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CHAPTER 9 GARBAGE COLLECTION
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Listin g 9-1. GC and Non-GC Property Implementations
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GC Environment @interface Doll : NSObject { NSColor *hairColor; NSColor *eyeColor; } @property (assign) NSColor *hairColor; @property (assign) NSColor *eyeColor; @end @implementation Doll @synthesize hairColor; - (NSColor*)eyeColor { return eyeColor; } - (void)setEyeColor:(NSColor*)color { eyeColor = color; } @end Non-GC Environment @interface Doll : NSObject { NSColor *hairColor; NSColor *eyeColor; } @property (retain) NSColor *hairColor; @property (retain) NSColor *eyeColor; @end @implementation Doll @synthesize hairColor; - (NSColor*)eyeColor { @synchronized(self) { return [[eyeColor retain] autorelease]; } }
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- (void)setEyeColor:(NSColor*)color { @synchronized(self) { if (eyeColor!=color) { [eyeColor release]; eyeColor = [color retain]; } } } @end
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When all references to an object are gone, the object is eligible to be collected and destroyed. In Objective-C, just as in Java, the garbage collector sends each collectable object a -finalize message before it is destroyed. The rules for well-behaved finalize methods are very similar to those in Java, so I ll just summarize them here: Don t attempt to perform time-consuming clean-up or resource recovery. That should be done before the object is forgotten. Don t disconnect object graphs or set instance variables to nil in an attempt to help the garbage collector (it s redundant). Don t attempt to remove the object from collections or view hierarchies (it s redundant). All weak references to the object will be disconnected before it is sent a -finalize message. It s generally safe to send messages to other objects, but keep it to a minimum. Objects may be finalized in any order, so your object should be prepared to receive messages (from other collectable objects) before or after receiving -finalize. Objects receiving the -finalize message should not attempt to resurrect collectable objects or attempt to resurrect themselves by creating a strong reference to self. Only one -finalize message is sent to each object. An object s -finalize methods must be thread safe.
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There are situations where your application wants to maintain a reference to an object but does not want to prevent the garbage collector from collecting it. The typical situation is a cache or pool of objects (let s say they are graphic images) that are used by many other objects. A single cache or pool of resource objects makes it convenient for individual objects to obtain references to those common resource objects. When all of the objects in the application are done with a resource, they all forget the object.
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CHAPTER 9 GARBAGE COLLECTION
Ideally, the resource object should now be reclaimed, but the single reference from the pool to the object keeps the resource object from being collected. This is solved using a weak reference. A weak reference is a pointer to an object that the garbage collector does not traverse when building the set of reachable objects. From the garbage collector s perspective, it is not a reference and does not prevent the object from being collected. In Java, weak references are established via java.lang.ref.WeakReference objects. To create a weak reference, a WeakReference object is created to hold the reference to the weakly referenced object, as shown in Listing 9-2. In Objective-C, appending the __weak modifier to any object pointer creates a weak reference.
Listin g 9-2. Creating Weak References
Java String name = "Clarence"; WeakReference weakName = new WeakReference(name); name = null; ... name = (String)weakName.get(); if (name!=null) { // ... name has not been collected } Objective-C __weak NSString *name = @"Clarence"; ... if (name!=nil) { // ... name has not been collected } A __weak object pointer is set to nil by the garbage collector whenever it determines that there are no other strong references to that object and the object is eligible for collection. The garbage collector guarantees that all __weak references to an object are set to nil before the object is finalized and destroyed. For the sake of clarity, all non-weak references are strong references. Objective-C does not support soft references or phantom references. Nor are there any reference queues, so your objects are not notified when the garbage collector decides to collect an object. To make it easy to manage groups of objects via weak references, both Java and Objective-C provide specialized collections that hold weak references to a set of objects, gracefully removing them when they are collected. These are listed in Table 9-1.
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