Flex：用openItems属性进行Tree控件的展开与关闭

<?xml version="1.0" encoding="utf-8"?>
<!-- http://yecon.blog.hexun.com/28902341_d.html -->
<!-- http://www.slsay.com -->
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
        layout="vertical"
        verticalAlign="middle"
        backgroundColor="white">
    <mx:Script>
        <![CDATA[
            private function openAllNodes():void {
                tree.openItems = dp..node;
            }
            private function closeAllNodes():void {
                tree.openItems = [];
            }
        ]]>
    </mx:Script>
    <mx:XML id="dp">
        <root>
            <node label="Parent 1">
                <node label="Child 1" />
                <node label="Child 2">
                    <node label="Grandchild 1" />
                    <node label="Grandchild 2" />
            &nbs