code: function(fn, excludeThis) {
              var children = this.getChildren();
              if (!excludeThis) fn(this);
              for (var i = 0; i < children.length; i++) {
                  children[i].preTraverse(fn);
              }
          }
docs: 先序遍历当前节点树
  @param  {Function} fn 遍历函数