+ (instancetype)sharedInstance
{
    static MyClass *sharedInstance = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        sharedInstance = [[MyClass alloc] init];
    });
    return sharedInstance;
}

Singleton *inst1 = [Singleton sharedInstance];
Singleton *inst2 = [Singleton sharedInstance];

static MyClass *sharedInstance = nil;