code: void changeDragViewViewAlpha() {
      if (enableHorizontalAlphaEffect) {
        float alpha = 1 - getHorizontalDragOffset();
        if (alpha == 0) {
          alpha = 1;
        }
        ViewHelper.setAlpha(dragView, alpha);
      }
    }
docs: Modify dragged view alpha based on the horizontal position while the view is being
  horizontally dragged.