DataSingleton单例

#pragma 单例模式定义
static DataSingleton * instance = nil;
+(DataSingleton *) Instance
{
    @synchronized(self)
    {
        if(nil == instance)
        {
            [self new];
        }
    }
    return instance;
}
+(id)allocWithZone:(NSZone *)zone
{
    @synchronized(self)
    {
        if(instance == nil)
        {
            instance = [super allocWithZone:zone];
            return instance;
        }
    }
    return nil;
}
+ (id)copyWithZone:(NSZone *)zone {
    return self;
}

- (id)retain {
    return self;
}

- (unsigned)retainCount {
    return UINT_MAX;
}

- (oneway void)release {
}

- (id)autorelease {
    return self;
}

这种写法已经足够严谨，同时考虑了内存池以及多线程的特殊情况。Apple的官方示例，但没有包含多线程考虑。

需要注意的是instance的初始化使用了[self new]，它是alloc+init的简要写法；调用alloc由于历史原因，objc会默认调用 allocWithZone:，

更为简洁明快的实现方式可以参考：http://eschatologist.net/blog/?p=178

 1 // One way to do this would be to create your singleton instance in +initialize since it will always be run, on a single thread, before any other methods in your class
 2 @implementation SomeManager
 3 
 4 static id sharedManager = nil;
 5 
 6 + (void)initialize {
 7     if (self == [SomeManager class]) {
 8         sharedManager = [[self alloc] init];
 9     }
10 }
11 
12 + (id)sharedManager {
13     return sharedManager;
14 }
15 
16 @end

关于alloc和allocWithZone可以参考：http://www.justinyan.me/post/1306