NSDocument abstract class defines the interface for macOS documents. A document is an object that can internally represent data displayed in a window and that can read data from and write data to a file or file package. Documents create and manage one or more window controllers and are in turn managed by a document controller. Documents respond to first-responder action messages to save, revert, and print their data.
- macOS 10.10+
Conceptually, a document is a container for a body of information identified by a name under which it is stored in a disk file. In this sense, however, the document is not the same as the file but is an object in memory that owns and manages the document data. In the context of AppKit, a document is an instance of a custom
NSDocument subclass that knows how to represent internally, in one or more formats, persistent data that is displayed in windows.
A document can read that data from a file and write it to a file. It is also the first-responder target for many menu commands related to documents, such as Save, Revert, and Print. A document manages its window’s edited status and is set up to perform undo and redo operations. When a window is closing, the document is asked before the window delegate to approve the closing.
NSDocument class is designed to be subclassed. That is, the
NSDocument class is abstract, and your app must create at least one
NSDocument subclass in order to use the document architecture. To create a useful
NSDocument subclass, you must override some methods, and you can optionally override others.
NSDocument class itself knows how to handle document data as undifferentiated lumps; although it understands that these lumps are typed, it knows nothing about particular types. In their overrides of the data-based reading and writing methods, subclasses must add the knowledge of particular types and how data of the document’s native type is structured internally. Subclasses are also responsible for the creation of the window controllers that manage document windows and for the implementation of undo and redo. The
NSDocument class takes care of much of the rest, including generally managing the state of the document.
See Document-Based App Programming Guide for Mac for more information about creating subclasses of
NSDocument, particularly the list of primitive methods that subclasses must override and those that you can optionally override.
Document Saving Behavior
NSDocument class implements document saving in a way that preserves, when possible, various attributes of each document, including:
Location of the document’s icon in its parent folder’s Icon View Finder window
Value of the document’s Show Extension setting
Care is also taken to save documents in a way that does not break any user-created aliases that may point to documents. As a result, some methods in any class of
NSDocument may be invoked with parameters that do not have the same meaning as they did in early releases of macOS. It is important that overrides of
write(to:ofType:for:originalContentsURL:) make no assumptions about the file paths passed as parameters, including:
The location to which the file is being written. This location might be a hidden temporary directory.
The name of the file being written. It is possible that this file has no obvious relation to the document name.
The relation of any file being passed, including the original file, to the value in
When updating your app to link against OS X v10.5, keep in mind that it is usually more appropriate to invoke in your app code one of the
save... methods than one of the
write... methods. The
write... methods are there primarily for you to override. The
saveToURL:ofType:forSaveOperation:error: method that is meant always to be invoked during document saving, sets the
fileModificationDate property with the file’s new modification date after it has been written (for
Likewise, it’s usually more appropriate to invoke in your app code one of the
revert... methods than one of the
read... methods. The
read... methods are there primarily for you to override. The
revert(toContentsOf:ofType:) method that is meant always to be invoked during rereading of an open document, sets the
fileModificationDate property with the file’s modification date after it has been read.
NSDocument class implements the file coordination support that is required for an iCloud-enabled, document-based Mac app (see How iCloud Document Storage Works in iCloud Design Guide). In addition, this class’s methods for moving and renaming documents, new in OS X v10.8, ensure that these operations are performed in a safe manner for iCloud-enabled apps.
In macOS 10.6 and later,
NSDocument supports the ability to open multiple documents concurrently. However, this support requires the cooperation of the document object. If your document subclass is able to read specific document types independently of other similar documents, you should override the
canConcurrentlyReadDocuments(ofType:) class method and return
true for the appropriate document types. If specific document types rely on shared state information, however, you should return
false for those types.