Flex的树形控件(Tree Control)中如何通过点击行进行展开/缩进的

<?xml version="1.0" encoding="utf-8"?>
<!-- http://blog.flexexamples.com/2008/04/05/opening-branches-by-clicking-rows-in-a-tree-control-in-flex/ -->
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
        layout="vertical"
        verticalAlign="middle"
        backgroundColor="white" viewSourceURL="srcview/index.html">

    <mx:Script>
        <![CDATA[
            import mx.collections.ICollectionView;
            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);
                }
            }

            private function tree_labelFunc(item:XML):String {
                var children:ICollectionView;
                var suffix:String = "";
                if (tree.dataDescriptor.isBranch(item)) {
                    children = tree.dataDescriptor.getChildren(item);
                    suffix = " (" + children.length + ")";
                }
                return item[tree.labelField] + suffix;
            }
        ]]>
    </mx:Script>

    <mx:XML id="dp">
        <root>
            <folder label="One">
                <folder label="One.A">
                    <item label="One.A.1" />
                    <item label="One.A.2" />
                    <item label="One.A.3" />
                    <item label="One.A.4" />
                    <item label="One.A.5" />
                </folder>
                <item label="One.1" />
                <item label="One.2" />
            </folder>
            <folder label="Two">
                <item label="Two.1" />
                <folder label="Two.A">
                    <item label="Two.A.1" />
                    <item label="Two.A.2" />
                </folder>
            </folder>
        </root>
    </mx:XML>

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

</mx:Application>