flex tree 自动显示横向滚动条

package com.anydata.consumer.components
{
 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
 */
 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);
  }
 }
}