默认的，Tree控件在初始化后是收缩的，你也许不确定如何初始化控件时展开数，并且选定指定的节点。下边的例子，展示了如何实现它。在这个程序中，initTree()方法，在Tree控件被创建后调用。这个方法展开Tree控件的根节点，并且设置selectedIndex属性为指定节点的索引号。

<?xml version="1.0"?>
<mx:Application xmlns:mx=http://www.adobe.com/2006/mxml viewSourceURL="src/index.html">    <mx:Script>
        <![CDATA[
            import flash.events.*;
            import mx.events.*;
            import mx.controls.*;
            private function initTree():void {
                XMLTree1.expandItem(MailBox.getItemAt(0), true);
                XMLTree1.selectedIndex = 2;
            }
        ]]>
    </mx:Script>
    <mx:Tree id="XMLTree1" width="150" height="170"
             labelField="@label" creationComplete="initTree();">
        <mx:XMLListCollection id="MailBox">
            <mx:XMLList>
                <node label="Mail" data="100">
                    <node label="Inbox" data="70"/>
                    <node label="Personal Folder" data="10">
                        <node label="Business" data="2"/>
                        <node label="Demo" data="3"/>
                        <node label="Saved Mail" data="5" />
                    </node>
                    <node label="Sent" data="15"/>
                    <node label="Trash" data="5"/>
                 </node>
            </mx:XMLList>
        </mx:XMLListCollection>
</mx:Tree>
</mx:Application>