title: What's the difference between ACTION_CANCEL and ACTION_UP in MotionEvent?
body: <p>I want to track a finger touch on the screen. So what I did was to start recording the position when <code>MotionEvent</code> triggers <code>ACTION_DOWN</code>, but how do I know when the action is finished, at <code>ACTION_CANCEL</code>, or <code>ACTION_UP</code>?</p>
  
  <p>What's the exact difference between them?</p>