code: @Override public void updateScale(float verticalDragOffset) {
      ViewHelper.setScaleX(getView(), 1 - verticalDragOffset / getXScaleFactor());
      ViewHelper.setScaleY(getView(), 1 - verticalDragOffset / getYScaleFactor());
    }
docs: Uses Nineoldandroids to change the scale.
  
  @param verticalDragOffset used to calculate the new scale.