flex tree 自动显示横向滚动条

从原始Tree继承出新的类AutoSizeTree

AutoSizeTree.as

package view.component
{
	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);
		}
	}
}

测试页面? aa.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"          
labelField="@label"
width="100%" height="100%" 
horizontalScrollPolicy="auto" >
</cmp:AutoSizeTree>
</mx:Application>