code: function (callback, context) {
          callback.call(context, this);
          var _children = this._children;
          for(var i = 0, len = _children.length; i < len; i++) {
              _children[i].traverse(callback, context);
          }
      }
docs: Depth first traverse all its descendant scene nodes.
  
  **WARN** Don't do `add`, `remove` operation in the callback during traverse.
  @param {Function} callback
  @param {Node} [context]