Cocoa-cocoa如何实现单态(singleton)类实例？

Question

苹果官方建议使用以下方式来实现单态模式：

    static MyGizmoClass *sharedGizmoManager = nil;
     
    + (MyGizmoClass*)sharedManager
    {
        @synchronized(self)  {
            if  (sharedGizmoManager == nil) {
                [[self alloc] init]; // assignment not done here
            }
        }
        return sharedGizmoManager;
    }
     
    + (id)allocWithZone:(NSZone *)zone
    {
        @synchronized(self) {
            if (sharedGizmoManager == nil) {
                sharedGizmoManager = [super allocWithZone:zone];
                return sharedGizmoManager;  // assignment and return on first allocation
            }
        }
        return nil; //on subsequent allocation attempts return nil
    }
     
    - (id)copyWithZone:(NSZone *)zone
    {
        return self;
    }
     
    -  (id)retain
    {
        return self;
    }
     
    -  (unsigned)retainCount
    {
        return UINT_MAX;  //denotes an object that cannot be released
    }
     
    - (void)release
    {
        //do nothing
    }
     
    - (id)autorelease
    {
        return self;
    }

Answer 1

我现在在用的实现：

static Sample* __sharedSampleManager = nil;

@implementation Sample

+(Sample*) sharedSampleManager
{
return __sharedSampleManager;
}

+(void) initialize
{
static BOOL __bInitialize = false;
if(!__bInitialize)
{
__bInitialize = true;
__sharedSampleManager = [[self alloc] init];
}
}

-(void) dealloc
{
[super dealloc];
[__sharedSampleManager release];
__sharedSampleManager = nil;
}
@end

线程安全:
定义宏:

#define DEFINE_SHARED_INSTANCE_USING_BLOCK(block)
static dispatch_once_t pred = 0;
__strong static id _sharedObject = nil;
dispatch_once(&pred, ^{
_sharedObject = block();
});
return _sharedObject;

+(id)sharedInstance
{
DEFINE_SHARED_INSTANCE_USING_BLOCK(^{
return [[self alloc] init];
});
}