code: function(nodes) {
                  var i, node, matrix, treeBox;
                  var box = new kity.Box();
                  if (!(nodes instanceof Array)) nodes = [ nodes ];
                  for (i = 0; i < nodes.length; i++) {
                      node = nodes[i];
                      matrix = node.getLayoutTransform();
                      treeBox = node.getContentBox();
                      if (node.isExpanded() && node.children.length) {
                          treeBox = treeBox.merge(this.getTreeBox(node.children));
                      }
                      box = box.merge(matrix.transformBox(treeBox));
                  }
                  return box;
              }
docs: 工具方法：计算给定的节点的子树所占的布局区域
  
  @param  {MinderNode} nodes 需要计算的节点
  
  @return {Box} 计算的结果