[NSThread sleepForTimeInterval:1] 8.1崩溃 9.1不崩 为啥

  

搞不懂这段代码为啥 8.1会崩溃 9.1不会崩

  UIAlertController *alertVC = [UIAlertControlleralertControllerWithTitle:str message:messageStrpreferredStyle:UIAlertControllerStyleAlert];

    [selfpresentViewController:alertVC animated:YEScompletion:^{

     [NSThreadsleepForTimeInterval:1];

            [_session startRunning];

            [alertVC dismissViewControllerAnimated:YEScompletion:nil];

    }];

    //改成下面这段就没问题了

    UIAlertController *alertC = [UIAlertControlleralertControllerWithTitle:str message:messageStrpreferredStyle:UIAlertControllerStyleAlert];

    [selfpresentViewController:alertC animated:YEScompletion:^{

        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(1.0 * NSEC_PER_SEC)),dispatch_get_main_queue(), ^{

            [_session startRunning];

            [alertC dismissViewControllerAnimated:YEScompletion:nil];

        });

    }];