Flex中如何通过includeInLayout和visible属性设置Form显示隐藏

通过includeInLayout和visible属性设置 属性释放真正隐藏

<?xml version="1.0" encoding="utf-8"?> 
<mx:Application name="FormItem_includeInLayout_test" 
        xmlns:mx="http://www.adobe.com/2006/mxml" 
        layout="vertical" 
        verticalAlign="middle" 
        backgroundColor="white"> 
  
    <mx:Style> 
        Form { 
            indicatorGap: 0; 
            paddingLeft: 0; 
            paddingRight: 0; 
            paddingTop: 0; 
            paddingBottom: 0; 
        } 
    </mx:Style> 
  
    <mx:ApplicationControlBar dock="true"> 
        <mx:Button id="bccButton" 
                label="Show BCC" 
                toggle="true" 
                selected="false" /> 
        <mx:Button id="fromButton" 
                label="Show From" 
                toggle="true" 
                selected="false" /> 
    </mx:ApplicationControlBar> 
  
    <mx:HBox width="100%"> 
        <mx:Button id="sendButton" 
                label="Send" 
                height="100%" /> 
        <mx:Form width="100%"> 
            <mx:FormItem label="From:" 
                    includeInLayout="{fromButton.selected}" 
                    visible="{fromButton.selected}" 
                    width="100%"> 
                <mx:ComboBox id="fromComboBox" 
                        width="100%" /> 
            </mx:FormItem> 
            <mx:FormItem label="To:" width="100%"> 
                <mx:TextInput id="toTextInput" 
                        width="100%" /> 
            </mx:FormItem> 
            <mx:FormItem label="CC:" width="100%"> 
                <mx:TextInput id="ccTextInput" 
                        width="100%" /> 
            </mx:FormItem> 
            <mx:FormItem label="BCC:" 
                    includeInLayout="{bccButton.selected}" 
                    visible="{bccButton.selected}" 
                    width="100%"> 
                <mx:TextInput id="bccTextInput" 
                        width="100%" /> 
            </mx:FormItem> 
            <mx:FormItem label="Subject:" 
                    width="100%"> 
                <mx:TextInput id="subjectTextInput" 
                        width="100%" /> 
            </mx:FormItem> 
        </mx:Form> 
    </mx:HBox> 
  
    <mx:TextArea id="bodyTextArea" 
            width="100%" 
            height="100%" /> 
  
</mx:Application>