combobox下拉框打开后高亮显示选中项

<?xml version="1.0" encoding="utf-8"?> 
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" 
        layout="vertical" 
        verticalAlign="top" 
        backgroundColor="white"> 
  
    <mx:Script> 
        <![CDATA[ 
            import mx.events.DropdownEvent; 
  
            private function comboBox_open(evt:DropdownEvent):void { 
                comboBox.dropdown.setStyle("useRollOver", checkBox.selected); 
            } 
        ]]> 
    </mx:Script> 
  
    <mx:Array id="arr"> 
        <mx:Object label="One" /> 
        <mx:Object label="Two" /> 
        <mx:Object label="Three" /> 
        <mx:Object label="Four" /> 
        <mx:Object label="Five" /> 
        <mx:Object label="Six" /> 
        <mx:Object label="Seven" /> 
        <mx:Object label="Eight" /> 
        <mx:Object label="Nine" /> 
        <mx:Object label="Ten" /> 
    </mx:Array> 
  
    <mx:ApplicationControlBar dock="true"> 
        <mx:Form styleName="plain"> 
            <mx:FormItem label="useRollOver:"> 
                <mx:CheckBox id="checkBox" /> 
            </mx:FormItem> 
        </mx:Form> 
    </mx:ApplicationControlBar> 
  
    <mx:ComboBox id="comboBox" 
            dataProvider="{arr}" 
            open="comboBox_open(event);" /> 
  
</mx:Application>