这是我放在一起的一个宏:

http://github.com/cjhanson/Objective-C-Optimized-Singleton

它基于马特·加拉格尔的作品 但是将实现改为使用谷歌的Dave MacLachlan所描述的方法swizzling。

我欢迎评论/贡献。

我在sharedInstance上有一个有趣的变化，它是线程安全的，但在初始化后不锁定。我还不确定是否可以按照要求修改顶部的答案，但我将其提出以供进一步讨论:

// Volatile to make sure we are not foiled by CPU caches
static volatile ALBackendRequestManager *sharedInstance;

// There's no need to call this directly, as method swizzling in sharedInstance
// means this will get called after the singleton is initialized.
+ (MySingleton *)simpleSharedInstance
{
    return (MySingleton *)sharedInstance;
}

+ (MySingleton*)sharedInstance
{
    @synchronized(self)
    {
        if (sharedInstance == nil)
        {
            sharedInstance = [[MySingleton alloc] init];
            // Replace expensive thread-safe method 
            // with the simpler one that just returns the allocated instance.
            SEL origSel = @selector(sharedInstance);
            SEL newSel = @selector(simpleSharedInstance);
            Method origMethod = class_getClassMethod(self, origSel);
            Method newMethod = class_getClassMethod(self, newSel);
            method_exchangeImplementations(origMethod, newMethod);
        }
    }
    return (MySingleton *)sharedInstance;
}

static MyClass *sharedInst = nil;

+ (id)sharedInstance
{
    @synchronize( self ) {
        if ( sharedInst == nil ) {
            /* sharedInst set up in init */
            [[self alloc] init];
        }
    }
    return sharedInst;
}

- (id)init
{
    if ( sharedInst != nil ) {
        [NSException raise:NSInternalInconsistencyException
            format:@"[%@ %@] cannot be called; use +[%@ %@] instead"],
            NSStringFromClass([self class]), NSStringFromSelector(_cmd), 
            NSStringFromClass([self class]),
            NSStringFromSelector(@selector(sharedInstance)"];
    } else if ( self = [super init] ) {
        sharedInst = self;
        /* Whatever class specific here */
    }
    return sharedInst;
}

/* These probably do nothing in
   a GC app.  Keeps singleton
   as an actual singleton in a
   non CG app
*/
- (NSUInteger)retainCount
{
    return NSUIntegerMax;
}

- (oneway void)release
{
}

- (id)retain
{
    return sharedInst;
}

- (id)autorelease
{
    return sharedInst;
}

我的方法很简单:

static id instanceOfXXX = nil;

+ (id) sharedXXX
{
    static volatile BOOL initialized = NO;

    if (!initialized)
    {
        @synchronized([XXX class])
        {
            if (!initialized)
            {
                instanceOfXXX = [[XXX alloc] init];
                initialized = YES;
            }
        }
    }

    return instanceOfXXX;
}

如果单例已经初始化，LOCK块将不会被输入。第二次检查if(!initialized)是为了确保当前线程获得LOCK时它还没有初始化。

扩展一下@robbie-hanson的例子…

static MySingleton* sharedSingleton = nil;

+ (void)initialize {
    static BOOL initialized = NO;
    if (!initialized) {
        initialized = YES;
        sharedSingleton = [[self alloc] init];
    }
}

- (id)init {
    self = [super init];
    if (self) {
        // Member initialization here.
    }
    return self;
}

这也适用于非垃圾收集环境。

@interface MySingleton : NSObject {
}

+(MySingleton *)sharedManager;

@end


@implementation MySingleton

static MySingleton *sharedMySingleton = nil;

+(MySingleton*)sharedManager {
    @synchronized(self) {
        if (sharedMySingleton == nil) {
            [[self alloc] init]; // assignment not done here
        }
    }
    return sharedMySingleton;
}


+(id)allocWithZone:(NSZone *)zone {
    @synchronized(self) {
        if (sharedMySingleton == nil) {
            sharedMySingleton = [super allocWithZone:zone];
            return sharedMySingleton;  // assignment and return on first allocation
        }
    }
    return nil; //on subsequent allocation attempts return nil
}


-(void)dealloc {
    [super dealloc];
}

-(id)copyWithZone:(NSZone *)zone {
    return self;
}


-(id)retain {
    return self;
}


-(unsigned)retainCount {
    return UINT_MAX;  //denotes an object that cannot be release
}


-(void)release {
    //do nothing    
}


-(id)autorelease {
    return self;    
}


-(id)init {
    self = [super init];
    sharedMySingleton = self;

    //initialize here

    return self;
}

@end