防止Block的循环引用 weakself strongself

1. __weak typeof(self)weakSelf=self;  
2.   dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{   
3.       __strong typeof(weakSelf)strongSelf=weakSelf;   
4.       [strongSelf doSomething];      
5.   });