NSDiscardableContent Protocol Reference

Adopted by
Conforms to
Framework
/System/Library/Frameworks/Foundation.framework
Availability
Available in OS X v10.6 and later.
Declared in
NSObject.h

Overview

You implement this protocol when a class’s objects have subcomponents that can be discarded when not being used, thereby giving an application a smaller memory footprint.

An NSDiscardableContent object’s life cycle is dependent upon a “counter” variable. An NSDiscardableContent object is a purgeable block of memory that keeps track of whether or not it is currently being used by some other object. When this memory is being read, or is still needed, its counter variable will be greater than or equal to 1. When it is not being used, and can be discarded, the counter variable will be equal to 0.

When the counter is equal to 0, the block of memory may be discarded if memory is tight at that point in time. In order to discard the content, call discardContentIfPossible on the object, which will free the associated memory if the counter variable equals 0.

By default, NSDiscardableContent objects are initialized with their counter equal to 1 to ensure that they are not immediately discarded by the memory-management system. From this point, you must keep track of the counter variable’s state. Calling the beginContentAccess method increments the counter variable by 1, thus ensuring that the object will not be discarded. When you no longer need the object, decrement its counter by calling endContentAccess.

The Foundation framework includes the NSPurgeableData class, which provides a default implementation of this protocol.

Tasks

Accessing Content

Discarding Content

Instance Methods

beginContentAccess

Returns a Boolean value indicating whether the discardable contents are still available and have been successfully accessed. (required)

- (BOOL)beginContentAccess
Return Value

YES if the discardable contents are still available and have now been successfully accessed; otherwise, NO.

Discussion

Call this method if the object’s memory is needed or is about to be used. This method increments the counter variable, thus protecting the object’s memory from possibly being discarded. The implementing class may decide that this method will try to recreate the contents if they have been discarded and return YES if the re-creation was successful. Implementors of this protocol should raise exceptions if the NSDiscardableContent objects are used when the beginContentAccess method has not been called on them.

Availability
  • Available in OS X v10.6 and later.
Declared In
NSObject.h

discardContentIfPossible

Called to discard the contents of the receiver if the value of the accessed counter is 0. (required)

- (void)discardContentIfPossible
Discussion

This method should only discard the contents of the object if the value of the accessed counter is 0. Otherwise, it should do nothing.

Availability
  • Available in OS X v10.6 and later.
Declared In
NSObject.h

endContentAccess

Called if the discardable contents are no longer being accessed. (required)

- (void)endContentAccess
Discussion

This method decrements the counter variable of the object, which will usually bring the value of the counter variable back down to 0, which allows the discardable contents of the object to be thrown away if necessary.

Availability
  • Available in OS X v10.6 and later.
Declared In
NSObject.h

isContentDiscarded

Returns a Boolean value indicating whether the content has been discarded. (required)

- (BOOL)isContentDiscarded
Return Value

YES if the content has been discarded; otherwise, NO.

Availability
  • Available in OS X v10.6 and later.
Declared In
NSObject.h