问Flex4 ContextMenu()未显示添加的项目

EN

<?xml version="1.0" encoding="utf-8"?>
<!-- http://blog.flexexamples.com/2007/08/20/using-a-custom-context-menu-with-the-flex-datagrid-control/ -->
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
        layout="vertical"
        verticalAlign="middle"
        backgroundColor="white"
        creationComplete="init()">

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

            [Bindable]
            private var cm:ContextMenu;

            private var alert:Alert;

            private function init():void {
                var cmi:ContextMenuItem = new ContextMenuItem("View item...", true);
                cmi.addEventListener(ContextMenuEvent.MENU_ITEM_SELECT, contextMenuItem_menuItemSelect);

                cm = new ContextMenu();
                cm.hideBuiltInItems();
                cm.customItems = [cmi];
                cm.addEventListener(ContextMenuEvent.MENU_SELECT, contextMenu_menuSelect);
            }

            private function contextMenu_menuSelect(evt:ContextMenuEvent):void {
                dataGrid.selectedIndex = lastRollOverIndex;
            }

            private function contextMenuItem_menuItemSelect(evt:ContextMenuEvent):void {
                var obj:Object = dataGrid.selectedItem;
                alert = Alert.show("Property A: " + obj.@propertyA + "\\n" + "Property B: " + obj.@propertyB, obj.@label, Alert.OK);
            }
        ]]>
    </mx:Script>

    <mx:XML id="itemsXML">
        <items>
            <item label="Item 1" data="i001" propertyA="Item 1.A" propertyB="Item 1.B" />
            <item label="Item 2" data="i002" propertyA="Item 2.A" propertyB="Item 2.B" />
            <item label="Item 3" data="i003" propertyA="Item 3.A" propertyB="Item 3.B" />
            <item label="Item 4" data="i004" propertyA="Item 4.A" propertyB="Item 4.B" />
            <item label="Item 5" data="i005" propertyA="Item 5.A" propertyB="Item 5.B" />
            <item label="Item 6" data="i006" propertyA="Item 6.A" propertyB="Item 6.B" />
            <item label="Item 7" data="i007" propertyA="Item 7.A" propertyB="Item 7.B" />
            <item label="Item 8" data="i008" propertyA="Item 8.A" propertyB="Item 8.B" />
        </items>
    </mx:XML>

    <mx:Number id="lastRollOverIndex" />

    <mx:DataGrid id="dataGrid"
            width="400"
            dataProvider="{itemsXML.item}"
             contextMenu="{cm}"
             itemRollOver="lastRollOverIndex = event.rowIndex">
        <mx:columns>
            <mx:DataGridColumn id="labelCol"
                    dataField="@label"
                    headerText="Label:" />

            <mx:DataGridColumn id="propACol"
                    dataField="@propertyA"
                    headerText="Property A:" />

            <mx:DataGridColumn id="propBCol"
                    dataField="@propertyB"
                    headerText="Property B:" />
        </mx:columns>
    </mx:DataGrid>

    <mx:Label text="{dataGrid.selectedItem.@label}" />

</mx:Application>