code: void changeBackgroundAlpha() {
      Drawable background = getBackground();
      if (background != null) {
        int newAlpha = (int) (ONE_HUNDRED * (1 - getVerticalDragOffset()));
        background.setAlpha(newAlpha);
      }
    }
docs: Modify the background alpha if has been configured to applying an alpha effect when the view
  is dragged.