<?xml version="1.0" encoding="utf-8"?>
<!-- http://blog.flexexamples.com/2008/02/11/determining-if-a-check-box-menu-item-is-toggled-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:Style>
        PopUpButton {
            popUpStyleName: MyCustomPopUpStyleName;
        }

        .MyCustomPopUpStyleName {
            fontWeight: normal;
            textAlign: left;
        }

        .redModal {
            modalTransparencyColor: red;
            modalTransparency: 0.8;
        }

        .greenModal {
            modalTransparencyColor: haloGreen;
            modalTransparency: 0.8;
        }
    </mx:Style>

    <mx:Script>
        <![CDATA[
            import mx.events.MenuEvent;
            import mx.controls.Menu;
            import mx.controls.Alert;

            private var menu:Menu;
            private function init():void {
                menu = new Menu();
                menu.variableRowHeight = true;
                menu.dataProvider = arr;
                menu.addEventListener(MenuEvent.CHANGE, menu_change);
                popUpButton.popUp = menu;
            }

            private function menu_change(evt:MenuEvent):void {
                switch (menu.dataDescriptor.getType(evt.item)) {
                    case "check":
                        if (menu.dataDescriptor.isToggled(evt.item)) {
                            application.styleName = "greenModal";
                            Alert.show("\"" + evt.item.label + "\" was checked");
                        } else {
                            application.styleName = "redModal";
                            Alert.show("\"" + evt.item.label + "\" was not checked");
                        }
                        break;
                }
            }
        ]]>
    </mx:Script>

    <mx:Array id="arr">
        <mx:Object label="Option 1"
                type="check"
                toggled="true" />
        <mx:Object label="Option 2"
                type="check"
                toggled="true" />
    </mx:Array>

    <mx:PopUpButton id="popUpButton"
            label="Click to open..."
            openAlways="true"
            initialize="init();" />

</mx:Application>