Flex如何通过点击行还不是三角箭头来展开树

import mx.events.ListEvent;

private function tree_itemClick(evt:ListEvent):void {
                var item:Object = Tree(evt.currentTarget).selectedItem;
                if (tree.dataDescriptor.isBranch(item)) {
                    tree.expandItem(item, !tree.isItemOpen(item), true);
                }
            }


<mx:Tree id="tree"
            showRoot="false"
            labelField="@label"
            labelFunction="tree_labelFunc"
            width="300"
            rowCount="6"
            itemClick="tree_itemClick(event);" />