code: public static MotionDirection getVerticalMotionDirection(MotionEvent e1, MotionEvent e2) {
          return getVerticalMotionDirection(e1, e2, DEFAULT_THRESHOLD);
      }
docs: Calculate the vertical move motion direction.
  @param e1 start point of the motion.
  @param e2 end point of the motion.
  @return the motion direction for the vertical axis.