, you won't get any compile-
me errors, but your applicaon will strangely fail to run.
The include element can also be used to assign or override several aributes of the root
included element. These include the element android:id, and any of the android:
layout aributes. This allows you to reuse the same layout le in several parts of your
applicaon, but with dierent layout aributes and a dierent ID. You can even include the
same layout le several mes on the same screen, but with a dierent ID for each instance.
If we were to change our main.xml le to include each of the tabs from other layout
resources, the le would look something more like this:
<?xml version="1.0" encoding="UTF-8"?>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/
android"
android:layout_width="fill_parent"
android:layout_height="fill_parent">
<include
android:id="@+id/review"
layout="@layout/review"/>
<include
android:id="@+id/photos"
layout="@layout/photos"/>
<include
android:id="@+id/reservation"
layout="@layout/reservations"/>
</FrameLayout>
Merging layouts
The include element is very ne and well when you want to include a single View or
ViewGroup into a larger layout structure. However, what if you want to include mulple
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.

Developing with Specialized Android Widgets
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However, having an addional ViewGroup just for the sake of an include can adversely
aect 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
<merge>. 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
le named main.xml, with a LinearLayout that included a user_editor.xml
layout resource, then the code would look something like this:
<LinearLayout android:orientation="vertical">
<include layout="@layout/user_editor"/>
<Button android:id="@+id/save"
android:text="Save User"
android:layout_width="fill_parent"
android:layout_height="wrap_content"/>
</LinearLayout>
The simple implementaon of the user_editor.xml looks something like this:
<LinearLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="fill_parent"
android:layout_height="wrap_content">
<TextView android:text="User Name:"
android:layout_width="fill_parent"
android:layout_height="wrap_content"/>
<EditText android:id="@+id/user_name"
android:layout_width="fill_parent"
android:layout_height="wrap_content"/>
<!-- the rest of the editor -->
</LinearLayout>
However, when this is included into the main.xml le, we embed the user_editor.
xml LinearLayout into the main.xml LinearLayout, resulng in two LinearLayout
objects with idencal layout aributes. Obviously it would be much beer to simply put
the TextView and EditView from user_editor.xml directly into the main.xml
LinearLayout element. This is exactly what the <merge> tag is used for. If we now re-write
the user_editor.xml le using the <merge> tag instead of a LinearLayout, it looks
like this:
<merge xmlns:android="http://schemas.android.com/apk/res/android">
<TextView android:text="User Name:"
android:layout_width="fill_parent"
android:layout_height="wrap_content"/>

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<EditText android:id="@+id/user_name"
android:layout_width="fill_parent"
android:layout_height="wrap_content"/>
<!-- the rest of the editor -->
</merge>
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 le. Beware of
layouts that have too many nested ViewGroup objects, as they are almost certain to give
trouble (more than about ten levels of nesng is likely to cause your applicaon 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 applicaon 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 aach 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 les. In situaons with very
large layout structures, this can consume a huge amount of your applicaon memory, and
even cause your applicaon 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-inialize
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 unl 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 aributes set on a ViewStub will be passed on to its inated View object. You
can also assign a separate ID to the inated layout. If we wanted to include each of our tabs
in a ViewStub, the main.xml le would look something like this:
<?xml version="1.0" encoding="UTF-8"?>
<FrameLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="fill_parent">
<ViewStub android:id="@+id/review"
Downloa d f r o m W o w ! e B o o k < w w w.woweb o o k . c o m >

Developing with Specialized Android Widgets
[ 100 ]
android:inflatedId="@+id/inflated_review"
android:layout="@layout/review"/>
<ViewStub android:id="@+id/photos"
android:inflatedId="@+id/inflated_photos"
android:layout="@layout/photos"/>
<ViewStub android:id="@+id/reservations"
android:inflatedId="@+id/inflated_reservations"
android:layout="@layout/reservations"/>
</FrameLayout>
Note that unlike the include tag, the ViewStub requires the android XML namespace for
its layout aribute. Aer 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
inated layout object using the android:inflatedId reference.
Have a go hero – separate the tabs
Extract each of the tabs into its own layout resource le, 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 specic
tab's content.
Summary
Tabs are a great way of breaking an Activity into dierent 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 me.
The RatingBar and SeekBar are two dierent methods of capturing, or displaying numeric
data to the user. While they are closely related, and both funcon in the same way, each
class is used to address dierent types of data. Keep in mind the limitaons 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 dierent
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 funcon is to change the View
objects returned from the Adapter implementaon.

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When it comes to date and me capturing, try to sck 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
beer 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 acvity stack, and the
lifecycle of an Android applicaon. We'll invesgate how Intent objects and the acvity
stack can be used as a way to keep applicaons more usable. Also, we shall learn about
improving the reuse of Activity classes.

4
Leveraging Activities and Intents
In many ways Android applicaon 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 nd 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 lile deeper for.
The Acvity Stack is much like a single-direconal web browser history. When
you launch an Activity using the startActivity method, you eecvely
hand control back to the Android system. When the user pushes the hardware
"Back" buon on their phone, the default acon is to pop the top Activity o
the stack, and display the one underneath (not always the one that started it).
In this chapter we'll explore a lile of how Android runs an applicaon 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 applicaon, and allow you to reuse more of your applicaon 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
pracces to follow when building an Activity class, and how to behave nicely within the
connes of an Android applicaon.

Leveraging Acvies and Intents
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We've already encountered the "Acvity Stack" in Chapter 1, Developing a Simple Acvity
and Chapter 2, Presenng Data for Views where we constructed Intent objects to
launch specic Activity classes. When you used the hardware "Back" buon, you were
automacally 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 applicaon state
Exploring the relaonship 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
applicaon. 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
specicaons. However, most of them appear underpowered when compared to the top-of-
the-range devices. For those devices that do have good specicaons, 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 applicaons and services sharing a very limited device.
An Activity may be garbage-collected any me it is not visible to the user. This means
it may be your applicaon that is running, but because the user is looking at a dierent
Activity, any non-visible or background Activity objects may be shut down or garbage-
collected 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 applicaon with two Activity
instances is shown in the following gure. When the "Main Acvity" is paused, it becomes
eligible for garbage-collecon by the system. If this happens, it will rst store its state in a
temporary locaon, 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 aempt to store their state before they are made available for
garbage-collecon. However, this state is only stored for the lifeme of
the applicaon. When the applicaon 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 eecvely trying to take the
control away from Android, which will always create problems for you. If for example, you
developed an applicaon with only one Activity class, and used mulple layout resources
or your own custom ViewGroup objects to represent dierent screens, you would also have
to take control of the hardware "Back" buon on the device in order to allow the user to
go backwards. Your applicaon is released in the Android market, and a few months later a
handset manufacturer decides to put a "Forward" buon onto their new phone (in the same
style as the "Forward" buon on a web-browser). The Android system would be patched to
handle this change to the device, but your applicaon would not be. As a result, your users
get frustrated with your applicaon because "it doesn't work properly".
Using Bundle objects
In the onCreate method of the Activity class, we've been accepng a Bundle parameter
named saveInstanceState, as you may have guessed. It's where state informaon 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 conguraon 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 exisng
instance, and then creates a new instance of the Activity (with the new conguraon
parameters) with the Bundle that the state informaon was saved in.
The Bundle class is eecvely a Map<String, ?> 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 serializaon 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 me in a user interface, you don't need to store the layout
informaon, since that can be recreated from the layout le.
Being a key-value structure, a Bundle is more future-proof and exible than a
serialized object. You can leave out values that are set to their defaults, reducing
the size of the Bundle.

Leveraging Acvies and Intents
[ 106 ]
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 informaon 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 Acvity crashes
If an Activity class throws an uncaught excepon, the user will get
the dreaded Force Close dialog box. Android will aempt to recover
from these errors by terminang the Virtual Machine, and re-opening
the root acvity, 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 menoned earlier, the Activity class' default
funconality will aempt to save each View object with an ID within a Bundle. This is
another good reason to sck 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 cluers 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 buon to tell the applicaon
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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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 starng 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 le 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:
<TextView android:text=
"I'm thinking of a number between 1 and 10. Can you guess what
it is?"
android:layout_width="fill_parent"
android:layout_height="wrap_content"/>
5. Create a new EditText where the users will enter their guess. Use the
android:numeric aribute of TextView to enforce only integer input:
<EditText
android:id="@+id/number"
android:numeric="integer"
android:layout_width="fill_parent"
android:layout_height="wrap_content"/>
6. Add a Button that the users can click on to submit their guess:
<Button android:id="@+id/guess"
android:text="Guess!"
android:layout_width="fill_parent"
android:layout_height="wrap_content"/>
7. Now open the GuessActivity.java le in your editor or IDE.
8. Make the GuessActivity class implement OnClickListener:
public class GuessActivity
extends Activity implements OnClickListener {
9. Create a eld variable to store the number the user is supposed to guess:
private int number;

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10. Create a ulity 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 aer 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 aempt to fetch the stored Number from it:
number = savedInstanceState.getInt("Number", random());
13. If the Bundle is null, the Acvity 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 rst 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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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 automacally disappear aer 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 enrely non-interacve.
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 le, the call to super.onSaveInstanceState will handle
the storage of the EditText widget's exact state (potenally including "selecon" and
"focus" state).

Leveraging Acvies and Intents
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In the onCreate method, the example rst checks to make sure that the Bundle is not
null. If Android is aempng 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 aempt 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, eliminang the need for
us to check such a condion.
As a quick side-note, the example made use of the android:numeric aribute 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 aects the so-
keyboard. 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 acon (along with some parameter data), while
not specifying how the acon 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 acon. It may use the default "browser" applicaon, 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 applicaon may depend on the exact implementaon 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 acon we want carried out. Generally, an implicit Intent
will have much more informaon content, due to the following reasons:
To allow the system to make a good selecon 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 oen requires more informaon about how it is
expected to behave
Intent objects are what really make Android dierent from other (more tradional)
operang systems. They level the playing eld between applicaons, 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 applicaon.
Each Activity instance holds onto the Intent object that started it. In Chapter 1,
Developing a Simple Acvity, we made use of the Activity.getIntent() method
to fetch some parameters from the Intent object, which in turn told us which queson
to ask the user.
Dening Intent actions
The rst thing looked at in an implicit Intent is its acon. The acon denes what the
Intent "does", but not how it does it, or what it does it to. The Intent class denes a long
series of constants which represent common acons. These common acons always have
some form of backing logic, which is generally dened by the phone system. Thus they are
always available to be used by an applicaon.
For example, you wanted to present the users with the dialler applicaon, 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 acon value of an Intent is matched against one of the acons dened for
an Activity. An Activity may have any number of acons that it may perform,
and they're all specied as part of an applicaon's AndroidManifest.xml le. For
example, you wanted to dene an askQuestion acon and bind it to an Activity,
your AndroidManifest.xml le would contain an Activity entry which would look
something like this:
<activity
android:name=".AskQuestionActivity"

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android:label="Ask Question">
<intent-filter>
<action android:name="questions.askQuestion"/>
<category android:name="android.intent.category.DEFAULT"/>
</intent-filter>
</activity>
An Activity may have any number of <intent-filter> elements, each dening a
dierent type of match to perform on an Intent. The Activity with the closest match
to any given Intent is chosen to perform the acon requested by the Intent object.
Passing data in an Intent
Presenng the user with the dialler applicaon 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 acon, it also provides a default space for us to tell it what
we want the acon 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 555-
1234"? 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
Somemes a Uri doesn't allow enough data to be specied. 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 Acvity, we used the extra data to keep track of which queson we
were asking the user.
When dening generic Activity classes (such as le viewers), it's a good idea to work on
a three phase fall-back system when looking for operaonal data:
Any custom (non-standard) parameters can be passed in extra elds (and none of
them should be mandatory)
Inspect the data Uri to see what informaon you should be working with
If no data Uri is specied, fall-back gracefully to a logical default, and provide some
funconality to the user

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Have a go hero – generic questions and answers
Go back to the example queson and answer applicaon from Chapter 1, Developing a
Simple Acvity. Rework the QuestionActivity class to use the data Uri to specify the
queson ID (by name) instead of the extra parameters.
Also, allow for the full queson to be passed in using "extra" parameters—a parameter
Question for the queson text to ask the user, and a parameter Answers, specifying
a string array of possible answers to the given queson.
Using advanced Intent features
An Intent object is designed to indicate a single acon as requested by the user. It's a self-
contained request, and in some ways it is quite similar to an HTTP request, containing both,
the acon to carry out, and the resource upon which the acon should be carried out, and
any addional informaon that may be required.
In order to nd the Activity (service or broadcast receiver) that will handle an
Intent, the system makes use of intent-lters (as we discussed briey earlier). Each
intent-lter indicates a single type of acon that could be carried out by the Activity.
When two or more Activity implementaons 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 implementaons should be used to handle the Intent. The default
behavior is to ask the users which of the Activity implementaons they wish to use.
Getting data back from an Intent
An Intent is not alwa