Changing a VBox container’s background alpha in Flex

<?xml version="1.0" encoding="utf-8"?>
<!-- http://blog.flexexamples.com/2008/01/05/changing-a-vbox-containers-background-alpha-in-flex/ -->
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
        layout="vertical"
        verticalAlign="middle"
        backgroundColor="white">
 
    <mx:Style>
        .containerVBox {
            backgroundColor: haloBlue;
            borderThickness: 1;
            borderColor: black;
            borderStyle: solid;
            paddingLeft: 5;
            paddingRight: 5;
            paddingTop: 5;
            paddingBottom: 5;
        }
    </mx:Style>
 
    <mx:Script>
        <![CDATA[
            [Bindable]
            [Embed("Fx.png")]
            private var flexLogo:Class;
 
            private function backImage_change(evt:Event):void {
                if (backImage.selected) {
                    vBox.setStyle("backgroundImage", flexLogo);
                } else {
                    vBox.setStyle("backgroundImage", null);
                }
            }
 
            private function backColor_change(evt:Event):void {
                if (backColor.selected) {
                    vBox.setStyle("backgroundColor", colorPicker.selectedColor);
                } else {
                    vBox.setStyle("backgroundColor", null);
                }
            }
        ]]>
    </mx:Script>
 
    <mx:String id="lorem" source="lorem.txt" />
 
    <mx:ApplicationControlBar dock="true">
        <mx:Form styleName="plain">
            <mx:FormItem label="backgroundAlpha:">
                <mx:HSlider id="slider"
                        minimum="0.0"
                        maximum="1.0"
                        value="1.0"
                        liveDragging="true"
                        tickInterval="0.1" />
            </mx:FormItem>
            <mx:FormItem label="show backgroundImage:">
                <mx:CheckBox id="backImage"
                        selected="true"
                        change="backImage_change(event);" />
            </mx:FormItem>
            <mx:FormItem label="show backgroundColor:">
                <mx:CheckBox id="backColor"
                        selected="true"
                        change="backColor_change(event);" />
            </mx:FormItem>
            <mx:FormItem label="backgroundColor:">
                <mx:ColorPicker id="colorPicker"
                        selectedColor="white"
                        enabled="{backColor.selected}"
                        change="backColor_change(event);" />
            </mx:FormItem>
        </mx:Form>
    </mx:ApplicationControlBar>
 
    <mx:VBox styleName="containerVBox">
        <mx:VBox id="vBox"
                backgroundImage="{flexLogo}"
                backgroundAlpha="{slider.value}"
                backgroundColor="white"
                backgroundAttachment="fixed"
                width="500"
                height="250">
            <mx:Text width="100%" text="{lorem}" />
        </mx:VBox>
    </mx:VBox>
 
</mx:Application>