onTouchEvent

Android onTouchEvent

Required video you should watch! : https://www.youtube.com/watch?v=SYoN-OvdZ3M
(Android onTouchEvent Part 1)

How Android Handles Touches

• Each user touch event is wrapped up as MotionEvent
• Describe user's current action
  • ACTION_DOWN
  • ACTION_UP
  • ACTION_MOVE
  • ACTION_POINTER_DOWN
  • ACTION_POINTER_UP
  • ACTION_CANCEL
• Event metadata included
  • Touch location
  • Number of pointers (fingers)
  • Event time
• A 'gesture' is defined as begining with ACTION_DOWN and ending with ACTION_UP

---

• Activity.dispatchTouchEvent()
  • Always first to be called
  • Sends event to root view attached to Window
  • onTouchEvent()
    • Called if no views consume the event
    • Always last to be called
• View.dispatchTouchEvent()
  • Sends event to listener first, if exists
    • View.OnTouchListener.onTouch()
  • If not consumed, processes touch itself
    • View.onTouchEvent()

How a ViewGroup's simple dispatchTouch looks like?

public boolean dispatchTouchEvent(MotionEvent ev) {
    if (!onInterceptTouchEvent()) {
        for (int i = children.size(); i >= 0; i--){
            if (children.get(i).dispatchTouchEvent(ev))
                return true;
        }
        return super.dispatchTouchEvent(ev);
    }
}