weakSelf和strongSelf

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

 

 

weak typeof(self)weakSelf=self;
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{

        __strong typeof(weakSelf)strongSelf=weakSelf;
        
        [strongSelf doSomething];
        
    });

 

weakSelf是爲了block不持有self，避免循環引用，而再聲明一個strongSelf是由於一旦進入block執行，就不容許self在這個執行過程當中釋放。block執行完後這個strongSelf會自動釋放，沒有循環引用問題。