确定过渡是否完成

Question

我有一个带按钮的外部设备。按此按钮时，它会切换视图控制器。当我慢慢按下按钮时，它们转换得很好。但是，当我按下按钮太快时，应用程序崩溃并显示错误：

Terminating app due to uncaught exception NSInternalInconsistencyException, reason: Attempting to begin a modal transition from <SViewController: (numbers)> to <VViewController: (numbers)> while a transition is already in progress. Wait for viewDidAppear/viewDidDisappear to know the current transition has completed

我不知道如何解决这个问题。这是我用来在视图控制器之间切换的代码。我在每个视图控制器中放置了这个if语句（当然是切换视图控制器）：

 if ([data isEqualToString: @"Switch1"]) {
        UIStoryboard *sb = [UIStoryboard storyboardWithName:@"MainStoryboard_iPhone" bundle:nil];
        UIViewController *vc = [sb instantiateViewControllerWithIdentifier:@"VViewController"];
        vc.modalTransitionStyle = UIModalTransitionStyleCrossDissolve;
        [self presentViewController:vc animated:YES completion:NULL];
    }

错误说是检查viewDidLoad', but I have no idea what code to put in viewDidLoad`以确定转换是否已完成，如果没有，则阻止它再次尝试转换。