flex tree 自动显示水平滚动条

package com.common.tree {
import flash.events.Event;

import mx.controls.Tree;
import mx.core.ScrollPolicy;
import mx.core.mx_internal;

/**
* 自动显示横向滚动条
* @author chensong
*
*/
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);
}
}
}