Checking that an item is selected in the Spark DropDownList control in Flex 4

<?xml version="1.0" encoding="utf-8"?> 
<!-- http://blog.flexexamples.com/2010/03/12/checking-that-an-item-is-selected-in-the-spark-dropdownlist-control-in-flex-4/ -->
<s:Application name="Spark_DropDownList_requiresSelection_test"
        xmlns:fx="http://ns.adobe.com/mxml/2009" 
        xmlns:s="library://ns.adobe.com/flex/spark" 
        xmlns:mx="library://ns.adobe.com/flex/mx"> 
    <s:layout>
        <s:HorizontalLayout horizontalAlign="center" verticalAlign="middle" />
    </s:layout>
 
    <s:DropDownList id="ddl" prompt="Please select an item" requireSelection="true">
        <s:dataProvider>
            <s:ArrayList source="[one,two,three,four,five,six,seven]" />
        </s:dataProvider>
    </s:DropDownList>
 
</s:Application>

<?xml version="1.0" encoding="utf-8"?>
<!-- http://blog.flexexamples.com/2010/03/12/checking-that-an-item-is-selected-in-the-spark-dropdownlist-control-in-flex-4/ -->
<s:Application name="Spark_DropDownList_selectedIndex_NumberValidator_test"
        xmlns:fx="http://ns.adobe.com/mxml/2009" 
        xmlns:s="library://ns.adobe.com/flex/spark" 
        xmlns:mx="library://ns.adobe.com/flex/mx"> 
    <s:layout>
        <s:HorizontalLayout horizontalAlign="center" verticalAlign="middle" />
    </s:layout>
 
    <fx:Script>
        <![CDATA[
            import mx.controls.Alert;
            import mx.events.ValidationResultEvent;
 
            protected function button1_clickHandler(evt:MouseEvent):void {
                numValidator.validate();
            }
 
            protected function numValidator_validHandler(evt:ValidationResultEvent):void {
                Alert.show("DropDownList item selected: " + ddl.selectedItem.toString(), evt.type);
            }
 
            protected function numValidator_invalidHandler(evt:ValidationResultEvent):void {
                Alert.show(ddl.prompt, evt.type);
            }
        ]]>
    </fx:Script>
 
    <fx:Declarations>
        <mx:NumberValidator id="numValidator"
                source="{ddl}"
                property="selectedIndex"
                minValue="0"
                lowerThanMinError="{ddl.prompt}"
                valid="numValidator_validHandler(event);"
                invalid="numValidator_invalidHandler(event);" />
    </fx:Declarations>
 
    <s:DropDownList id="ddl" prompt="Please select an item">
        <s:dataProvider>
            <s:ArrayList source="[one,two,three,four,five,six,seven]" />
        </s:dataProvider>
    </s:DropDownList>
 
    <s:Button label="validate" click="button1_clickHandler(event);" />
 
</s:Application>