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I'm very new to Android programming, and trying to understand touch events with nested views. To start, here's a description of my app:

I have a relative layout that I've added via the GUI editor. Everything is default. I've also created a class called ClipGrid that extends ScrollView. Nested inside that, I make a HorizontalScrollView. Inside of that, I make a TableLayout and it's rows. The rows contain buttons.

The end result is a grid of buttons. It displays 4x4 at once, but can scroll either direction to display other buttons.

I call it to the screen from my main activity like this: ClipGrid clip_grid = new ClipGrid(this); setContentView(clip_grid);

I did that just for testing purposes, and I think I will have to change it later when I want to add other views to my relativelayout. But I think it might have implications for touch events.

in the end, I want to detect when the grid has been moved and snap the newly viewable 4x4 grid of buttons to the edge of my layout when the user lifts their finger. I'm just not sure how to go about implementing this and any help would be appreciated. Thanks.

ultra
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1 Answers1

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The way touch events are handled is kind of a cascading effect that starts from the top view and goes down to the lower nested views. Basically, Android will pass the event to each view until true is returned.

The general way you could implement the onTouchEvent event of a View would be:

@Override
public boolean onTouchEvent(MotionEvent event) {
  boolean actionHandled = false;
  final int action = event.getAction();

  switch(action & MotionEventCompat.ACTION_MASK) {
    case MotionEvent.ACTION_DOWN:
      // user first touches view with pointer
      break;
    case MotionEvent.ACTION_MOVE:
      // user is still touching view and moving pointer around
      break;
    case MotionEvent.ACTION_UP:
      // user lifts pointer
      break;
  }

  // if the action was not handled by this touch, send it to other views
  if (!actionHandled) 
     actionHandled |= super.onTouch(v, MotionEvent.event);

  return actionHandled;
}
DeeV
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  • Thanks for replying. So, say I want my table layout to handle the touch events. Since it's nested inside a horizontal scrollview and a scrollview, do I return false from those parent views (and true from the table layout) or not even implement the event? as a side note, is there a better way to achieve what I'm trying to do? – ultra Aug 13 '12 at 13:44
  • If the View doesn't handle touch events, then don't implement it. They return false by default. From what it sounds like, you may want to look in to `ViewPager`. Aside from that, you may want to detect the swiping events in the `ScrollView`. It gives and knows the dimensions of it's children. It would also be better if it controlled the movements rather than the children. With `ScrollViews`, you can also use `onInterceptTouchEvent()` which basically steals the touch event from the top views to handle it before giving the events to the children. – DeeV Aug 13 '12 at 13:54
  • thanks, this got me going in the right direction and now i have it working. turns out, i have to implement an onTouchListener from both the scrollview or its nested horizontal scrollview, otherwise it'll only detect scrolling in one direction. also, returning true from either of these will actually stop the table layout from scrolling. – ultra Aug 13 '12 at 23:29