> For the complete documentation index, see [llms.txt](https://ryukiedev.gitbook.io/wiki/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://ryukiedev.gitbook.io/wiki/ios/duo-xian-cheng/wei-shi-mo-yong-dispatchonce-shi-xian-dan-li.md).

# 03.为什么用dispatch-once实现单例

> 面试问到了 自己说到了这点(猜的是线程安全的查了一下确实是这样)

```
+ (XXXX *)sharedInstance
{
    static id sharedInstance = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        sharedInstance = [[self alloc] init];
    });
    return sharedInstance;
}
```

`dispatch_once`是线程安全的,可以确保

## 参考

> <http://www.cnblogs.com/hellocby/archive/2012/08/24/2654488.html> <https://blog.csdn.net/qqMCY/article/details/88648000> 单例alloc 、 copy的处理