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。

@interface MySingleton : NSObject
{
}

+ (MySingleton *)sharedSingleton;
@end

@implementation MySingleton

+ (MySingleton *)sharedSingleton
{
  static MySingleton *sharedSingleton;

  @synchronized(self)
  {
    if (!sharedSingleton)
      sharedSingleton = [[MySingleton alloc] init];

    return sharedSingleton;
  }
}

@end

(来源)

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;
}

如何

static MyClass *gInstance = NULL;

+ (MyClass *)instance
{
    if (gInstance == NULL) {
        @synchronized(self)
        {
            if (gInstance == NULL)
                gInstance = [[self alloc] init];
        }
    }

    return(gInstance);
}

这样就避免了初始化后的同步成本?

关于单例宏代码的详细解释在Cocoa With Love博客上

http://cocoawithlove.com/2008/11/singletons-appdelegates-and-top-level.html。

这不应该是线程安全的，避免第一次调用后昂贵的锁定吗?

+ (MySingleton*)sharedInstance
{
    if (sharedInstance == nil) {
        @synchronized(self) {
            if (sharedInstance == nil) {
                sharedInstance = [[MySingleton alloc] init];
            }
        }
    }
    return (MySingleton *)sharedInstance;
}