【剑指Offer学习】【面试题02：实现Singleton 模式】

https://www.jianshu.com/p/edab2673ee6c

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

    return sharedInstance;
}

dispatch_once 内部实现

https://www.jianshu.com/p/b25535a7403e

https://xiaozhuanlan.com/topic/7916538240