code: function ( index ) {
  
              var shape = this.pointShapes[ index ],
              // 当前的包含的数据点
                  bezierPoint = this.points[ index ],
                  forward = bezierPoint.getForward(),
                  vertex = bezierPoint.getVertex();
  
              shape.line.setPoint1( vertex.x, vertex.y ).setPoint2( forward.x, forward.y );
              shape.forward.setCenter( forward.x, forward.y );
              shape.backward.setCenter( vertex.x, vertex.y );
  
              this._stroke( shape.vertex, "white" );
  
              // 清理前一次的辅助点
              if ( index > 1 ) {
  
                  this._clearShapePoint( index - 1 );
  
              }
  
          }
docs: 根据指定的索引， 把该索引所对应的点更新成辅助点 一个辅助点是当前曲线区间的起点， 它显示出该辅助点的forward控制点和连线