在UIScrollView中使用UISwipeGestureRecognizer，无法检测到右键滑动，只检测左侧滑动

I have a UIView demoView in a UIScrollView. I want to scroll the UIScroll view vertically and don't scroll horizontally. I add UISwipeGestureRecognizer to the UIView. The code is (self here is the UIScrollView): in a UIScrollView. I w