Thread safe Singleton 寫法

http://stackoverflow.com/questions/2199106/thread-safe-instantiation-of-a-singleton

+(MyClass *)sharedInstance

{

static MyClass *sharedInstance = nil;

static dispatch_once_t pred;

dispatch_once(&pred, ^{

sharedInstance = [MyClass alloc];

sharedInstance = [sharedInstance init];

});

return sharedInstance;

}