如何让Flex的Tree显示水平滚动条
论坛一直有人问为何设置了twaver.controls.Tree的horizontalScrollPolicy为 ScrollPolicy.AUTO，可是还是没有横向滚动条。这是由于Adobe出于性能的考虑，没有计算maxHorizontalScrollPosition的值。本文贴出了解决方案，效果如下：
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从原始Tree继承出新的类AutoSizeTree：
package {import flash.events.Event;import mx.core.ScrollPolicy;import mx.core.mx_internal;import twaver.DataBox;import twaver.controls.Tree;public class AutoSizeTree extends Tree {public function AutoSizeTree(dataBox:DataBox=null) {super(dataBox);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);if (diffWidth <= 0)maxHorizontalScrollPosition = NaN;elsemaxHorizontalScrollPosition = diffWidth;super.updateDisplayList(unscaledWidth, unscaledHeight);}}}测试代码如下：
<?xml version="1.0" encoding="utf-8"?><mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"xmlns:tw="http://www.servasoftware.com/2009/twaver/flex"applicationComplete="init()" xmlns:local="*"><mx:Script><![CDATA[import twaver.*;private function init():void {var box:ElementBox = new ElementBox();var group:Group = new Group();group.name = "Group";box.add(group);for(var i:int=0; i<20; i++){var node:Node = new Node();node.name = "Node with long long long long long long name " + Utils.randomInt(10000000);group.addChild(node);box.add(node);}tree.dataBox = box;tree.callLater2(function():void {tree.expandAll();});}]]></mx:Script><local:AutoSizeTree id="tree" width="300" height="100%"/></mx:Application>如果您数据量少，性能不是大问题，不妨试一试，完整工程代码见附件：TestAutoSizeTree
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