问题
I want to track a finger touch on the screen. So what I did was to start recording the position when MotionEvent triggers ACTION_DOWN, but how do I know when the action is finished, at ACTION_CANCEL, or ACTION_UP?
What's the exact difference between them?
回答1:
MotionEvent:
ACTION_UP: A pressed gesture has finished, the motion contains the final release location as well as any intermediate points since the last down or move event.
ACTION_CANCEL: The current gesture has been aborted.
ACTION_CANCEL occurs when the parent takes possession of the motion, for example when the user has dragged enough across a list view that it will start scrolling instead of letting you press the buttons inside of it. You can find out more about it at the viewgroup documentation: onInterceptTouchEvent.
so use ACTION_CANCEL when the action is dragged out of the parent, and ACTION_UP otherwise.
回答2:
In general ACTION_UP is triggered by user when the guest is finished to definitely indicate that the user completed interacting with movement.
On the other hand ACTION_CANCEL is called by Android system to indicate that one of views took control using onInterceptTouchEvent() which returned true as a result system finished propagating the touch event.
Please take a look at a diagram
来源：https://stackoverflow.com/questions/5240719/whats-the-difference-between-action-cancel-and-action-up-in-motionevent