Objective-C单例模式的常用实现

+ (YourClass *)sharedInstance
{
    static dispatch_once_t once;
    static YourClass *sharedInstance = nil;

    dispatch_once(&once, ^
    {
        sharedInstance = [[self alooc] init];
    });

    return sharedInstance;
}

如果被多个线程调用，该函数会同步等等直至代码块完成。

AccountManager *accountManager = [AccountManager sharedManager];

AccountManager *accountManager = [[AccountManager alloc] init];