Flex中如何利用FocusManager类的setFocus函数设置TextInput的焦

<?xml version="1.0" encoding="utf-8"?>
<s:Application xmlns:fx="http://ns.adobe.com/mxml/2009" 
               xmlns:s="library://ns.adobe.com/flex/spark" 
               xmlns:mx="library://ns.adobe.com/flex/mx" 
               minWidth="955" 
               minHeight="600">
    <fx:Declarations>
        <!-- 将非可视元素（例如服务、值对象）放在此处 -->
    </fx:Declarations>
    <!-- Flex中如何利用FocusManager类的setFocus函数设置TextInput的焦点的例子 -->
    <fx:Script>
        <![CDATA[
        import mx.controls.Alert;
        
        private function submitButton_click(evt:MouseEvent):void {
        Alert.show(evt.currentTarget.label,Object(focusManager.getFocus()).name);
        }
        
        private function resetButton_click(evt:MouseEvent):void {
        username.text = "";
        password.text = "";
        Alert.show(evt.currentTarget.label,Object(focusManager.getFocus()).name);
        }
        ]]>
    </fx:Script>
    
    <mx:ApplicationControlBar dock="true">
        <mx:Button label="Set focus to Username"
                   click="focusManager.setFocus(username);" />
        <mx:Button label="Set focus to Password"
                   click="focusManager.setFocus(password);" />
    </mx:ApplicationControlBar>
    
    <mx:Form id="form"
             defaultButton="{submitButton}" left="300">
        <mx:FormItem label="Username:">
            <mx:TextInput id="username" />
        </mx:FormItem>
        <mx:FormItem label="Password:">
            <mx:TextInput id="password"
                          displayAsPassword="true" />
        </mx:FormItem>
        <mx:FormItem direction="horizontal"
                     horizontalAlign="right"
                     width="100%">
            <mx:Button id="submitButton"
                       label="Submit"
                       click="submitButton_click(event);" />
            <mx:Button id="resetButton"
                       label="Reset"
                       click="resetButton_click(event);" />
        </mx:FormItem>
    </mx:Form>
    
</s:Application>