Based App Programming Guide For Mac Doc OSX

DocBasedAppProgrammingGuideForOSX

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Document-Based App Programming Guide for Mac

Document-Based App

Programming Guide for

Mac

Contents

About the Cocoa Document Architecture 7

At a Glance 7

The Model-View-Controller Pattern Is Basic to a Document-Based App 8

Xcode Supports Coding and Configuring Your App 8

You Must Subclass NSDocument 8

NSDocument Provides Core Behavior and Customization Opportunities 8

Prerequisites 9

See Also

See Document-Based App Programming Guide for iOS for information about how to develop a document-based

app for iOS using the UIDocument class.

For information about iCloud, see iCloud Design Guide.

File Metadata Search Programming Guide describes how to conduct searches using the NSMetadataQuery

class and related classes. You use metadata queries to locate an app’s documents stored in iCloud.

For information about how to publish your app in the App Store, see App Distribution Guide .

About the Cocoa Document Architecture

Prerequisites

Documents are containers for user data that can be stored in files locally and in iCloud. In a document-based

design, the app enables users to create and manage documents containing their data. One app typically

handles multiple documents, each in its own window, and often displays more than one document at a time.

For example, a word processor provides commands to create new documents, it presents an editing environment

in which the user enters text and embeds graphics into the document, it saves the document data to disk or

iCloud, and it provides other document-related commands, such as printing and version management. In

Cocoa, the document-based app design is enabled by a subsystem called the document architecture, which

is part of of the AppKit framework.

Documents in OS X

There are several ways to think of a document. Conceptually, a document is a container for a body of information

that can be named and stored in a file. In this sense, the document is an object in memory that owns and

manages the document data. To users, the document is their information—such as text and graphics formatted

on a page. In the context of Cocoa, a document is an instance of a custom NSDocument subclass that knows

how to represent internally persistent data that it can display in a window. This document object knows how

to read document data from a file and create an object graph in memory for the document data model. It also

knows how to modify that data model consistently and write the document data back out to disk. So, the

document object mediates between different representations of document data, as shown in Figure 1-1.

Figure 1-1 Document file, object, and data model

Designing a Document-Based App

Using iCloud, documents can be shared automatically among a user’s computers and iOS devices. The system

synchronizes changes to the document data without user intervention. See Storing Documents in iCloud (page

12) for more information.

The Document Architecture Provides Many Capabilities for Free

The document-based style of app is one design choice among several that you should consider when you

design your app. Other choices include single-window utility apps, such as Calculator, and library-style “shoebox”

apps, such as iPhoto. It’s important to choose the basic app style early in the design process because

development takes quite different paths depending on that choice. If it makes sense for your users to create

multiple discrete sets of data, each of which they can edit in a graphical environment and store in files, then

you should plan to develop a document-based app.

The Cocoa document architecture provides a framework for document-based apps to do the following things:

●Create new documents. The first time the user chooses to save a new document, it presents a dialog in

which the user names and saves the document in a disk file in a user-chosen location.

●Open existing documents stored in files. A document-based app specifies the types of document files

it can read and write, as well as read-only and write-only types. It can represent the data of different

document types internally and display the data appropriately.

●Automatically save documents. Document-based apps can adopt autosaving in place, and its documents

are automatically saved at appropriate times so that the data the user sees on screen is effectively the

same as that saved on disk. Saving is done safely, so that an interrupted save operation does not leave

data inconsistent. To avoid automatic saving of inadvertent changes, old files are locked from editing until

explicitly unlocked by the user.

●Asynchronously read and write document data. Reading and writing are done asynchronously on a

background thread, so that lengthy operations do not make the app’s user interface unresponsive. In

addition, reads and writes are coordinated using the NSFilePresenter protocol and the

NSFileCoordinator class to reduce version conflicts.

●Manage multiple versions of documents. Autosave creates versions at regular intervals, and users can

manually save a version whenever they wish. Users can browse versions and revert the document’s contents

to a chosen version using a Time Machine–like interface. The version browser is also used to resolve version

conflicts from simultaneous iCloud updates.

●Print documents. Users can specify various page layouts in the print dialog and page setup dialog.

●Track changes and set the document’s edited status. The document manages its edited status and

implements multilevel undo and redo.

●Validate menu items. The document enables or disables menu items automatically, depending on its

edited status and the applicability of the associated action methods.

Designing a Document-Based App

The Document Architecture Provides Many Capabilities for Free

●Handle app and window delegation. Notifications are sent and delegate methods called at significant

life cycle events, such as when the app terminates.

Cocoa’s document architecture implements most of its capabilities in three classes. These classes interoperate

to provide an extensible app infrastructure that makes it easy for you to create document-based apps. Table

1-1 briefly describes these classes.

Table 1-1 Primary classes in the document architecture

PurposeClass

Creates, presents, and stores document dataNSDocument

Manages a window in which a document is displayedNSWindowController

Manages all of the document objects in the appNSDocumentController

See The Classes That Support Document-Based Apps (page 16) for more detailed information.

Storing Documents in iCloud

The iCloud storage technology enables you to share documents and other app data among multiple computers

that run your document-based app. If you have a corresponding iOS version of your app, you can share your

documents and app data with your iOS devices as well. Once your app sets up the proper connections, iCloud

automatically pushes documents and changes to all the devices running an instance of your app with no

explicit user intervention.

There are two kinds of storage in iCloud: document storage and key-value data storage. Document storage is

designed for storing large amounts of data such as that in a document file. Key-value storage is designed for

small amounts of app data such as configuration data. For example, you might store the text and illustrations

for a book in document storage, and you might store the reader’s page location in key-value storage. That

way, whenever the user opens the document on any device, the correct page is displayed.

Documents and key-value data designated for storage in iCloud are transferred to iCloud and to the user’s

other computers as soon as possible. On iOS devices, only file metadata is transferred from iCloud to devices

as soon as possible, while the file data itself is transferred on demand. Once data has been stored initially in

iCloud, only changes are transferred thereafter, to make synchronization most efficient.

NSDocument implements file coordination, version management, and conflict resolution among documents,

so it provides the easiest path to using iCloud. For details explaining how to handle document storage in

iCloud, see Moving Document Data to and from iCloud (page 40).

Designing a Document-Based App

Storing Documents in iCloud

The Document Architecture Supports App Sandbox

The document architecture helps document-based apps adopt App Sandbox, an access control technology

that provides a last line of defense against stolen, corrupted, or deleted user data if malicious code exploits

your app. The NSDocument class automatically works with Powerbox to make items available to your app

when the user opens and saves documents or uses drag and drop. NSDocument also provides support for

keeping documents within your sandbox if the user moves them using the Finder. For more information about

App Sandbox, see App Sandbox Design Guide.

Considerations for Designing Your Document Data Model

Your document data model is an object or graph of interconnected objects that contain the data displayed

and manipulated by your document objects.

Cocoa Uses the Model-View-Controller Design Pattern

The Cocoa document architecture and many other technologies throughout Cocoa utilize the

Model-View-Controller (MVC) design pattern. Model objects encapsulate the data specific to an app and

manipulate and process that data. View objects display data from the app’s model objects and enable the

editing of that data by users. Controller objects act as intermediaries between the app’s view objects and

model objects. By separating these behaviors into discrete objects, your app code tends to be more reusable,

the object interfaces are better defined, and your app is easier to maintain and extend. Perhaps most importantly,

MVC-compliant app objects fit seamlessly into the document architecture.

A Data Model Corresponds to a Document Type

A document object is a controller dedicated to managing the objects in the document’s data model. Each

document object is a custom subclass of NSDocument designed specifically to handle a particular type of data

model. Document-based apps are able to handle one or more types of documents, each with its own type of

data model and corresponding NSDocument subclass. Apps use an information property list file, which is

stored in the app’s bundle and named, by default, <appName>-Info.plist, to specify information that can

be used at runtime. Document-based apps use this property list to specify the document types the app can

edit or view. For example, when the NSDocumentController object creates a new document or opens an

existing document, it searches the property list for such items as the document class that handles a document

type, the uniform type identifier (UTI) for the type, and whether the app can edit or only view the type. For

more information about creating a property list for types of documents, see Complete the Information Property

List (page 29).

Designing a Document-Based App

The Document Architecture Supports App Sandbox

Data Model Storage

Any objects that are part of the persistent state of a document should be considered part of that document’s

model. For example, the Sketch sample app has a subclass of NSDocument named SKTDocument. Objects of

this class have an array of SKTGraphic objects containing the data that defines the shapes Sketch can draw,

so they form the data model of the document. Besides the actual SKTGraphic objects, however, the

SKTDocument object contains some additional data that should technically be considered part of the model,

such as the order of the graphics within the document’s array, which determines the front-to-back ordering

of the SKTGraphic objects.

Like any document-based app, Sketch is able to write the data from its data model to a file and vice versa. The

reading and writing are the responsibility of the SKTDocument object. Sketch implements the NSDocument

data-based writing method that flattens its data model objects into an NSData object before writing it to a

file. Conversely, it also implements the data-based NSDocument reading method to reconstitute its data model

in memory from an NSData object it reads from one of its document files.

There are three ways you can implement data reading and writing capabilities in your document-based app:

●Reading and writing native object types. NSDocument has methods that read and write NSData and

NSFileWrapper objects natively. You must override at least one writing method to convert data from

the document model’s internal data structures into an NSData object or NSFileWrapper object in

preparation for writing to a file. Conversely, you must also override at least one reading method to convert

data from an NSData or NSFileWrapper object into the document model’s internal data structures in

preparation for displaying the data in a document window. See Creating the Subclass of NSDocument (page

35) for more details about document reading and writing methods.

●Using Core Data. If you have a large data set or require a managed object model, you may want to use

NSPersistentDocument to create a document-based app that uses the Core Data framework. Core Data

is a technology for object graph management and persistence. One of the persistent stores provided by

Core Data is based on SQLite. Although Core Data is an advanced technology requiring an understanding

of Cocoa fundamental design patterns and programming paradigms, it brings many benefits to a

document-based app, such as:

●Incremental reading and writing of document data

●Data compatibility for apps with iOS and OS X versions

For more information, see Core Data Starting Point.

●Custom object formats. If you need to read and write objects without using NSData and NSFileWrapper,

you can override other NSDocument methods to do so, but your code needs to duplicate what NSDocument

does for you. Naturally, this means your code will have greater complexity and a greater possibility of

error.

Designing a Document-Based App

Considerations for Designing Your Document Data Model

Handling a Shared Data Model in OS X and iOS

Using iCloud, a document can be shared between document-based apps in OS X and iOS. However, there are

differences between the platforms that you must take into consideration. For an app to edit the same document

in iOS and OS X, the document-type information should be consistent. Other cross-platform considerations

for document-data compatibility are:

●Some technologies are available on one platform but not the other. For example, if you use rich text format

(RTF) as a document format in OS X, it won’t work in iOS because its text system doesn’t have built-in

support for rich text format (although you can implement that support in your iOS app).

●The default coordinate system for each platform is different, which can affect how content is drawn. See

“Default Coordinate Systems and Drawing in iOS” in Drawing and Printing Guide for iOS for a discussion

of this topic.

●If you archive a document’s model object graph, you may need to perform suitable conversions using

NSCoder methods when you encode and decode the model objects.

●Some corresponding classes are incompatible across the platforms. That is, there are significant differences

between the classes representing colors (UIColor and NSColor), images (UIImage and NSImage), and

Bezier paths (UIBezierPath and NSBezierPath). NSColor objects, for example, are defined in terms

of a color space (NSColorSpace), but there is no color space class in UIKit.

These cross-platform issues affect the way you store document data in the file that is shared between OS X

and iOS as an iCloud document. Both versions of your app must be able to reconstitute a usable in-memory

data model that is appropriate to its platform, using the available technologies and classes, without losing any

fidelity. And, of course, both versions must be able to convert their platform-specific data model structures

into the shared file format.

One strategy you can use is to drop down to a lower-level framework that is shared by both platforms. For

example, on the iOS side, UIColor defines a CIColor property holding a Core Image object representing the

color; on the OS X side, your app can create an NSColor object from the CIColor object using the

colorWithCIColor: class method.

Designing a Document-Based App

Considerations for Designing Your Document Data Model

There are three major classes in the document architecture: NSDocumentController,NSDocument, and

NSWindowController. Objects of these classes divide and orchestrate the work of creating, saving, opening,

and managing the documents of an app. They are arranged in a tiered one-to-many relationship, as depicted

in Figure 2-1. An app can have only one NSDocumentController object, which creates and manages one

or more NSDocument objects (one for each New or Open operation). In turn, an NSDocument object creates

and manages one or more NSWindowController objects, one for each of the windows displayed for a

document. In addition, some of these objects have responsibilities analogous to NSApplication and NSWindow

delegates, such as approving major events like closing and quitting.

Figure 2-1 Relationships among NSDocumentController,NSDocument, and NSWindowController objects

The Classes That Support Document-Based Apps

A Cocoa app includes a number of key objects in addition to the three major types of objects of the document

architecture. Figure 2-2 shows how these objects fit into the overall Cocoa object infrastructure.

Figure 2-2 Key objects in a document-based app

NSDocumentController Creates and Manages Documents

An app’s NSDocumentController object manages the documents in an app. In the MVC design pattern, an

NSDocumentController object is a high-level controller. It has the following primary responsibilities:

●Creates empty documents in response to the New item in the File menu

●Creates documents initialized with data from a file in response to the Open item in the File menu

The Classes That Support Document-Based Apps

NSDocumentController Creates and Manages Documents

●Tracks and manages those documents

●Handles document-related menu items, such as Open Recent

When a user chooses New from the File menu, the NSDocumentController object gets the appropriate

NSDocument subclass from the app’s Information property list and allocates and initializes an instance of this

class. Likewise, when the user chooses Open, the NSDocumentController object displays the Open dialog,

gets the user’s selection, finds the NSDocument subclass for the file, allocates an instance of the class, and

initializes it with data from the file. In both cases, the NSDocumentController object adds a reference to

the document object to an internal list to help manage its documents.

Most of the time, you can use NSDocumentController as is to manage your app’s documents.

NSDocumentController is hard-wired to respond appropriately to certain app events, such as when the app

starts up, when it terminates, when the system is shutting down, and when documents are opened or printed.

Alternatively, you can create a custom delegate object and implement the delegate methods corresponding

to the same events (see NSApplicationDelegate Protocol Reference).

NSDocument Presents and Stores Document Data

NSDocument is the base class for document objects in the app architecture—you must create an NSDocument

subclass for each type of document your app handles. When your app is running, it has an NSDocument-based

object for each open document. In the MVC design pattern, NSDocument isamodel controller because it

manages the data model, that is, the persistent data associated with its document. An NSDocument object

has the following responsibilities:

●Manages the display and capture of the data in its windows (with the assistance of its window controllers)

●Loads and stores (that is, reads and writes) the persistent data associated with its document

●Responds to action messages to save, print, revert, and close documents

●Runs and manages the Save and Page Setup dialogs

A fully implemented NSDocument object also knows how to track its edited status, perform undo and redo

operations, print document data, and validate its menu items. Although these behaviors aren’t completely

provided by default, the NSDocument object does assist the developer in implementing each, in the following

ways:

●For tracking edited status, NSDocument provides a method for updating a change counter.

●For undo and redo operations, NSDocument lazily creates an NSUndoManager instance when one is

requested, responds appropriately to Undo and Redo menu commands, and updates the change counter

when undo and redo operations are performed.

The Classes That Support Document-Based Apps

NSDocument Presents and Stores Document Data

●For printing, NSDocument facilitates the display of the Page Setup dialog and the subsequent modification

of the NSPrintInfo object used in printing. To do this, subclasses of NSDocument must override

printOperationWithSettings:error:.

●To validate menu items, NSDocument implements validateUserInterfaceItem: to manage the

enabled state and titles of the menu items Revert Document and Save (which becomes Save a Version

after the document is first saved). If you want to validate other menu items, you can override this method,

but be sure to invoke the superclass implementation. For more information on menu item validation, see

NSUserInterfaceValidations Protocol Reference.

When designing your document objects, you should always maintain a clean separation between these

data-handling activities of the document object itself and the code for managing the visual presentation of

that data. The document object is responsible for the data, including the reading and writing of that data to

disk. The visual presentation of that data is the responsibility of the associated window controller object.

Keeping a clean separation between these two activities makes for a more modular design that can be updated

more easily in the future.

Nonetheless, managing the document’s data and its user interface are closely related, which is why the document

object owns and manages its window controllers. The document object also manages its menu, which is part

of the user interface, because the state of its user commands—what commands are available and whether

they are enabled—is determined by the state of the document data.

An NSDocument object should not contain or require the presence of any objects that are specific to the app’s

user interface. Although a document can own and manage NSWindowController objects—which present

the document visually and allow the user to edit it—it should not depend on these objects being there. For

example, it might be desirable to have a document open in your app without having it visually displayed.

For details about subclassing NSDocument, see Creating the Subclass of NSDocument (page 35).

If you have a large data set or require a managed object model, you may want to use NSPersistentDocument,

a subclass of NSDocument, to create a document-based app that uses Core Data. For more information, see

Core Data Starting Point .

NSWindowController Manages One Document Window

An NSWindowController object manages one window associated with a document. That window is typically

stored in a nib file. As such, in the MVC design pattern, it is a view controller. When an NSWindowController

object receives a request from its owning NSDocument object, it loads the nib file containing a window, displays

the window, and sets itself as the File’s Owner of the nib file. It also assumes responsibility for closing windows

properly.

The Classes That Support Document-Based Apps

NSWindowController Manages One Document Window

A window controller keeps track of its window using its window outlet. The window outlet should be connected

to the window for which your window controller is responsible, as shown in Figure 2-3.

Figure 2-3 Window outlet of window controller

Although not required, it’s often convenient to set up your window controller as the delegate of the window

it manages. In your nib file, connect the delegate outlet of the window your window controller is managing

to the object that represents your window controller—specifically, the File’s Owner object.

Note: NSWindowController does not depend on being the controlled window's delegate to do

its job, and it doesn't implement any NSWindow delegate methods. A subclass of

NSWindowController, however, is a fine place to put implementations of NSWindow delegate

methods, and if you do so you'll probably need to connect the delegate outlet of the window to the

File's Owner of the nib file as described. But you do not have to do so for NSWindowController

itself to work properly.

The Xcode document-based app template does not subclass NSWindowController, and you do not need

to do so if you are writing a simple app. However, if you are writing an app with more advanced requirements,

as is typical, you will almost certainly want to do so. In addition, subclassing NSWindowController promotes

better encapsulation of your view and model code. For more information, see You Should Subclass

NSWindowController (page 22).

The Classes That Support Document-Based Apps

NSWindowController Manages One Document Window

Subclassing Objects in the Document Architecture

You can create a document-based app without writing much code. You have only to create a document project,

compose the human interface, implement a subclass of NSDocument, and add any other custom classes or

behavior required by your app. However, most app requirements are more complex, so you can customize the

default object architecture through subclassing and delegation, as described in this section.

Table 2-1 summarizes the object architecture and subclass requirements of a document-based app.

Table 2-1 Document architecture objects and subclasses

SubclassingNumber of objectsClass

Required1 per documentNSDocument

Optional (but recommended)1 per windowNSWindowController

Optional (and unlikely)1 per appNSDocumentController

You Must Subclass NSDocument

Every app that uses the document architecture must create at least one subclass of NSDocument. To create a

document-based Cocoa app, you choose the Xcode template for a Cocoa application presented in the New

Project dialog and select the option Create Document-Based Application in the next pane. When you do this,

you get a new app project that already contains a subclass of NSDocument and nib files for your document

and app menu. Minimal or empty method implementations are provided for:

●Reading and writing document data. Comments explain what you need to fill in, how to handle an error

condition, and alternate reading and writing methods to override instead. The method bodies include

code that throws an “unimplemented method” exception if you don’t change anything.

●Initialization of the document object. The implementation contains the proper Cocoa initialization pattern,

which calls the superclass initializer and provides a place for subclass-specific initialization.

●Returning the document nib file name. This code overrides the windowNibName method to return the

nib file name used for documents of this type. Comments explain situations where you should do alternate

overrides.

●Post-nib-loading code. This override provides a place for code to be executed after the document window

nib file is loaded. For example, objects in the nib cannot be initialized until after the nib is loaded.

●Opting into autosaving. By leaving this override as written in the template to return YES, you ensure that

your document saves its data to disk automatically.

The Classes That Support Document-Based Apps

Subclassing Objects in the Document Architecture

See Creating the Subclass of NSDocument (page 35) for information about implementing the required methods

in your NSDocument subclass.

You Should Subclass NSWindowController

Even if your document has only one window, it may be complex enough that you’d like to split up some of

the logic in the controller layer to have a view controller as well as a model controller object. In this case, you

should subclass NSWindowController as well as NSDocument. In this way, you can add specific knowledge

of the app’s view layer that the window controller is responsible for managing. Any outlets and actions, and

any other behavior that is specific to the management of the user interface, goes into the NSWindowController

subclass. Especially for larger apps, splitting the controller duties between two classes makes a lot of sense.

This strategy allows you to have documents that are open, but not onscreen, to avoid having to allocate

memory and other resources of a front-end that may not be used in some circumstances.

Reasons to Subclass NSWindowController

If your document requires or allows multiple windows for a single document, that is a good reason to subclass

NSWindowController. For example, a CAD program could need to present front, top, and side views, as well

as a rendered 3D view of a document. When it does, you might want to have one or more subclasses of

NSWindowController to manage the different kinds of windows that your document needs, and so you

must create one of each in makeWindowControllers.

Some apps need only one window for a document but want to allow the user to create several copies of the

window for a single document (sometimes this is called a multiple-view document) so that the user can have

each window scrolled to a different position or displayed differently, such as at a different scale. In this case,

your makeWindowControllers override would create only one NSWindowController object, and there

would be a menu command or other control that allows the user to create others.

Another reason to subclass NSWindowController is to customize your document window titles. To customize

a document's window title properly, subclass NSWindowController and override

windowTitleForDocumentDisplayName:. If your app requires even deeper customization, override

synchronizeWindowTitleWithDocumentName.

How to Subclass NSWindowController

Once you've decided to subclass NSWindowController, you need to change the default document-based

app setup. First, add any Interface Builder outlets and actions for your document's user interface to the

NSWindowController subclass instead of to the NSDocument subclass. The NSWindowController subclass

instance should be the File’s Owner for the nib file because that creates better separation between the

The Classes That Support Document-Based Apps

Subclassing Objects in the Document Architecture

view-related logic and the model-related logic. Some menu actions can still be implemented in the NSDocument

subclass. For example, Save and Revert Document are implemented by NSDocument, and you might add other

menu actions of your own, such as an action for creating new views on a document.

Second, instead of overriding windowNibName in your NSDocument subclass, override

makeWindowControllers.InmakeWindowControllers, create at least one instance of your custom

NSWindowController subclass and use addWindowController: to add it to the document. If your document

always needs multiple controllers, create them all here. If a document can support multiple views but by default

has one, create the controller for the default view here and provide user actions for creating other views.

You should not force the windows to be visible in makeWindowControllers.NSDocument does that for you

if it’s appropriate.

An NSWindowController Subclass Manages Nib Files

An NSWindowController object expects to be told what nib file to load (through its initWithWindowNib...

methods) because it is a generic implementation of the default behavior for all window controllers. However,

when you write a subclass of NSWindowController, that subclass is almost always designed to control the

user interface contained in a particular nib file, and your subclass would not work with a different nib file. It is

therefore inconvenient and error-prone for the instantiator of the subclass to have to tell it which nib file to

load.

This problem is solved by overriding the init method to call the superclass’s initWithWindowNibName:

method with the correct nib name. Then instantiators just use init, and the controller has the correct nib file.

You can also override the initWithWindowNib... methods to log an error, as shown in Figure 2-4, because

no instantiator should ever try to tell your subclass which nib file to use. It is a good idea for any

The Classes That Support Document-Based Apps

Subclassing Objects in the Document Architecture

NSWindowController subclass designed to work with a specific nib file to use this technique. You should

do otherwise only if you are extending just the basic functionality of NSWindowController in your subclass

and have not tied that functionality to any particular nib file.

Figure 2-4 Loading a nib file that is controller specific

An NSWindowController object without an associated NSDocument object is useful by itself.

NSWindowController can be used as the base class for auxiliary panel controllers in order to gain the use

of its nib management abilities. One common standalone use of NSWindowController subclasses is as

controllers for shared panels such as find panels, inspectors, or preferences panels. For example, the Sketch

sample app uses NSWindowController subclasses for its various secondary panels. In this case, you can make

an NSWindowController subclass that implements a “shared-instance” method to create a singleton window

controller object. For example, you could create a PreferencesController subclass with a

sharedPreferenceController class method that creates a single instance the first time it is called and

returns that same instance on all subsequent calls.

Because your subclass derives from NSWindowController, you can just tell it the name of your preferences

nib file and it handles loading the nib file and managing the window automatically. You add your own outlets

and actions, as usual, to hook up the specific user interface for your panel and add methods to manage the

panel’s behavior.

You Rarely Need to Subclass NSDocumentController

Most apps do not need to subclass NSDocumentController. Almost anything that can be done by subclassing

can be done just as easily by the app’s delegate. However, it is possible to subclass NSDocumentController

if you need to.

The Classes That Support Document-Based Apps

Subclassing Objects in the Document Architecture

For example, if you need to customize the Open dialog, an NSDocumentController subclass is needed. You

can override the NSDocumentController method runModalOpenPanel:forTypes: to customize the

dialog or add an accessory view. The addDocument: and removeDocument: methods are provided for

subclasses that want to know when documents are opened or closed.

There are two ways to subclass NSDocumentController:

●You can make an instance of your subclass in your app’s main nib file. This instance becomes the shared

instance.

●You can create an instance of your subclass in your app delegate’s applicationWillFinishLaunching:

method.

The first NSDocumentController object to be created becomes the shared instance. The AppKit framework

creates the shared instance (using the NSDocumentController class) during the “finish launching” phase of

app startup. So if you need a subclass instance, you must create it before AppKit does.

The Classes That Support Document-Based Apps

Subclassing Objects in the Document Architecture

It is possible to put together a document-based app without having to write much code. You have only to

create a document project, compose the human interface, complete the information property list for your

document types, implement a subclass of NSDocument, and add any other custom classes or behavior required

by your app.

If you intend to sell your app through the Mac App Store or use iCloud storage, you also need to create an

explicit App ID, create provisioning profiles, and enable the correct entitlements for your app. These procedures

are explained in App Distribution Guide.

Xcode Provides a Document-Based App Template

To expedite the development of document-based apps, Xcode provides a Cocoa Application template, which

has the option to make the app document based. The template provides the following things:

●A skeletal NSDocument subclass implementation. The document subclass implementation (.m) file

includes commented blocks for important methods, including an init method that initializes and returns

self. This method provides a location for subclass-specific initialization. The template also includes a fully

implemented windowNibName method that returns the name of the document window nib file. An

override of windowControllerDidLoadNib: provides a place for code to be executed after the

document’s window nib has finished loading. In addition, the template includes skeletal implementations

of the dataOfType:error: and readFromData:ofType:error: basic writing and reading methods;

these methods throw an exception if you don’t supply a working implementation. Finally, the template

includes an override of the autosavesInPlace class method that returns YES to turn on automatic

saving of changes to your documents.

●A nib file for the app’s document. This nib file is named with your NSDocument subclass name with the

extension .xib. The subclass of NSDocument is made File’s Owner of the nib file. It has an outlet named

window connected to its window object, which in turn has a delegate outlet connected to the File’s Owner,

as shown in Figure 2-3 (page 20). The window has only one user interface object in it initially, a text field

with the words "Your document contents here".

●The app’s menu bar nib file. The menu bar nib file, named MainMenu.xib, contains an app menu (named

with the app’s name), a File menu (with all of its associated document commands), an Edit menu (with

text editing commands and Undo and Redo menu items), and Format, View, Window, and Help menus

App Creation Process Overview

(with their own menu items representing commands). These menu items are connected to the appropriate

first-responder action methods. For example, the About menu item is connected to the

orderFrontStandardAboutPanel: action method that displays a standard About window.

See Review Your App Menu Bar Commands (page 28) for more information about the menu bar nib file

provided by the Xcode app templates.

●The app's information property list. The <appName>-Info.plist file contains placeholder values for

global app keys, as well as for the CFBundleDocumentTypes key, whose associated value is a dictionary

containing key-value pairs specifying information about the document types the app works with, including

the NSDocument subclass for each document type.

The following sections describe the process of selecting and utilizing the document-based app template.

Create the Project

To create your project in Xcode, choose File > New > New Project. Select the Cocoa Application icon from the

OS X Application choices. In the next pane, select the Create Document-Based Application option, as shown

in Figure 3-1. In this pane you also name your app, give your NSDocument subclass a prefix, and specify your

documents’ filename extension, in addition to other options. If you intend to use Core Data for your data

model, select the Use Core Data option, which automatically inserts NSPersistentDocument as the immediate

superclass of your document subclass.

Figure 3-1 New Project dialog

The final pane of the New Project dialog enables you to place your project in the file system and create a source

control repository if you wish. For more details about the Xcode project creation process, see Start a Project

in Xcode Overview .

App Creation Process Overview

Create the Project

Without writing any additional code, you can compile and run the app. When you first launch the app, you

see an untitled document with an empty window. The File menu commands all do something reasonable,

such as bringing up a Save dialog or Open dialog. Because you have not yet defined any types or implemented

loading and saving, you can't open or save anything, and the default implementations throw an exception.

Create Your Document Window User Interface

To create the user interface for your document window, in the project navigator area, click the nib file named

with your NSDocument subclass name with the extension .xib. This opens the file in Interface Builder, an

Xcode editor that provides a graphical interface for the creation of user interface files. You can drag user

interface elements onto the document window representation from the Interface Builder Object library in the

utility area. If the objects in the document window require outlets and actions, add them to your NSDocument

subclass. Connect these actions and outlets via the File’s Owner icon in the list of placeholders in the Interface

Builder dock. If your document objects interact with other custom objects, such as model objects that perform

specialized computations, define those objects in Interface Builder and make any necessary connections to

them.

Step-by-step instructions for connecting menu items to action methods in your code are given in Edit User

Interfaces in Xcode Overview .

Review Your App Menu Bar Commands

Table 3-1 lists the File menu first-responder action connections that exist in the template.

Table 3-1 File Menu commands in the document-based app template

First-responder actionFile menu command

newDocument:New

openDocument:Open

clearRecentDocuments:Open Recent > Clear Menu

performClose:Close

saveDocument:Save/Save a Version

revertDocumentToSaved:Revert Document

runPageLayout:Page Setup

App Creation Process Overview

Create Your Document Window User Interface

First-responder actionFile menu command

printDocument:Print

After a document has been saved for the first time, the Save command changes to Save a Version. In applications

that have enabled autosaving in place, the Save As and Save All items in the File menu are hidden, and a

Duplicate menu item is added. The template has similar ready-made connections for the Edit, Format, View,

Window, and Help menus.

Warning: If your app does not support any of the supplied actions, such as printing, for example, you

must remove the associated menu items from the nib. Otherwise, when a user chooses the action,

your app could raise an exception or crash.

For your app’s custom menu items that are not already connected to action methods in objects or placeholder

objects in the nib file, there are two common techniques for handling menu commands in an OS X app:

●Connect the corresponding menu item to a first responder method.

●Connect the menu item to a method of your custom app object or your app delegate object.

Of these two techniques, the first is more common because many menu commands act on the current document

or its contents, which are part of the responder chain. The second technique is used primarily to handle

commands that are global to the app, such as displaying preferences or creating a new document. In addition

to implementing action methods to respond to your menu commands, you must also implement the methods

of the NSMenuValidation protocol to enable the menu items for those commands.

For more information about menu validation and other menu topics, see Application Menu and Pop-up List

Programming Topics .

Complete the Information Property List

You need to configure the project’s information property list so that the app knows what kinds of documents

it can handle. You specify this information in the Xcode information property list file, which is shown in Figure

3-2. The property list file is stored in the app’s bundle and named <appName>-Info.plist by default.

When the NSDocumentController object creates a new document or opens an existing document, it searches

the property list for such items as the document class that handles a document type, the uniform type identifier

(UTI) for the type, and whether the app can edit or only view the type. Similarly, Launch Services uses information

App Creation Process Overview

Complete the Information Property List

about the icon file for the type and to know which app to launch when the user double-clicks a document file.

Document type information is associated with the CFBundleDocumentTypes key as an array of dictionaries,

each of which contains the key-value pairs that define the document type.

Xcode provides a property list file with every Mac app project. The property list editor appears when you select

the Info.plist file in the project navigator or select the target and choose the Info pane of the project editor.

In the Info pane, there’s a list of target properties. You can edit the property values and add new key-value

pairs. By default, Xcode displays a user-friendly version of each key name. To see the actual key names that

are in the Info.plist file, Control-click an item in the editor and choose Show Raw Keys/Values from the

contextual menu that appears.

Figure 3-2 The information property list editor

For a new document-based app, you should create a document type with a name and extension that make

sense for your app. You can add more types as well, one for each of the document types your app handles.

The app’s most important document type must be listed first in the list of types. This is the type that

NSDocumentController uses by default when the user asks for a new document.

App Creation Process Overview

Complete the Information Property List

The most important document type value is its Uniform Type Identifier (UTI), a string that uniquely identifies

the type of data contained in the document for all apps and services to rely upon. A document’s UTI corresponds

to the LSItemContentTypes key in the information property list. The UTI is used as the programmatic type

name by NSDocument and NSDocumentController. By using UTIs, apps avoid much of the complexity

previously required to handle disparate kinds of file-type information in the system, including filename

extensions, MIME types, and HFS type codes (OS types).

A document UTI can be defined by the system, as shown in System-Declared Uniform Type Identifiers in Uniform

Type Identifiers Reference , or a document-based app can declare its own proprietary UTI. Such custom UTIs

must also be exported to make the system aware of them, as described in Export Custom Document Type

Information (page 32).

To declare a document type in Xcode, perform the following steps:

1. Select the project in the project navigator.

2. Select the target and click the Info tab.

3. Click the Add (+) button at the bottom right of the editor area and choose Add Document Type from the

pop-up menu.

4. Click the triangle next to “Untitled” to disclose the property fields.

Alternatively, you can select the Info.plist file in the project navigator, click in the editor area, and choose

Editor > Add Item to add document type properties directly to the property list file, as shown in Figure 3-2 (page

30). Choose Editor > Show Raw Keys & Values to reveal the actual key names.

Add the properties shown in Table 3-2.

Table 3-2 Properties defining a document type (CFBundleDocumentTypes)

ValueXcode field

(Info.plist

identifier)

Key

An array of UTI strings. Typically, only one is

specified per document type. The UTI string must

be spelled out explicitly.

IdentifierLSItemContentTypes

A string specifying the NSDocument subclass

name corresponding to this document type.

Class (Cocoa

NSDocument

Class)

NSDocumentClass

A string specifying the role the app with respect

to this document type. Possible values are Editor,

Viewer, Shell, Quick Look Generator, or None.

RoleCFBundleTypeRole

App Creation Process Overview

Complete the Information Property List

ValueXcode field

(Info.plist

identifier)

Key

An array of strings specifying UTIs that define a

supported file type to which this document can

export its content.

(Exportable Type

UTIs)

NSExportableTypes

A Boolean value specifying whether the

document is distributed as a bundle. If NO, omit

this value.

Bundle

(Document is a

package or

bundle)

LSTypeIsPackage

A string specifying the name of the icon resource

file (extension .icns) to associate with this

document type. An icon resource file contains

multiple images at different resolutions.

Icon (Icon File

Name)

CFBundleTypeIconFile

A string specifying the abstract name of the

document type.

Name (Document

Type Name)

CFBundleTypeName

A string specifying how Launch Services ranks

this app among those that declare themselves

editors or viewers of documents of this type.

Possible values, in order of precedence, are

Owner,Alternate, and None.

Handler rankLSHandlerRank

For more information about these and other document type keys, see “CFBundleDocumentTypes” in Information

Property List Key Reference.

Export Custom Document Type Information

If you define a custom document type with its own UTI, you must export the UTI. To declare a document type

in Xcode, perform the following steps:

1. Select the project in the project navigator area.

2. Select the target and click the Info tab.

3. Click the Add (+) button at the bottom right of the editor area and choose Add Exported UTI from the

pop-up menu.

4. Click the triangle next to “Untitled” to disclose the property fields.

App Creation Process Overview

Export Custom Document Type Information

Add the properties shown in Table 3-3.

Table 3-3 Properties defining an exported document type (UTExportedTypeDeclarations)

ValueXcode field (Info.plist

identifier)

Key

A string describing this document type.DescriptionUTTypeDescription

The exported document type’s UTI.IdentifierUTTypeIdentifier

A string specifying the name of the document

type’s icon file.

Icon (Icon file name)UTTypeIconFile

An array of strings representing the UTIs to

which the document type conforms.

Conforms to (Conforms

to UTIs)

UTTypeConformsTo

An array of strings named

public.filename-extension containing

filename extensions corresponding to the

document type.

Extensions (Equivalent

Types)

UTTypeTag-

Specification

For more information about these and other exported type property keys, see Declaring New Uniform Type

Identifiers in Uniform Type Identifiers Overview .

For information about document types in alternate document-based app designs, see Multiple Document

Types Use Multiple NSDocument Subclasses (page 62) and Additional Document Type Considerations (page

63).

Implement the NSDocument Subclass

Every document-based app that uses the document architecture must create at least one subclass of

NSDocument. You must override some NSDocument methods (among several choices), and you should override

several others in certain situations. Details explaining how to implement your NSDocument subclass are in

Creating the Subclass of NSDocument (page 35).

Create Any Additional Custom Classes

The Cocoa document architecture, as embodied primarily in NSDocument,NSDocumentController, and

NSWindowController, provides an operating framework for apps, including sophisticated document handling

mechanisms. However, you must add the behaviors that differentiate your app and suit it to its particular

App Creation Process Overview

Implement the NSDocument Subclass

purpose. Much customized behavior can be implemented in your NSDocument subclass, in delegate methods,

custom classes added to your project, and subclasses of NSDocumentController and NSWindowController

if you need to extend the capabilities of either of those classes. Generally, you should use custom classes to

encapsulate the program logic of your data model and controllers, maintaining a healthy MVC separation.

For more information about app design, see Mac App Programming Guide.

App Creation Process Overview

Create Any Additional Custom Classes

The NSDocument subclass provides storage for the model and the ability to load and save document data. It

also has any outlets and actions required for the user interface. The NSDocument object automatically creates

an NSWindowController object to manage that nib file, but the NSDocument object serves as the File’s

Owner proxy object for the nib file.

When you subclass NSDocument, you must override certain key methods and implement others to do at least

the following things:

●Read data of existing documents from files

●Write document data to files

●Initialize new documents

●Put documents into iCloud and remove them

In particular, you must override one reading and one writing method. In the simplest case, you can override

the data-based reading and writing methods, readFromData:ofType:error: and dataOfType:error:.

Reading Document Data

Opening existing documents stored in files is one of the most common operations document-based apps

perform. Your override’s responsibility is to load the file data into your app’s data model.

If it works for your application, you should override the data-based reading method,

readFromData:ofType:error:. Overriding that method makes your work easier because it uses the default

document-reading infrastructure provided by NSDocument, which can handle multiple cases on your behalf.

Creating the Subclass of NSDocument

Note: You should disable undo registration during document reading.

How to Override the Data-Based Reading Method

You can override the readFromData:ofType:error: method to convert an NSData object containing

document data into the document’s internal data structures and display that data in a document window. The

document architecture calls readFromData:ofType:error:, passing in the NSData object, during its

document initialization process.

Listing 4-1 shows an example implementation of the readFromData:ofType:error: document-reading

method. This example assumes that the app has an NSTextView object configured with an NSTextStorage

object to hold the text view’s data. The NSDocument object has a setMString: accessor method for the

document’s NSAttributedString data model, declared as a property named mString.

Listing 4-1 Data-based document-reading method implementation

- (BOOL)readFromData:(NSData *)data ofType:(NSString *)typeName

error:(NSError **)outError {

BOOL readSuccess = NO;

NSAttributedString *fileContents = [[NSAttributedString alloc]

initWithData:data options:NULL documentAttributes:NULL

error:outError];

if (!fileContents && outError) {

*outError = [NSError errorWithDomain:NSCocoaErrorDomain

code:NSFileReadUnknownError userInfo:nil];

}

if (fileContents) {

readSuccess = YES;

[self setMString:fileContents];

}

return readSuccess;

}

If you need to deal with the location of the file, override the URL reading and writing methods instead. If your

app needs to manipulate document files that are file packages, override the file-wrapper reading and writing

methods instead. For information about overriding the URL-based and file-wrapper-based reading methods,

see Overriding the URL and File Package Reading Methods (page 57).

Creating the Subclass of NSDocument

Reading Document Data

The flow of messages during document data reading is shown in Figure 6-5 (page 69).

It’s Easy to Support Concurrent Document Opening

A class method of NSDocument,canConcurrentlyReadDocumentsOfType:, enables your NSDocument

subclass to load documents concurrently, using background threads. This override allows concurrent reading

of multiple documents and also allows the app to be responsive while reading a large document. You can

override canConcurrentlyReadDocumentsOfType: to return YES to enable this capability. When you do,

initWithContentsOfURL:ofType:error: executes on a background thread when opening files via the

Open dialog or from the Finder.

The default implementation of this method returns NO. A subclass override should return YES only for document

types whose reading code can be safely executed concurrently on non-main threads. If a document type relies

on shared state information, you should return NO for that type.

Don’t Rely on Document-Property Getters in Overrides of Reading Methods

Don’t invoke fileURL,fileType,orfileModificationDate from within your overrides. During reading,

which typically happens during object initialization, there is no guarantee that NSDocument properties like

the file’s location or type have been set yet. Your overridden method should be able to determine everything

it needs to do the reading from the passed-in parameters. During writing, your document may be asked to

write its contents to a different location or using a different file type.

If your override cannot determine all of the information it needs from the passed-in parameters, consider

overriding another method. For example, if you see the need to invoke fileURL from within an override of

readFromData:ofType:error:, you should instead override readFromURL:ofType:error: and use the

passed-in URL value.

Writing Document Data

In addition to implementing a document-reading method, you must implement a document-writing method

to save your document data to disk. In the simplest case, you can override the data-based writing method,

dataOfType:error:. If it works for your application, you should override dataOfType:error:. Overriding

that method makes your work easier because it uses the default document-reading infrastructure provided

by NSDocument. The responsibility of your override of the dataOfType:error: method is to create and

return document data of a supported type, packaged as an NSData object, in preparation for writing that data

to a file.

Creating the Subclass of NSDocument

Writing Document Data

Listing 4-2 shows an example implementation of dataOfType:error:. As with the corresponding example

implementation document-reading method, this example assumes that the app has an NSTextView object

configured with an NSTextStorage object to hold the document’s data. The document object has an outlet

property connected to the NSTextView object and named textView. The document object also has synthesized

mString and setMString: accessors for the document’s NSAttributedString data model, declared as a

property named mString.

Listing 4-2 Data-based document-writing method implementation

- (NSData *)dataOfType:(NSString *)typeName error:(NSError **)outError {

NSData *data;

[self setMString:[self.textView textStorage]]; // Synchronize data model with

the text storage

NSMutableDictionary *dict = [NSDictionary

dictionaryWithObject:NSRTFTextDocumentType

forKey:NSDocumentTypeDocumentAttribute];

[self.textView breakUndoCoalescing];

data = [self.mString dataFromRange:NSMakeRange(0, [self.mString length])

documentAttributes:dict error:outError];

if (!data && outError) {

*outError = [NSError errorWithDomain:NSCocoaErrorDomain

code:NSFileWriteUnknownError userInfo:nil];

}

return data;

}

The override sends the NSTextView object a breakUndoCoalescing message when saving the text view’s

contents to preserve proper tracking of unsaved changes and the document’s dirty state.

If your app needs access to document files, you can override writeToURL:ofType:error: instead. If your

document data is stored in file packages, you can override fileWrapperOfType:error: instead. For

information about overriding the other NSDocument writing methods, see Overriding the URL and File Package

Writing Methods (page 60).

The actual flow of messages during this sequence of events is shown in detail in Figure 6-6 (page 70).

Creating the Subclass of NSDocument

Writing Document Data

Initializing a New Document

The init method of NSDocument is the designated initializer, and it is invoked by the other initializers

initWithType:error: and initWithContentsOfURL:ofType:error:. If you perform initializations that

must be done when creating new documents but not when opening existing documents, override

initWithType:error:. If you have any initializations that apply only to documents that are opened, override

initWithContentsOfURL:ofType:error:. If you have general initializations, override init. In all three

cases, be sure to invoke the superclass implementation as the first action.

If you override init, make sure that your override never returns nil. Returning nil could cause a crash (in

some versions of AppKit) or present a less than useful error message. If, for example, you want to prevent the

creation or opening of documents under circumstances unique to your app, override a specific

NSDocumentController method instead. That is, you should control this behavior directly in your app-level

logic (to prevent document creation or opening in certain cases) rather than catching the situation after

document initialization has already begun.

Note: If you don’t want to open an untitled document when the app is launched or activated,

implement the app delegate method applicationShouldOpenUntitledFile: to return NO. If

you do want to open an untitled document when launched, but don't want to open an untitled

document when the app is already running and activated from the dock, you can instead implement

the delegate’s applicationShouldHandleReopen:hasVisibleWindows: method to return

NO.

Implement awakeFromNib to initialize objects unarchived from the document’s window nib files (but not the

document itself).

Creating the Subclass of NSDocument

Initializing a New Document

Moving Document Data to and from iCloud

The iCloud storage technology enables you to share documents and other app data among multiple computers

that run your document-based app. If you have an iOS version of your document-based app that shares the

same document data formats, documents can be shared among iOS devices as well, as shown in Figure 4-1.

Changes made to the file or directory on one device are stored locally and then pushed to iCloud using a local

daemon. The transfer of files to and from each device is transparent to your app.

Figure 4-1 Sharing document data via iCloud

Access to iCloud is controlled using entitlements, which your app configures through Xcode. If these entitlements

are not present, your app is prevented from accessing files and other data in iCloud. In particular, the container

identifiers for your app must be declared in the

com.apple.developer.ubiquity-container-identifiers entitlement. For information about how to

configure your app’s entitlements, see Developing for the App Store and Tools Workflow Guide for Mac.

Creating the Subclass of NSDocument

Moving Document Data to and from iCloud

All files and directories stored in iCloud must be managed by an object that adopts the NSFilePresenter

protocol, and all changes you make to those files and directories must occur through an NSFileCoordinator

object. The file presenter and file coordinator prevent external sources from modifying the file at the same

time and deliver relevant notifications to other file presenters. NSDocument implements the methods of the

NSFilePresenter protocol and handles all of the file-related management for you. All your app must do is

read and write the document data when told to do so. Be sure you override autosavesInPlace to return

YES to enable file coordination in your NSDocument object.

Determining Whether iCloud Is Enabled

Early in the execution of your app, before you try to use any other iCloud interfaces, you must call the

NSFileManager method URLForUbiquityContainerIdentifier: to determine whether iCloud storage

is enabled. This method returns a valid URL when iCloud is enabled (and the specified container directory is

available) or nil when iCloud is disabled. URLForUbiquityContainerIdentifier: also returns nil if you

specify a container ID that the app isn't allowed to access or that doesn't exist. In that case, the NSFileManager

object logs a message to the console to help diagnose the error.

Listing 4-3 illustrates how to determine whether iCloud is enabled for the document’s file URL, presenting an

error message to the user if not, and setting the value of the document’s destination URL to that of its iCloud

container otherwise (in preparation for moving the document to iCloud using the

setUbiquitous:itemAtURL:destinationURL:error: method).

Listing 4-3 Determining whether iCloud is enabled

NSURL *src = [self fileURL];

NSURL *dest = NULL;

NSURL *ubiquityContainerURL = [[[NSFileManager defaultManager]

URLForUbiquityContainerIdentifier:nil]

URLByAppendingPathComponent:@"Documents"];

if (ubiquityContainerURL == nil) {

NSDictionary *dict = [NSDictionary dictionaryWithObjectsAndKeys:

NSLocalizedString(@"iCloud does not appear to be configured.", @""),

NSLocalizedFailureReasonErrorKey, nil];

NSError *error = [NSError errorWithDomain:@"Application" code:404

userInfo:dict];

[self presentError:error modalForWindow:[self windowForSheet] delegate:nil

didPresentSelector:NULL contextInfo:NULL];

return;

}

Creating the Subclass of NSDocument

Moving Document Data to and from iCloud

dest = [ubiquityContainerURL URLByAppendingPathComponent:

[src lastPathComponent]];

Because the message specifies nil for the container identifier parameter,

URLForUbiquityContainerIdentifier: returns the first container listed in the

com.apple.developer.ubiquity-container-identifiers entitlement and creates the corresponding

directory if it does not yet exist. Alternatively, you could specify your app’s container identifier—a concatenation

of team ID and app bundle ID, separated by a period for the app’s primary container identifier, or a different

container directory. For example, you could declare a string constant for the container identifier, as in the

following example, and pass the constant name with the message.

static NSString *UbiquityContainerIdentifier = @"A1B2C3D4E5.com.domainname.appname";

The method also appends the document’s filename to the destination URL.

Searching for Documents in iCloud

Apps should use NSMetadataQuery objects to search for items in iCloud container directories. Metadata

queries return results only when iCloud storage is enabled and the corresponding container directories have

been created. For information about how to create and configure metadata search queries, see File Metadata

Search Programming Guide . For information about how to iterate directories using NSFileManager, see File

System Programming Guide.

Moving a Document into iCloud Storage

To save a new document to the iCloud container directory, first save it locally and then call the NSFileManager

method setUbiquitous:itemAtURL:destinationURL:error: to move the document file to iCloud.

Warning: Do not call setUbiquitous:itemAtURL:destinationURL:error: from your app’s

main thread. Doing so can trigger a deadlock with any file presenter monitoring the file, and it can

take an indeterminate amount of time to complete. Instead, call the method in a block running in a

dispatch queue other than the main-thread queue.

Listing 4-4 shows an example implementation of a method that moves a file to iCloud storage. It assumes the

source and destination URLs from Listing 4-3 (page 41).

Creating the Subclass of NSDocument

Moving Document Data to and from iCloud

Listing 4-4 Moving a document to iCloud

dispatch_queue_t globalQueue =

dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

dispatch_async(globalQueue, ^(void) {

NSFileManager *fileManager = [[NSFileManager alloc] init];

NSError *error = nil;

// Move the file.

BOOL success = [fileManager setUbiquitous:YES itemAtURL:src

destinationURL:dest error:&error];

dispatch_async(dispatch_get_main_queue(), ^(void) {

if (! success) {

[self presentError:error modalForWindow:[self windowForSheet]

delegate:nil didPresentSelector:NULL contextInfo:NULL];

}

});

[self setFileURL:dest];

[self setFileModificationDate:nil];

After a document file has been moved to iCloud, as shown in Listing 4-4, reading and writing are performed

by the normal NSDocument mechanisms, which automatically manage the file access coordination required

by iCloud.

Removing a Document from iCloud Storage

To move a document file from an iCloud container directory, follow the same procedure described in Moving

a Document into iCloud Storage (page 42), except switch the source URL (now the document file in the iCloud

container directory) and the destination URL (the location of the document file in the local file system). In

addition, the first parameter of the setUbiquitous:itemAtURL:destinationURL:error: method should

now be NO.

For clarity in this example, the URL of the file in iCloud storage is named cloudsrc and the local URL to which

the file is moved is named localdest.

dispatch_queue_t globalQueue =

dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

dispatch_async(globalQueue, ^(void) {

Creating the Subclass of NSDocument

Moving Document Data to and from iCloud

NSFileManager *fileManager = [[NSFileManager alloc] init];

NSError *error = nil;

// Move the file.

BOOL success = [fileManager setUbiquitous:NO itemAtURL:cloudsrc

destinationURL:localdest error:&error];

dispatch_async(dispatch_get_main_queue(), ^(void) {

if (! success) {

[self presentError:error modalForWindow:[self windowForSheet]

delegate:nil didPresentSelector:NULL contextInfo:NULL];

}

});

For more information about iCloud, see iCloud Design Guide.

NSDocument Handles Conflict Resolution Among Document Versions

NSDocument handles conflict resolution automatically, so you do not need to implement it yourself. If a conflict

comes in while the document is open, NSDocument presents a sheet asking the user to resolve the conflict

(or ignore, which marks it as resolved and accepts the automatic winner of the conflict, usually the one with

the most recent modification date). Clicking Resolve invokes the Versions user interface (see Users Can Browse

Document Versions (page 50)) with only the conflicting versions visible. The user can choose a particular

version and click Restore to make it the winner of the conflict, or just select Done to accept the automatic

winner.

Even after the conflict is resolved, NSDocument always keeps the conflicting versions, and they can be accessed

normally through Versions.

Optional Method Overrides

The areas described by items in the following sections require method overrides in some situations. And, of

course, you must implement any methods that are special to your NSDocument subclass. More options for

your NSDocument subclass are described in Alternative Design Considerations (page 57).

Creating the Subclass of NSDocument

Optional Method Overrides

Window Controller Creation

NSDocument subclasses must create their window controllers. They can do this indirectly or directly. If a

document has only one nib file with one window in it, the subclass can override windowNibName to return

the name of the window nib file. As a consequence, the document architecture creates a default

NSWindowController instance for the document, with the document as the nib file’s owner. If a document

has multiple windows, or if an instance of a custom NSWindowController subclass is used, the NSDocument

subclass must override makeWindowControllers to create these objects.

If your document has only one window, the project template provides a default implementation of the

NSDocument method windowNibName:

- (NSString *)windowNibName {

return @"MyDocument";

}

If your document has more than one window, or if you have a custom subclass of NSWindowController,

override makeWindowControllers instead. Make sure you add each created window controller to the list of

such objects managed by the document using addWindowController:.

Window Nib File Loading

You can implement windowControllerWillLoadNib: and windowControllerDidLoadNib: to perform

any necessary tasks related to the window before and after it is loaded from the nib file. For example, you may

need to perform setup operations on user interface objects, such as setting the content of a view, after the

app’s model data has been loaded. In this case, you must remember that the NSDocument data-reading

methods, such as readFromData:ofType:error:, are called before the document’s user interface objects

contained in its nib file are loaded. Of course, you cannot send messages to user interface objects until after

the nib file loads. So, you can do such operations in windowControllerDidLoadNib:.

Here is an example:

- (void)windowControllerDidLoadNib:(NSWindowController *)windowController {

[super windowControllerDidLoadNib:windowController];

[textView setAllowsUndo:YES];

if (fileContents != nil) {

[textView setString:fileContents];

fileContents = nil;

}

Creating the Subclass of NSDocument

Optional Method Overrides

Printing and Page Layout

A document-based app can change the information it uses to define how document data is printed. This

information is encapsulated in an NSPrintInfo object. If you want users to be able to print a document, you

must override printOperationWithSettings:error:, possibly providing a modified NSPrintInfo object.

Warning: If your app does not support printing, you must remove the printing-related menu items

from the menu bar nib file (MainMenu.nib) provided when you create a document-based application

using the Cocoa Application template in Xcode.

Modifying the Save Dialog Accessory View

By default, when NSDocument runs the Save dialog, and the document has multiple writable document types,

it inserts an accessory view near the bottom of the dialog. This view contains a pop-up menu of the writable

types. If you don’t want this pop-up menu, override shouldRunSavePanelWithAccessoryView to return

NO. You can also override prepareSavePanel: to do any further customization of the Save dialog.

Validating Menu Items

NSDocument implements validateUserInterfaceItem: to manage the enabled state of the Revert

Document and Save menu items. If you want to validate other menu items, you can override this method, but

be sure to invoke the superclass implementation. For more information on menu item validation, see Application

Menu and Pop-up List Programming Topics .

Creating the Subclass of NSDocument

Optional Method Overrides

The Cocoa document architecture, and NSDocument in particular, provide support for many core behaviors

of Mac apps.

Documents Are Automatically Saved

In OS X v10.7 and later, users don’t need to save documents explicitly or be concerned about losing unsaved

changes. Instead, the system automatically writes document data to disk as necessary. Your NSDocument

subclass opts into this behavior by overriding the autosavesInPlace class method to return YES. The ideal

baseline for save-less documents is this: The document data that users see in an app window is identical to

the document data on disk at all times. For practical reasons, the system does not attempt to save every change

immediately, but it saves documents often enough and at the correct times to ensure that the document in

memory and the one on disk are effectively the same.

Part of the implementation of save-less documents is file coordination, a mechanism that serializes access to

files among processes to prevent inconsistencies due to non-sequential reading and writing. Apps use file

coordination so that users don’t need to remember to save document changes before causing the document’s

file to be read by another app. Document-based Cocoa apps use file coordination automatically.

Autosaving in Place Differs From Autosaving Elsewhere

Automatic document saving is supported by the implementation of autosaving in place . Autosaving in place

and autosaving elsewhere both protect against the user losing work due to app crashes, kernel panics, and

power failures. However, autosaving in place differs from autosaving elsewhere in that it overwrites the actual

Core App Behaviors

document file rather than writing a new file next to it containing the autosaved document contents. (Autosaving

in place performs a safe save by writing to a new file first, then moving it into the place of the document file

when done.) Autosaving in place is illustrated in Figure 5-1.

Figure 5-1 Autosaving in place

The document architecture still uses autosaving elsewhere to save untitled documents that have content but

have not been explicitly saved and named by the user. In this case, untitled documents are autosaved in

~/Library/Autosave Information. In addition, NSDocument saves earlier revisions of documents

elsewhere, giving the user access to previous versions.

The saveless-documents model automates crash protection but preserves the ability for users to save documents

explicitly. It also automates maintenance of multiple older versions. Users can save immediately in the traditional

way (by choosing File > Save a Version or pressing Command-S). For an untitled document, an explicit Save

command presents a dialog enabling the user to name the document and specify the location where it is to

be written to disk.

You should not invoke the autosavesInPlace method to find out whether autosaving is being done. Instead,

the document architecture passes one of two new autosaving-related enumerators as an

NSSaveOperationType parameter to your overrides of the NSDocument methods beginning with save...

and write..., and you can examine those values. The autosave enumerators are

NSAutosaveInPlaceOperation and NSAutosaveElsewhereOperation. The old NSAutosaveOperation

enumerator is equivalent to NSAutosaveElsewhereOperation and is deprecated in OS X v10.7.

Core App Behaviors

Documents Are Automatically Saved

Consider Autosaving Performance

Before you enable autosaving, consider the saving performance of your app. If your app saves quickly, there

is little reason not to enable it. But if your app saves slowly, enabling autosaving could cause periodic blocking

of your user interface while saving is happening. So, for example, if you have already implemented the autosaving

behavior introduced in OS X v10.4 (sending setAutosavingDelay: to the NSDocumentController object

with a nonzero value), then your app’s saving performance is probably acceptable, and opting into autosaving

in place is as simple as overriding autosavesInPlace to return YES. Otherwise, you may first need to address

any issues with your document model or saving logic that could hinder saving performance.

Safety Checking Prevents Unintentional Edits

When saving happens without user knowledge, it becomes easier for unintentional edits to get saved to disk,

resulting in potential data loss. To help prevent autosaving unintentional edits, NSDocument performs safety

checking to determine when a user has opened a document to read it, but not edit it. For example, if the

document has not been edited for some period of time, it is locked for editing and opened only for reading.

(The period after editing when the document is locked is an option in the Time Machine system preference.)

NSDocument also checks for documents that are in folders where the user typically does not edit documents,

such as the ~/Downloads folder.

When an edit is made to the document, NSDocument offers the user the choice of canceling the change,

creating a new document with the change, or allowing editing. A document that is preventing edits displays

Locked in the title bar. The user can explicitly enable editing of the document by clicking on the Locked label

and choosing Unlock in the pop-up menu. A document that has been changed since it was last opened and

is therefore being actively autosaved in place displays Edited in the titlebar instead of Locked.

An app can programmatically determine when a document is locked in read-only “viewing mode” by sending

it the isInViewingMode message. You can use this information to prevent certain kinds of user actions or

changes when the user is viewing an old document revision. Another useful feature for managing locked

documents is NSChangeDiscardable. You can use this constant to specify that a particular editing change

is non-critical and can be thrown away instead of prompting the user. For example, changing the slide in a

Keynote document would normally cause some data to be saved in the document, but Keynote declares that

change to be discardable, so the user viewing a locked document can change slides without being prompted

to unlock it.

Document Saving Can Be Asynchronous

In OS X v10.7 and later, NSDocument can save asynchronously, so that document data is written to a file on a

background thread. In this way, even if writing is slow, the app’s user interface remains responsive. You can

override the method canAsynchronouslyWriteToURL:ofType:forSaveOperation: to return YES to

Core App Behaviors

Document Saving Can Be Asynchronous

enable asynchronous saving. In this case, NSDocument creates a separate writing thread and invokes

writeSafelyToURL:ofType:forSaveOperation:error: on it. However, the main thread remains blocked

until an object on the writing thread invokes the unblockUserInteraction method.

When unblockUserInteraction is invoked, the app resumes dequeueing user interface events and the

user is able to continue editing the document, even if the writing of document data takes some time. The right

moment to invoke unblockUserInteraction is when an immutable snapshot of the document’s contents

has been taken, so that writing out the snapshot of the document’s contents can continue safely on the writing

thread while the user continues to edit the document on the main thread.

Some Autosaves Can Be Cancelled

For various reasons, an app may not be able to implement asynchronous autosaving, or it may be unable to

take a snapshot of the document’s contents quickly enough to avoid interrupting the user’s workflow with

autosaves. In that case, the app needs to use a different strategy to remain responsive. The document

architecture supports the concept of cancellable autosaves for this purpose, which the app can implement

instead of asynchronous saving. At various times during an autosave operation, the app can check to see if

the user is trying to edit the document, usually by checking the event queue. If an event is detected, and if the

actual write to file has not yet begun, the app can cancel the save operation and simply return an

NSUserCancelledError error.

Some types of autosaves can be safely cancelled to unblock user interaction, while some should be allowed

to continue, even though they cause a noticeable delay. You can determine whether a given autosave can be

safely cancelled by sending the document an autosavingIsImplicitlyCancellable message. This

method returns YES when periodic autosaving is being done for crash protection, for example, in which case

you can safely cancel the save operation. It returns NO when you should not cancel the save, as when the

document is being closed, for example.

Users Can Browse Document Versions

The document architecture implements the Versions feature of OS X v10.7 in the behavior of NSDocument.

An NSDocument subclass adopts autosaving in place by returning YES from autosavesInPlace, as described

in Documents Are Automatically Saved (page 47), and adopting autosaving in turn enables version browsing.

After a document has been named and saved, the Save menu item is replaced by the “Save a Version” menu

item. This command saves a version of the document identified by date and time. And NSDocument sometimes

creates a version automatically during autosaving. The user can choose File > Revert Document, or choose

Core App Behaviors

Some Autosaves Can Be Cancelled

Browse All Revisions from the pop-up menu at the right of the title bar, to display a dialog enabling the user

to choose between the last saved version or an older version. Choosing an older version displays a Time

Machine–like user interface that selects among all of the document’s versions.

If the user chooses to restore a previous version, the current document contents are preserved on disk, if

necessary, and the file's contents are replaced with those of the selected version. Holding down the Option

key while browsing versions gives the user the option to restore a copy of a previous version, which does not

affect the current document contents. The user can also select and copy contents from a version and paste

them into the current document.

Windows Are Restored Automatically

The document architecture implements the Resume feature of OS X v10.7, so that individual apps need to

encode only information that is peculiar to them and necessary to restore the state of their windows.

The document architecture implements the following steps in the window restoration process; the steps

correlate to the numbers shown in Figure 5-2 (page 52):

1. The NSWindowController method setDocument: sets the restoration class of document windows to

the class of the shared NSDocumentController object. The NSWindow object invalidates its restorable

state whenever its state changes by sending invalidateRestorableState to itself.

2. At the next appropriate time, Cocoa sends the window an encodeRestorableStateWithCoder:

message, and the window encodes identification and status information into the passed-in encoder.

3. When the system restarts, Cocoa relaunches the app and sends the

restoreWindowWithIdentifier:state:completionHandler: message to the NSApp object.

Apps can override this method to do any general work needed for window restoration, such as substituting

a new restoration class or loading it from a separate bundle.

NSApp decodes the restoration class for the window, sends the

restoreWindowWithIdentifier:state:completionHandler: message to the restoration class

object, and returns YES.

4. The restoration class reopens the document and locates its window. Then it invokes the passed-in

completion handler with the window as a parameter.

Core App Behaviors

Windows Are Restored Automatically

5. Cocoa sends the restoreStateWithCoder: message to the window, which decodes its restorable state

from the passed-in NSCoder object and restores the details of its content.

Figure 5-2 Window restoration

Although the preceding steps describe only window restoration, in fact every object inheriting from

NSResponder has its own restorable state. For example, an NSTextView object stores the selected range (or

ranges) of text in its restorable state. Likewise, an NSTabView object records its selected tab, an NSSearchField

object records the search term, an NSScrollView object records its scroll position, and an NSApplication

object records the z-order of its windows. An NSDocument object has state as well. Although NSDocument

does not inherit from NSResponder, it implements many NSResponder methods, including the restoration

methods shown in Figure 5-2.

When the app is relaunched, Cocoa sends the restoreStateWithCoder: message to the relevant objects

in turn: first to the NSApplication object, then to each NSWindow object, then to the NSWindowController

object, then to the NSDocument object, and then to each view that has saved state.

The Document Architecture Provides Undo Support for Free

Undo support in the document architecture is built-in and straightforward to implement. By default, an

NSDocument object has its own NSUndoManager object. The NSUndoManager class enables you to construct

invocations that do the opposite of a previous action.

Core App Behaviors

The Document Architecture Provides Undo Support for Free

Important: Your document subclass should disable undo registration during document reading using the

[[self undoManager] disableUndoRegistration] message.

Implementing Undo

The key to implementing undo properly is to have well-defined primitives for changing your document. Each

model object, plus the NSDocument subclass itself, should define the set of primitive methods that can change

it. Each primitive method is then responsible for using the undo manager to enqueue invocations that undo

the action of the primitive method. For example, if you decide that setColor: is a primitive method for one

of your model objects, then inside of setColor: your object would do something like the following:

[[[myDocument undoManager] prepareWithInvocationTarget:self] setColor:oldColor]

This message causes the undo manager to construct and save an invocation. If the user later chooses Undo,

the saved invocation is invoked and your model object receives another setColor: message, this time with

the old color. You don’t have to keep track of whether commands are being undone to support redo. In fact,

the way redo works is by watching what invocations get registered as the undo is happening and recording

them on the redo stack.

Figure 5-3 Undo and redo stacks

Redo stack

Target

Selector

Arguments

You can use the setUndoManager: method if you need to use a subclass or otherwise need to change the

undo manager used by the document.

Because many discrete changes might be involved in a user-level action, all the undo registrations that happen

during a single cycle of the event loop are usually grouped together and are undone all at once. NSUndoManager

has methods that allow you to control the grouping behavior further if you need to.

Core App Behaviors

The Document Architecture Provides Undo Support for Free

Another aspect of good undo implementation is to provide action names so that the Undo and Redo menu

items can have more descriptive titles. Undo action names are best set in action methods instead of the change

primitives in your model objects because many primitive changes might go into one user action, or different

user actions might result in the same primitives being called in different ways. The Sketch example app

implements undo in action methods.

Implementing Partial Undo

Because the undo manager does multiple-level undo, do not implement undo for only a subset of the possible

changes to your document. The undo manager relies on being able to reliably take the document back through

history with repeated undos. If some changes get skipped, the undo stack state is no longer synchronized with

the contents of the document. Depending on your architecture, that situation can cause problems that range

from merely annoying to fatal.

For example, imagine that you have a drawing program that is able to undo a resize, but not a delete operation.

If the user selects a graphic and resizes it, the undo manager gets an invocation that can undo that resize

operation. Now the user deletes that graphic (which is not recorded for undo). If the user now tries to undo,

nothing happens (at the very least), because the graphic that was resized is no longer there and undoing the

resize can’t have any visual effect. At worst, the app might crash trying to send a message to a freed object.

So when you implement undo, remember that everything that causes a change to the document should be

undoable.

If there are some changes that you cannot undo, there are two ways to handle the situation when a user makes

such a change. If you can be absolutely sure that the change has no relationship to any other changes that

can happen to the document (that is, something totally independent of all the rest of the contents of the

document has changed), then you do not register any undo action for that change. On the other hand, if the

change does have some relationship to the rest of the document contents, remove all actions from the undo

manager when such a change takes place. Such changes then mark points of no return in your user experience.

When designing your app and document format, you should strive to avoid the need for these “point of no

return” operations.

Managing the Change Count

Because of undo support, the document must keep more information than just whether the document is dirty

or clean. If a user opens a file, makes five changes, and then chooses Undo five times, the document should

once again be clean. But if the user chooses Undo only four times, the document is still dirty.

The NSDocument object keeps a change count to deal with this. The change count can be modified by sending

an updateChangeCount: message with one of the supported change types. The supported change types

are NSChangeDone,NSChangeUndone, and NSChangeCleared. The NSDocument object itself clears the

Core App Behaviors

The Document Architecture Provides Undo Support for Free

change count whenever the user saves or reverts the document. If the document has an undo manager, it

observes the undo manager and automatically updates the change count when changes are done, undone,

or redone.

Not Supporting Undo

If you don't want to support undo at all, first send the setHasUndoManager: message with a parameter value

of NO to your document. This message causes the document never to get an undo manager.

Without an undo manager (and without undo support from your model objects), the document cannot

automatically track its dirty state. So, if you aren't implementing undo, you need to send an

updateChangeCount: message explicitly whenever your document is edited.

The Document Architecture Supports Robust Error Handling

Many NSDocument and NSDocumentController methods include as their last parameter an indirect reference

to an NSError object. These are methods that create a document, write a file, access a resource, or perform

a similar operation.

An example of an NSDocumentController method that takes an error parameter is

openUntitledDocumentAndDisplay:error:, which creates a new untitled document. In case of failure,

this method directly returns nil and, in the last parameter, indirectly returns an NSError object that describes

the error. Before calling such a method, client code that is interested in a possible error declares an NSError

object variable and passes the address of the variable in the error parameter. If the clients are not interested

in the error, they pass NULL in the error parameter.

Using NSError objects gives Cocoa apps the capability to present much more useful error messages to the

user, including detailed reasons for the error condition, suggestions for recovery, and even a mechanism for

attempting programmatic recovery. In addition, AppKit handles presenting the error to the user.

Important: Cocoa methods that take error parameters in the Cocoa error domain are guaranteed to return

NSError objects. So, if you override such a method, you must adhere to the following rule: A method that

takes an error:(NSError **)outError parameter must set the value of *outError to point to an

NSError object whenever the method returns a value that signals failure (typically nil or NO) and outError

!= NULL.

If you override a method that takes an error parameter and you call the superclass implementation, you don’t

need to set outError yourself. Pass it the error argument that your override received when invoked.

Core App Behaviors

The Document Architecture Supports Robust Error Handling

If you override such a method to prevent some action but you don’t want an error alert to be presented to the

user, return an error object whose domain is NSCocoaErrorDomain and whose code is

NSUserCancelledError. The AppKit framework presents errors through the NSApplication implementations

of the presentError: and

presentError:modalForWindow:delegate:didPresentSelector:contextInfo: methods declared

by NSResponder. Those implementations silently ignore errors whose domain is NSCocoaErrorDomain and

whose code is NSUserCancelledError. So, for example, if your override wanted to avoid presenting an error

to the user, it could set an error object as shown in the following fragment:

if (outError) {

*outError = [NSError errorWithDomain:NSCocoaErrorDomain

code:NSUserCancelledError userInfo:nil];

}

For detailed information about NSError handling see Error Handling Programming Guide.

Core App Behaviors

The Document Architecture Supports Robust Error Handling

Most document-based apps can use the information presented in other chapters of this document. However,

some apps have particular requirements necessitating alternate techniques, some of which are discussed in

this chapter.

Overriding the URL and File Package Reading Methods

There are situations in which the simplest solution for document reading, overriding the data-based reading

method, readFromData:ofType:error:, as described in Reading Document Data (page 35), is not sufficient.

In such cases, you can override another NSDocument reading method instead, such as the URL-based and file

package reading methods.

If your app needs access to the URL of a document file, you should override the readFromURL:ofType:error:

method instead of readFromData:ofType:error:, as in the example implementation shown in Listing 6-1.

This example assumes that the app has an NSTextView object configured with an NSTextStorage object

to display the document’s data. The NSDocument object has text and setText: accessors for the document’s

NSAttributedString data model.

Listing 6-1 URL-based document-reading method implementation

- (BOOL)readFromURL:(NSURL *)inAbsoluteURL ofType:(NSString *)inTypeName

error:(NSError **)outError {

BOOL readSuccess = NO;

NSAttributedString *fileContents = [[NSAttributedString alloc]

initWithURL:inAbsoluteURL options:nil

documentAttributes:NULL error:outError];

if (fileContents) {

readSuccess = YES;

[self setText:fileContents];

}

return readSuccess;

}

Alternative Design Considerations

If your app needs to manipulate directly a document file that is a file package, you should override the

readFromFileWrapper:ofType:error: method instead of readFromData:ofType:error:. For example,

if your document contains an image file and a text file, you can store both in a file package. A major advantage

of this arrangement is that if only one of those objects changes during an editing session, you don’t need to

save both objects to disk but can save just the changed one. Figure 6-1 shows a file package containing an

image file and an object archive.

Figure 6-1 File package containing an image

When opening a document, the method looks for the image and text file wrappers. For each wrapper, the

method extracts the data from it and keeps the file wrapper itself. The file wrappers are kept so that, if the

corresponding data hasn't been changed, they can be reused during a save and thus the source file itself can

be reused rather than rewritten. Keeping the file wrapper avoids the overhead of syncing data unnecessarily.

Listing 6-3 shows an override of the NSDocument file wrapper reading method

readFromFileWrapper:ofType:error:.

The example code in Listing 6-3 (and its corresponding file wrapper writing override shown in Listing 6-5 (page

61)) assume the existence of some auto-synthesized properties and constants, such as those shown in Listing

6-2; of course, a complete NSDocument implementation also requires some additional program logic.

Listing 6-2 File wrapper example properties and constants

@property (assign) IBOutlet NSTextView *textView;

@property (nonatomic, strong) NSImage *image;

@property (strong) NSString *notes;

@property (strong) NSFileWrapper *documentFileWrapper;

NSString *ImageFileName = @"Image.png";

NSString *TextFileName = @"Text.txt";

Alternative Design Considerations

Overriding the URL and File Package Reading Methods

NSStringEncoding TextFileEncoding = NSUTF8StringEncoding;

Listing 6-3 File wrapper document-reading method implementation

- (BOOL)readFromFileWrapper:(NSFileWrapper *)fileWrapper

ofType:(NSString *)typeName

error:(NSError **)outError {

NSDictionary *fileWrappers = [fileWrapper fileWrappers];

NSFileWrapper *imageFileWrapper = [fileWrappers objectForKey:ImageFileName];

if (imageFileWrapper != nil) {

NSData *imageData = [imageFileWrapper regularFileContents];

NSImage *image = [[NSImage alloc] initWithData:imageData];

[self setImage:image];

}

NSFileWrapper *textFileWrapper = [fileWrappers objectForKey:TextFileName];

if (textFileWrapper != nil) {

NSData *textData = [textFileWrapper regularFileContents];

NSString *notes = [[NSString alloc] initWithData:textData

encoding:TextFileEncoding];

[self setNotes:notes];

}

[self setDocumentFileWrapper:fileWrapper];

return YES;

}

If the data related to a file wrapper changes (a new image is added or the text is edited), the corresponding

file wrapper object is disposed of and a new file wrapper created on save. See Listing 6-5 (page 61) which

shows an override of the corresponding file writing method, fileWrapperOfType:error:.

Alternative Design Considerations

Overriding the URL and File Package Reading Methods

Overriding the URL and File Package Writing Methods

As with document reading, there are situations in which the simplest solution for document writing, overriding

the data-based writing method, dataOfType:error:, as described in Writing Document Data (page 37), is

not sufficient. In such cases, you can override another NSDocument writing method instead, such as the

URL-based and file package writing methods.

If your app needs access to the URL of a document file, you should override the NSDocument URL-based writing

method, writeToURL:ofType:error:, as shown in Listing 6-4. This example has the same assumptions as

Listing 6-1 (page 57).

Listing 6-4 URL-based document-writing method implementation

- (BOOL)writeToURL:(NSURL *)inAbsoluteURL ofType:(NSString *)inTypeName

error:(NSError **)outError {

NSData *data = [[self text] RTFFromRange:NSMakeRange(0,

[[self text] length]) documentAttributes:nil];

BOOL writeSuccess = [data writeToURL:inAbsoluteURL

options:NSAtomicWrite error:outError];

return writeSuccess;

}

If your override cannot determine all of the information it needs from the passed-in parameters, consider

overriding another method. For example, if you see the need to invoke fileURL from within an override of

writeToURL:ofType:error:, you should instead override

writeToURL:ofType:forSaveOperation:originalContentsURL:error:. Override this method if your

document writing machinery needs access to the on-disk representation of the document revision that is about

to be overwritten. This method is responsible for doing document writing in a way that minimizes the danger

of leaving the disk to which writing is being done in an inconsistent state in the event of a software crash,

hardware failure, or power outage.

If your app needs to directly manipulate a document file that is a file package, you should override the

fileWrapperOfType:error: method instead of dataOfType:error:. An example file wrapper writing

method implementation is shown in Listing 6-5. In this implementation, if the document was not read from a

file or was not previously saved, it doesn't have a file wrapper, so the method creates one. Likewise, if the

document file wrapper doesn’t contain a file wrapper for an image and the image is not nil, the method

creates a file wrapper for the image and adds it to the document file wrapper. And if there isn’t a wrapper for

the text file, the method creates one.

Alternative Design Considerations

Overriding the URL and File Package Writing Methods

Listing 6-5 File wrapper document-writing method override

- (NSFileWrapper *)fileWrapperOfType:(NSString *)typeName

error:(NSError **)outError {

if ([self documentFileWrapper] == nil) {

NSFileWrapper * documentFileWrapper = [[NSFileWrapper alloc]

initDirectoryWithFileWrappers:nil];

[self setDocumentFileWrapper:documentFileWrapper];

}

NSDictionary *fileWrappers = [[self documentFileWrapper] fileWrappers];

if (([fileWrappers objectForKey:ImageFileName] == nil) &&

([self image] != nil)) {

NSArray *imageRepresentations = [self.image representations];

NSData *imageData = [NSBitmapImageRep

representationOfImageRepsInArray:imageRepresentations

usingType:NSPNGFileType

properties:nil];

if (imageData == nil) {

NSBitmapImageRep *imageRep = nil;

@autoreleasepool {

imageData = [self.image TIFFRepresentation];

imageRep = [[NSBitmapImageRep alloc] initWithData:imageData];

}

imageData = [imageRep representationUsingType:NSPNGFileType

properties:nil];

}

NSFileWrapper *imageFileWrapper = [[NSFileWrapper alloc]

initRegularFileWithContents:imageData];

[imageFileWrapper setPreferredFilename:ImageFileName];

[[self documentFileWrapper] addFileWrapper:imageFileWrapper];

Alternative Design Considerations

Overriding the URL and File Package Writing Methods

}

if ([fileWrappers objectForKey:TextFileName] == nil) {

NSData *textData = [[[self textView] string]

dataUsingEncoding:TextFileEncoding];

NSFileWrapper *textFileWrapper = [[NSFileWrapper alloc]

initRegularFileWithContents:textData];

[textFileWrapper setPreferredFilename:TextFileName];

[[self documentFileWrapper] addFileWrapper:textFileWrapper];

}

return [self documentFileWrapper];

}

Incremental Data Reading and Writing

If your app has a large data set, you may want to read and write increments of your files as needed to ensure

a good user experience. Consider the following strategies:

●Use file packages. If your app supports document files that are file packages, then you can override the

file-wrapper reading and writing methods. File wrapper (NSFileWrapper) objects that represent file

packages support incremental saving. For example, if you have a file package containing text objects and

graphic objects, and only one of them changes, you can write the changed object to disk but not the

unchanged ones.

●Use Core Data. You can subclass NSPersistentDocument, which uses Core Data to store your document

data in a managed object context. Core Data automatically supports incremental reading and writing of

only changed objects to disk.

For more information about reading and writing files, see File System Programming Guide.

Multiple Document Types Use Multiple NSDocument Subclasses

The document architecture provides support for apps that handle multiple types of documents, each type

using its own subclass of NSDocument. For example, you could have an app that enables users to create text

documents, spreadsheets, and other types of documents, all in a single app. Such different document types

each require a different user interface encapsulated in a unique NSDocument subclass.

Alternative Design Considerations

Incremental Data Reading and Writing

If your multiple-document-type app opens only existing documents, you can use the default

NSDocumentController instance, because the document type is determined from the file being opened.

However, if your app creates new documents, it needs to choose the correct type.

The NSDocumentController action method newDocument: creates a new document of the first type listed

in the app’s array of document types configured in the Info.plist file. But automatically creating the first

type does not work for apps that support several distinct types of document. If your app cannot determine

which type to create depending on circumstances, you must provide a user interface allowing the user to

choose which type of document to create.

You can create your own new actions, either in your app’s delegate or in an NSDocumentController subclass.

You could create several action methods and have several different New menu items, or you could have one

action that asks the user to pick a document type before creating a new document.

Once the user selects a type, your action method can use the NSDocumentController method

makeUntitledDocumentOfType:error: to create a document of the correct type. After creating the

document, your method should add it to the document controller’s list of documents, and it should send the

document makeWindowControllers and showWindows messages.

Alternatively, if you subclass NSDocumentController, you can override the defaultType method to

determine the document type and return it when the user chooses New from the File menu.

Additional Document Type Considerations

If your app has some document types that it can read but not write, you can declare this by setting the role

for those types to Viewer instead of Editor in Xcode. If your app has some types that it can write but not

read, you can declare this by using the NSExportableTypes key. You can include the NSExportableTypes

key in the type dictionary for another type that your document class supports, usually the type dictionary for

the most native type for your document class. Its value is an array of UTIs defining a supported file type to

which this document can export its content.

The Sketch sample app uses this key to allow it to export TIFF and PDF images even though it cannot read

those types. Write-only types can be chosen only when doing Save As operations. They are not allowed for

Save operations.

Sometimes an app might understand how to read a type, but not how to write it, and when it reads documents

of that type, it should automatically convert them to another type that you can write. An example would be

an app that can read documents from an older version or from a competing product. It might want to read in

the old documents and automatically convert them to the new native format. The first step is to add the old

type as a read-only type. By doing this, your app is able to open the old files, but they come up as untitled

files.

Alternative Design Considerations

Additional Document Type Considerations

If you want to automatically convert them to be saved as your new type, you can override the readFrom...

methods in your NSDocument subclass to call super and then reset the filename and type afterwards. You

should use setFileType: and setFileURL: to set an appropriate type and name for the new document.

When setting the filename, make sure to strip the filename extension of the old type from the original filename,

if it is there, and add the extension for the new type.

Customizing the Save Dialog

By default, when NSDocument runs the Save dialog and the document has multiple writable document types,

NSDocument inserts an accessory view near the bottom of the dialog. This view contains a pop-up menu of

the writable types. If you don’t want this pop-up menu, override shouldRunSavePanelWithAccessoryView

to return NO. You can also override prepareSavePanel: to customize the Save dialog.

Customizing Document Window Titles

Subclasses of NSDocument sometimes override displayName to customize the titles of windows associated

with the document. That is rarely the right thing to do because the document’s display name is used in places

other than the window title, and the custom value that an app might want to use as a window title is often

not appropriate. For example, the document display name is used in the following places:

●Error alerts that may be presented during reverting, saving, or printing of the document

●Alerts presented during document saving if the document has been moved, renamed, or move to the

Trash

●The alert presented when the user attempts to close the document with unsaved changes

●As the default value shown in the "Save As:" field of Save dialog

To customize a document’s window title properly, subclass NSWindowController and override

windowTitleForDocumentDisplayName:. If your app requires even deeper customization, override

synchronizeWindowTitleWithDocumentName.

Customizing Document Closing

If a document has multiple windows, each window has its own window controller. For example, a document

might have a main data-entry window and a window that lists records for selection; each window would have

its own NSWindowController object.

Alternative Design Considerations

Customizing the Save Dialog

If you have multiple window controllers for a single document, you may want to explicitly control document

closing. By default, a document closes when its last remaining window controller closes. However, if you want

the document to close when a particular window closes—the document’s “main” window, for example—then

you can send the main window controller a setShouldCloseDocument: message with a value of YES.

Message Flow in the Document Architecture

The objects that form the document architecture interact to perform the activities of document-based apps,

and those interactions proceed primarily through messages sent among the objects via public APIs. This

message flow provides many opportunities for you to customize the behavior of your app by overriding

methods in your NSDocument subclass or other subclasses.

This section describes default message flow among major objects of the document architecture, including

objects sending messages to themselves; it leaves out various objects and messages peripheral to the main

mechanisms. Also, these messages are sent by the default implementations of the methods in question, and

the behavior of subclasses may differ.

Creating a New Document

The document architecture creates a new document when the user chooses New from the File menu of a

document-based app. This action begins a sequence of messages among the NSDocumentController object,

the newly created NSDocument object, and the NSWindowController object, as shown in Figure 6-2.

Figure 6-2 Creating a new document

The sequence numbers in Figure 6-2 refer to the following steps in the document-creation process:

Alternative Design Considerations

Message Flow in the Document Architecture

1. The user chooses New from the File menu, causing the newDocument: message to be sent to the document

controller (or an Apple event, for example, sends an equivalent message).

2. The openUntitledDocumentAndDisplay:error: method determines the default document type

(stored in the app’s Info.plist file) and sends it with the

makeUntitledDocumentOfType:error:message.

3. The makeUntitledDocumentOfType:error: method determines the NSDocument subclass

corresponding to the document type, instantiates the document object, and sends it an initialization

message.

4. The document controller adds the new document to its document list and, if the first parameter passed

with openUntitledDocumentAndDisplay:error: is YES, sends the document a message to create a

window controller for its window, which is stored in its nib file. The NSDocument subclass can override

makeWindowControllers if it has more than one window.

5. The document adds the new window controller to its list of window controllers by sending itself an

addWindowController: message.

6. The document controller sends the document a message to show its windows. In response, the document

sends the window controller a showWindow: message, which makes the window main and key.

If the first parameter passed with openUntitledDocumentAndDisplay:error: is NO, the document

controller needs to explicitly send the document makeWindowControllers and showWindows messages to

display the document window.

Opening a Document

The document architecture opens a document, reading its contents from a file, when the user chooses Open

from the File menu. This action begins a sequence of messages among the NSDocumentController,

NSOpenPanel,NSDocument, and NSWindowController objects, as shown in Figure 6-3 (page 67).

There are many similarities between the mechanisms for opening a document and creating a new document.

In both cases the document controller needs to create and initialize an NSDocument object, using the proper

NSDocument subclass corresponding to the document type; the document controller needs to add the

document to its document list; and the document needs to create a window controller and tell it to show its

window.

Alternative Design Considerations

Message Flow in the Document Architecture

Document Opening Message Flow

Opening a document differs from creating a new document in several ways. If document opening was invoked

by the user choosing Open from the File menu, the document controller must run an Open dialog to allow

the user to select a file to provide the contents of the document. An Apple event can invoke a different message

sequence. In either case, the document must read its content data from a file and keep track of the file’s

meta-information, such as its URL, type, and modification date.

Figure 6-3 Opening a document

The sequence numbers in Figure 6-3 refer to the following steps in the document-opening process:

1. The user chooses Open from the File menu, causing the openDocument: message to be sent to the

document controller.

2. The URL locating the document file must be retrieved from the user, so the NSDocumentController

object sends itself the URLsFromRunningOpenPanel message. After this method creates the Open dialog

and sets it up appropriately, the document controller sends itself the runModalOpenPanel:forTypes:

message to present the Open dialog to the user. The NSDocumentController object sends the

runModalForTypes: message to the NSOpenPanel object.

3. With the resulting URL, the NSDocumentController object sends itself the

openDocumentWithContentsOfURL:display:completionHandler: message.

Alternative Design Considerations

Message Flow in the Document Architecture

4. The NSDocumentController object sends itself the

makeDocumentWithContentsOfURL:ofType:error: message and sends the

initWithContentsOfURL:ofType:error: message to the newly created NSDocument object. This

method initializes the document and reads in its contents from the file located at the specified URL.

Document Initialization Message Flow (page 68) describes document initialization in this context.

5. When makeDocumentWithContentsOfURL:ofType:error: returns an initialized NSDocument object,

the NSDocumentController object adds the document to its document list by sending the

addDocument: message to itself.

6. To display the document’s user interface, the document controller sends the makeWindowControllers

message to the NSDocument object, which creates an NSWindowController instance and adds it to its

list using the addWindowController: message.

7. Finally, the document controller sends the showWindows message to the NSDocument object, which, in

turn, sends the showWindow: message to the NSWindowController object, making the window main

and key.

8. If the URLsFromRunningOpenPanel method returned an array with more than one URL, steps 3 through

7 repeat for each URL returned.

Document Initialization Message Flow

Steps in the document-initialization process for document creation are shown in Figure 6-4. Document

initialization in the context of document opening is noteworthy because it invokes the document's

location-based or data-based reading and writing methods, and you must override one of them. Steps in the

document-initialization process for document opening are shown in Figure 6-5 (page 69).

Figure 6-4 Document initialization for document creation

The sequence numbers in Figure 6-4 refer to the following steps in the document-initialization process:

1. The NSDocumentController object begins document initialization by sending the

initWithType:error: message to the newly created NSDocument object.

Alternative Design Considerations

Message Flow in the Document Architecture

2. The NSDocument object sends the init message to itself, invoking its designated initializer, then sets its

filetype by sending itself the message setFileType:.

Figure 6-5 Document initialization for document opening

The sequence numbers in Figure 6-5 refer to the following steps in the document-opening process:

1. The NSDocumentController object begins document initialization by sending the

initWithContentsOfURL:ofType:error: message to the newly created NSDocument object.

2. The NSDocument object sends the init message to itself, invoking its designated initializer, then sets its

metadata about the file it is about to open by sending itself the messages setFileURL:,setFileType:,

and setFileModificationDate:.

3. The NSDocument object reads the contents of the file by sending the readFromURL:ofType:error:

message to itself. That method gets a file wrapper from disk and reads it by sending the

readFromFileWrapper:ofType:error: message to itself. Finally, the NSDocument object puts the

file contents into an NSData object and sends the readFromData:ofType:error: message to itself.

Your NSDocument subclass must override one of the three document-reading methods

(readFromURL:ofType:error:,readFromData:ofType:error:, or

readFromFileWrapper:ofType:error:) or every method that may invoke

readFromURL:ofType:error:.

Alternative Design Considerations

Message Flow in the Document Architecture

Saving a Document

The document architecture saves a document—writes its contents to a file—when the user chooses one of

the Save commands or Export from the File menu. Saving is handled primarily by the document object itself.

Steps in the document-saving process are shown in Figure 6-6.

Figure 6-6 Saving a document

The sequence numbers in Figure 6-6 refer to the following steps in the document-saving process:

1. The user chooses Save As (document has never been saved) or Save a Version (document has been saved

before) from the File menu, causing the saveDocument: message to be sent to the NSDocument object.

2. The NSDocument object sends the saveDocumentWithDelegate:didSaveSelector:contextInfo:

message to itself.

Alternative Design Considerations

Message Flow in the Document Architecture

If the document has never been saved, or if the user has moved or renamed the document file, then the

NSDocument object runs a modal Save dialog to get the file location under which to save the document.

3. To run the Save dialog, the NSDocument object sends the

runModalSavePanelForSaveOperation:delegate:didSaveSelector:contextInfo: message

to itself. The document sends prepareSavePanel: to itself to give subclasses an opportunity to customize

the Save dialog, then sends runModal to the NSSavePanel object.

4. The NSDocument object sends the

saveToURL:ofType:forSaveOperation:delegate:didSaveSelector:contextInfo: and, in

turn, saveToURL:ofType:forSaveOperation:error: to itself.

5. The NSDocument object sends the writeSafelyToURL:ofType:forSaveOperation:error: message

to itself. The default implementation either creates a temporary directory in which the document writing

should be done, or renames the old on-disk revision of the document, depending on what sort of save

operation is being done, whether or not there’s already a copy of the document on disk, and the capabilities

of the file system to which writing is being done. Then it sends the

writeToURL:ofType:forSaveOperation:originalContentsURL:error: message to the document.

6. To write the document contents to the file, the NSDocument object sends itself the

writeToURL:ofType:error: message, which by default sends the document the

fileWrapperOfType:error: message. That method, in turn, sends the document the

dataOfType:error: message to create an NSData object containing the contents of the document.

(For backward compatibility, if the deprecated dataRepresentationOfType: is overridden, the document

sends itself that message instead.)

The NSDocument subclass must override one of its document-writing methods (dataOfType:error:,

writeToURL:ofType:error:,fileWrapperOfType:error:, or

writeToURL:ofType:forSaveOperation:originalContentsURL:error:).

7. The NSDocument object sends the

fileAttributesToWriteToURL:ofType:forSaveOperation:originalContentsURL:error:

message to itself to get the file attributes, if any, which it writes to the file. The method then moves the

just-written file to its final location, or deletes the old on-disk revision of the document, and deletes any

temporary directories.

8. The NSDocument object updates its location, file type, and modification date by sending itself the messages

setFileURL:,setFileType:, and setFileModificationDate: if appropriate.

Alternative Design Considerations

Message Flow in the Document Architecture

This table describes the changes to Document-Based App Programming Guide for Mac .

NotesDate

Added cross-references to iCloud Design Guide. Clarified example

implementations of fileWrapperOfType:error: and

readFromFileWrapper:ofType:error:.

2012-12-13

Rewrote the implementations of document-reading and writing methods.

Removed references to manual memory management in favor of ARC.

2012-07-23

New document that explains how to create document-based applications

using the Cocoa document architecture on OS X.

2012-01-09

Document Revision History

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