<?xml version="1.0"?>
<!-- http://blog.flexexamples.com/2008/01/18/aligning-labels-in-a-flex-popupbutton-controls-pop-up-menu/ -->
<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.ItemClickEvent;
            import mx.controls.Menu;
            import mx.events.MenuEvent;

            [Bindable]
            private var menu:Menu;

            private function initMenu():void {
                menu = new Menu();
                menu.dataProvider = arr;
            }

            private function toggleButtonBar_itemClick(evt:ItemClickEvent):void {
                var obj:CSSStyleDeclaration = StyleManager.getStyleDeclaration(".myCustomPopUpStyleName");
                obj.setStyle("textAlign", evt.label);
                popUpButton.open();
            }
        ]]>
    </mx:Script>

    <mx:Array id="arr">
        <mx:Object label="Button" />
        <mx:Object label="ButtonBar" />
        <mx:Object label="ColorPicker" />
        <mx:Object label="ComboBox" />
    </mx:Array>

    <mx:Style>
        .myCustomPopUpStyleName {
           textAlign: left;
        }
    </mx:Style>

    <mx:ApplicationControlBar dock="true">
        <mx:Form styleName="plain">
            <mx:FormItem label="textAlign:">
                <mx:ToggleButtonBar id="toggleButtonBar"
                        selectedIndex="0"
                        itemClick="toggleButtonBar_itemClick(event);">
                    <mx:dataProvider>
                        <mx:Array>
                            <mx:Object label="left" />
                            <mx:Object label="center" />
                            <mx:Object label="right" />
                        </mx:Array>
                    </mx:dataProvider>
                </mx:ToggleButtonBar>
            </mx:FormItem>
        </mx:Form>
    </mx:ApplicationControlBar>

    <mx:PopUpButton id="popUpButton"
            label="Select a control..."
            popUp="{menu}"
            popUpStyleName="myCustomPopUpStyleName"
            preinitialize="initMenu();" />

</mx:Application>