Creating a Singleton Instance

转自：http://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/CocoaFundamentals/CocoaObjects/CocoaObjects.html#//apple_ref/doc/uid/TP40002974-CH4-SW32

Some classes of the Foundation and AppKit frameworks create singleton objects. In a strict implementation, a singleton is the sole allowable instance of a class in the current process. But you can also have a more flexible singleton implementation in which a factory method always returns the same instance, but you can allocate and initialize additional instances.TheNSFileManagerclass fits this latter pattern, whereas theUIApplicationfits the former. When you ask for an instance ofUIApplication, it passes you a reference to the sole instance, allocating and initializing it if it doesn’t yet exist.

A singleton object acts as a kind of control center, directing or coordinating the services of the class. Your class should generate a singleton instance rather than multiple instances when there is conceptually only one instance (as with, for example,NSWorkspace). You use singleton instances rather than factory methods or functions when it is conceivable that there might be multiple instances someday.

To create a singleton as the sole allowable instance of a class in the current process, you need to have an implementation similar toListing 2-15. This code does the following:

• It declares a static instance of your singleton object and initializes it tonil.

• In your class factory method for the class (named something like “sharedInstance” or “sharedManager”), it generates an instance of the class but only if the static instance isnil.

• It overrides theallocWithZone:method to ensure that another instance is not allocated if someone tries to allocate and initialize an instance of your class directly instead of using the class factory method. Instead, it just returns the shared object.

• It implements the base protocol methodscopyWithZone:,release,retain,retainCount, andautoreleaseto do the appropriate things to ensure singleton status. (The last four of these methods apply to memory-managed code, not to garbage-collected code.)

Listing 2-15Strict implementation of a singleton

static MyGizmoClass *sharedGizmoManager = nil;

+ (MyGizmoClass*)sharedManager

{

    if (sharedGizmoManager == nil) {

        sharedGizmoManager = [[super allocWithZone:NULL] init];

    }

    return sharedGizmoManager;

}

+ (id)allocWithZone:(NSZone *)zone

{

    return [[self sharedManager] retain];

}

- (id)copyWithZone:(NSZone *)zone

{

    return self;

}

- (id)retain

{

    return self;

}

- (NSUInteger)retainCount

{

    return NSUIntegerMax;  //denotes an object that cannot be released

}

- (void)release

{

    //do nothing

}

- (id)autorelease

{

    return self;

}

If you want a singleton instance (created and controlled by the class factory method) but also have the ability to create other instances as needed through allocation and initialization, do not overrideallocWithZone:and the other methods following it as shown inListing 2-15.