Android: tearing effect while calling requestLayout() in OnTouchListener methods? a way to fix?

Question

I've tried adding some Drag&Drop mechanism to my project. the way I'm doing it is basically making two operations each time a touch event occurs:
first, updating the parameters I'm using inside onLayout() according to touch event
second, calling requestLayout() for refreshing.
(I've tried it both with OnTouchListener methods and with View's onTouchEvent() as in the code to be followed)

problem is, the result on screen wasn't so good. it gave the feeling like there is some tearing problem with the dragged view(the new drawing starts before the earlier ends)

the code looks something like this (a simplified version):

public class DragAndDrop extends ViewGroup
{
  View touchView;
  float touchX;
  float touchY;

  static int W = 180;
  static int H = 120;

  public DragAndDrop(Context context)
  {
    super(context);

    touchView = new View(context);
    touchView.setBackgroundColor(0xaaff9900);
    addView(touchView);
  }

  @Override
  protected void onLayout(boolean changed, int l, int t, int r, int b)
  {
    float width = r - l;
    float height = b - t;

    int centerX = (int) (touchX - W / 2);
    int centerY = (int) (touchY - H / 2);

    touchView.layout(centerX, centerY, centerX + W, centerY + H);
  }

  @Override
  public boolean onTouchEvent(MotionEvent event)
  {
    touchX = event.getX();
    touchY = event.getY();

    requestLayout();
    return true;
  }
}

after investigating I found the problem is with calling the requestLayout() method inside the touchListener methods.
I moved that call to a timer which fired it periodically and the result was much better.

does anyone else experienced that and knows of a better way doing it, without using a timer?
I prefer to avoid refreshing all the time more then actually needed.

Thanks for your help!

Answer 1

I think you're saying that your full version involved subclassing View and overriding onTouchEvent() in there, in which case see this answer. With event.getX() and event.getY() in my own similar drag implementation I saw move events that seemed to show my touch oscillating around my finger as it moved; with event.getRawX() and event.getRawY() the move was smooth like my true finger movement.

As an aside, with raw coordinates it's also probably best to save the coordinates of the ACTION_DOWN event and then recalculate the relative distance and requestLayout() each time you get an ACTION_MOVE event.