code: function() {
                  this._calcDragSources();
                  if (!this._dragSources.length) {
                      this._startPosition = null;
                      return false;
                  }
                  this._fadeDragSources(.5);
                  this._calcDropTargets();
                  this._calcOrderHints();
                  this._dragMode = true;
                  this._minder.setStatus("dragtree");
                  return true;
              }
docs: 进入拖放模式： 1. 计算拖放源和允许的拖放目标 2. 标记已启动