flex ComboBox 打开关闭验证

<?xml version="1.0" encoding="utf-8"?>
<!-- http://blog.flexexamples.com/2007/12/13/programmatically-opening-and-closing-combobox-controls-in-flex/ -->
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
        layout="vertical"
        verticalAlign="top"
        backgroundColor="white" viewSourceURL="srcview/index.html">

    <mx:Script>
        <![CDATA[
            import mx.events.ValidationResultEvent;
            import mx.controls.Alert;

            private function numberValidator_invalid(evt:ValidationResultEvent):void {
                comboBox.errorString = "Please select a thing";
                comboBox.open();
            }

            private function numberValidator_valid(evt:ValidationResultEvent):void {
                comboBox.errorString = "";
                Alert.show("Success");
            }

            private function validateCheckBox():void {
                numberValidator.validate(comboBox.selectedIndex);
            }
        ]]>
    </mx:Script>

    <mx:NumberValidator id="numberValidator"
            minValue="0"
            invalid="numberValidator_invalid(event);"
            valid="numberValidator_valid(event);" />

    <mx:ApplicationControlBar dock="true">
        <mx:Button label="validate"
                click="validateCheckBox();" />
        <mx:Button label="reset"
                click="comboBox.selectedIndex = -1;" />
    </mx:ApplicationControlBar>

    <mx:ComboBox id="comboBox"
            prompt="Please select a thing..."
            openDuration="2000"
            closeDuration="1000"
            close="validateCheckBox();">
        <mx:dataProvider>
            <mx:Array>
                <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:Array>
        </mx:dataProvider>
    </mx:ComboBox>

</mx:Application>