code: function getDirection(touch1, touch2) {
      var x = Math.abs(touch1.clientX - touch2.clientX),
          y = Math.abs(touch1.clientY - touch2.clientY);
  
      if (x >= y) {
        return touch1.clientX - touch2.clientX > 0 ? DIRECTION_LEFT : DIRECTION_RIGHT;
      }
      return touch1.clientY - touch2.clientY > 0 ? DIRECTION_UP : DIRECTION_DOWN;
    }
docs: do a small comparison to get the direction between two touches.
  @param {Touch} touch1
  @param {Touch} touch2
  @return {String} direction matches `DIRECTION_LEFT|RIGHT|UP|DOWN`