flex tree 自动显示横向滚动条

mxml

<?xml version="1.0" encoding="utf-8"?>

<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" xmlns:cmp="com.render.*"

    height="100%" width="100%" paddingTop="0" paddingBottom="0" paddingLeft="0" paddingRight="0">

    <cmp:AutoSizeTree

            id="checkTree"

            itemRenderer="com.render.CheckTreeRenderer"

            change="updataDataGrid()"

            labelField="@label"

             width="100%" height="100%" 

             horizontalScrollPolicy="auto" >

     </cmp:AutoSizeTree>

</mx:Application>

actionscript

package com.render

{

    import flash.events.Event;

    import mx.controls.Alert;

    import mx.controls.Tree;

    import mx.core.ScrollPolicy;

    import mx.core.mx_internal;

    public class AutoSizeTree extends Tree

    {

        public function AutoSizeTree()

        {

            super();

            horizontalScrollPolicy = ScrollPolicy.AUTO;

        }

        override public function get maxHorizontalScrollPosition():Number

        {

             if (isNaN(mx_internal::_maxHorizontalScrollPosition))        

                return 0;                         

            return mx_internal::_maxHorizontalScrollPosition;     

        }          

        override public function set maxHorizontalScrollPosition(value:Number):void

        {

            mx_internal::_maxHorizontalScrollPosition = value;            

            dispatchEvent(new Event("maxHorizontalScrollPositionChanged"));    

            scrollAreaChanged = true;           

            invalidateDisplayList();      

        }

        override protected function updateDisplayList(unscaledWidth:Number, unscaledHeight:Number):void

        {   

                var diffWidth:Number = measureWidthOfItems(0,0) - (unscaledWidth - viewMetrics.left - viewMetrics.right);

                var indentation:Number = getStyle("indentation");

                if (diffWidth <= 0)                  

                    maxHorizontalScrollPosition = NaN;

                else

                    maxHorizontalScrollPosition = diffWidth + indentation;                                   

                super.updateDisplayList(unscaledWidth, unscaledHeight);

        }

    }

}