当手势添加到view上的时候，手势会开始观察view和view上的subviews。
当手势被识别的时候，之前的touch将被取消同时不会再传递
当然这个可以通过设置cancelsTouchesInView为NO来取消或者开启，具体的可以看看Apple文档
> A gesture recognizer operates on touches hit-tested to a specific view and all of that view’s subviews.
> ......
> cancelsTouchesInView—If a gesture recognizer recognizes its gesture, it unbinds the remaining touches of that gesture from their view (so the window won’t deliver them). The window cancels the previously delivered touches with a (touchesCancelled:withEvent:) message. If a gesture recognizer doesn’t recognize its gesture, the view receives all touches in the multi-touch sequence.