android - Catch onTouch event by parent, handle it and pass to children

Question

I tried to understand how Android handle touch event and got a little bit confused. From what I understand touch event are send to the root view and pass down to the children. I have a FrameLayout that is a container for Fragment. First fragment view is a ScrollView, second one is some kind of Gallery (HorizontalListView) and the last one is also FrameLayout. Only one fragment in the layout each time. What I want to do is to identify user swipes on the screen, for the app use. I want to count the swipes and do something after some number of swipes.

I tried to put a OnTouchListener on the top FrameLayout but it doesn't get called when the child is the ScrollView or the Gallery. I tried to return false and also true in the end of onTouch, but I get same result - it's never being called.

How can I do it? I just want to "transparently" handle the touch events and passing them on like I didn't even touch them.

Answer 1

My understanding is that it actually goes the other direction. The Child views get their event triggered first (sort of). The root view get's it's dispatchTouchEvent() called, which propagates the event down to the children's onTouchEvent(), and then, depending on whether they return true or false, the parent's onTouchEvent() is called.

The normal solution for intercepting things like this is to override dispatchTouchEvent(MotionEvent ev) in one's activity like so:

@Override
public boolean dispatchTouchEvent (MotionEvent ev) {
  // Do your calcluations
  return super.dispatchTouchEvent(ev);
}

The documentation for this one is here. Note that you can also override that method in any ViewGroup (such as a FrameLayout, etc)