根据我下面的另一个回答，我认为你应该这样做:

+ (id)sharedFoo
{
    static dispatch_once_t once;
    static MyFoo *sharedFoo;
    dispatch_once(&once, ^ { sharedFoo = [[self alloc] init]; });
    return sharedFoo;
}

关于Objective-C中单例模式的深入讨论，请看这里:

在Objective-C中使用单例模式

你不想在self上同步…因为self对象还不存在!最终锁定一个临时id值。你想要确保没有其他人可以运行类方法(sharedInstance, alloc, allocWithZone:等)，所以你需要在类对象上同步:

@implementation MYSingleton

static MYSingleton * sharedInstance = nil;

+( id )sharedInstance {
    @synchronized( [ MYSingleton class ] ) {
        if( sharedInstance == nil )
            sharedInstance = [ [ MYSingleton alloc ] init ];
    }

    return sharedInstance;
}

+( id )allocWithZone:( NSZone * )zone {
    @synchronized( [ MYSingleton class ] ) {
        if( sharedInstance == nil )
            sharedInstance = [ super allocWithZone:zone ];
    }

    return sharedInstance;
}

-( id )init {
    @synchronized( [ MYSingleton class ] ) {
        self = [ super init ];
        if( self != nil ) {
            // Insert initialization code here
        }

        return self;
    }
}

@end

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;
}

根据我下面的另一个回答，我认为你应该这样做:

+ (id)sharedFoo
{
    static dispatch_once_t once;
    static MyFoo *sharedFoo;
    dispatch_once(&once, ^ { sharedFoo = [[self alloc] init]; });
    return sharedFoo;
}

我通常使用的代码大致类似于Ben Hoffstein的回答(这也是我从维基百科上得到的)。我使用它的原因是Chris Hanson在他的评论中提到的。

然而，有时我需要将一个单例放入NIB中，在这种情况下，我使用以下方法:

@implementation Singleton

static Singleton *singleton = nil;

- (id)init {
    static BOOL initialized = NO;
    if (!initialized) {
        self = [super init];
        singleton = self;
        initialized = YES;
    }
    return self;
}

+ (id)allocWithZone:(NSZone*)zone {
    @synchronized (self) {
        if (!singleton)
            singleton = [super allocWithZone:zone];     
    }
    return singleton;
}

+ (Singleton*)sharedSingleton {
    if (!singleton)
        [[Singleton alloc] init];
    return singleton;
}

@end

我把-retain(等等)的实现留给读者，尽管在垃圾收集环境中您只需要上面的代码。