code: function(x, y) {
                  var oldForward = this.forward.getPoint(), oldBackward = this.backward.getPoint(), oldVertex = this.vertex.getPoint(), //移动距离
                  distance = {
                      left: x - oldVertex.x,
                      top: y - oldVertex.y
                  };
                  // 更新
                  this.forward.setPoint(oldForward.x + distance.left, oldForward.y + distance.top);
                  this.backward.setPoint(oldBackward.x + distance.left, oldBackward.y + distance.top);
                  this.vertex.setPoint(x, y);
                  this.update();
              }
docs: @method moveTo()
  @for kity.BezierPoint
  @description 同步移动整个贝塞尔点，使顶点的移动到指定的坐标中。控制点的位置相对顶点坐标固定。
  
  @grammar moveTo() => {this}
  
  @param  {Number} x 顶点的目标 x 坐标
  @param  {Number} y 顶点的目标 y 坐标