UISwipeGestureRecognizerDirectionRight正在运行，但UISwipeGestureRecognizerDirectionDown无法正常工作

Question

如果我添加一个向下手势识别器，那么它不起作用，如果我将其更改为UISwipeGestureRecognizerDirectionRight，那么它可以工作。

    UISwipeGestureRecognizer *swipeDownRecognizer =
      [[UISwipeGestureRecognizer alloc] initWithTarget:_delegate
                                                action:@selector(statusViewDidDismiss:)];
    swipeDownRecognizer.direction = UISwipeGestureRecognizerDirectionDown;
    [swipeDownRecognizer setNumberOfTouchesRequired:1];
    [self addGestureRecognizer:swipeDownRecognizer];