苹果官方的单例写法

很多朋友喜欢用单例，苹果官方怎么写单例呢？

这个是苹果的示例代码SimpleNetworkStreams

http://developer.apple.com/library/ios/#samplecode/SimpleNetworkStreams/Introduction/Intro.html#//apple_ref/doc/uid/DTS40008979

+ (NetworkManager *)sharedInstance
{
    static dispatch_once_t  onceToken;
    static NetworkManager * sSharedInstance;

    dispatch_once(&onceToken, ^{
        sSharedInstance = [[NetworkManager alloc] init];
    });
    return sSharedInstance;
}