If you try and access the Application.application.stage property from the creationComplete event, you’ll get a run-time error (RTE) saying the following:

TypeError: Error #1009: Cannot access a property or method of a null object reference.
at main/init()
at main/___main_Application1_creationComplete()
at flash.events::EventDispatcher/dispatchEventFunction()
at flash.events::EventDispatcher/dispatchEvent()
at mx.core::UIComponent/dispatchEvent()
at mx.core::UIComponent/set initialized()
at mx.managers::LayoutManager/doPhasedInstantiation()
at Function/http://adobe.com/AS3/2006/builtin::apply()
at mx.core::UIComponent/callLaterDispatcher2()
at mx.core::UIComponent/callLaterDispatcher()

The example below is pretty bare with just a few Label controls and a Button control, but it should have enough of the required code to give you a push in the right direction.

You must have version 9,0,28,0 or greater of Flash Player installed to use full-screen mode. Download the latest version of Adobe Flash Player.

View MXML

<?xml version="1.0" encoding="utf-8"?>
<!-- http://blog.flexexamples.com/2007/08/07/creating-full-screen-flex-applications/ -->
<mx:Application name="FullScreen_test"
        xmlns:mx="http://www.adobe.com/2006/mxml"
        layout="vertical"
        applicationComplete="init(event)">

    <mx:Script>
        <![CDATA[
            import flash.display.StageDisplayState;

            private function init(evt:Event):void {
                /* Set up full screen handler. */
                Application.application.stage.addEventListener(FullScreenEvent.FULL_SCREEN, fullScreenHandler);
                dispState = Application.application.stage.displayState;
            }

            private function fullScreenHandler(evt:FullScreenEvent):void {
                dispState = Application.application.stage.displayState + " (fullScreen=" + evt.fullScreen.toString() + ")";
                if (evt.fullScreen) {
                    /* Do something specific here if we switched to full screen mode. */
                } else {
                    /* Do something specific here if we switched to normal mode. */
                }
            }

            private function toggleFullScreen():void {
                try {
                    switch (Application.application.stage.displayState) {
                        case StageDisplayState.FULL_SCREEN:
                            /* If already in full screen mode, switch to normal mode. */
                            Application.application.stage.displayState = StageDisplayState.NORMAL;
                            break;
                        default:
                            /* If not in full screen mode, switch to full screen mode. */
                            Application.application.stage.displayState = StageDisplayState.FULL_SCREEN;
                            break;
                    }
                } catch (err:SecurityError) {
                    // ignore
                }
            }
        ]]>
    </mx:Script>

    <mx:String id="dispState" />

    <mx:Label text="width={Application.application.width}" />
    <mx:Label text="height={Application.application.height}" />
    <mx:Label text="displayState={dispState}" />

    <mx:Button label="Toggle fullscreen" click="toggleFullScreen()" />

</mx:Application>

View source is enabled in the following example.

For more information on Full Screen Mode in Flash Player 9 (ActionScript 2.0 and ActionScript 3.0) check out the following article on the Adobe Developer Center: “Exploring full-screen mode in Flash Player 9″.

I *knew* I would have forgotten something. The real trick to using FullScreen support is to enable it in the JavaScript embed code, or <object /> and <embed /> tags… So, in your HTML wrapper, add the following property:

AC_FL_RunContent(
    "src", "main",
    "width", "100%",
    "height", "100%",
    "align", "middle",
    "id", "main",
    "quality", "high",
    "bgcolor", "#869ca7",
    "name", "main",
    "allowScriptAccess","sameDomain",
    "type", "application/x-shockwave-flash",
    "pluginspage", "http://www.adobe.com/go/getflashplayer",
    "allowFullScreen", "true"
);

Or, if you are using <object /> and <embed /> tags, you can use the following syntax instead:

<object classid="clsid:D27CDB6E-AE6D-11cf-96B8-444553540000"
        id="main" width="100%" height="100%"
        codebase="http://fpdownload.macromedia.com/get/flashplayer/current/swflash.cab">
        <param name="movie" value="main.swf" />
        <param name="quality" value="high" />
        <param name="bgcolor" value="#869ca7" />
        <param name="allowScriptAccess" value="sameDomain" />
        <param name="allowFullScreen" value="true" />

    <embed src="main.swf" quality="high" bgcolor="#869ca7"
        width="100%" height="100%" name="main" align="middle"
        play="true"
        loop="false"
        quality="high"
        allowScriptAccess="sameDomain"
        allowFullScreen="true"
        type="application/x-shockwave-flash"
        pluginspage="http://www.adobe.com/go/getflashplayer">
    </embed>

</object>

Also, I just learnt from my co-worker Raghu’s blog (“FLEXing My Muscle” and his “Error on adding FullScreenListener in creationComplete handler” post) that you can use the SystemManager class instead of all my references to Application.application. Thanks Raghu!

<?xml version="1.0" encoding="utf-8"?>
<!-- http://blog.flexexamples.com/2007/08/07/creating-full-screen-flex-applications/ -->
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" layout="vertical" applicationComplete="init()">

    <mx:Script>
        <![CDATA[
            import flash.display.StageDisplayState;
            import mx.managers.SystemManager;

            private function init():void {
                /* Set up full screen handler. */
                systemManager.stage.addEventListener(FullScreenEvent.FULL_SCREEN, fullScreenHandler);
                dispState = systemManager.stage.displayState;
            }

            private function fullScreenHandler(evt:FullScreenEvent):void {
                dispState = systemManager.stage.displayState + " (fullScreen=" + evt.fullScreen.toString() + ")";
                if (evt.fullScreen) {
                    /* Do something specific here if we switched to full screen mode. */
                } else {
                    /* Do something specific here if we switched to normal mode. */
                }
            }

            private function toggleFullScreen():void {
                try {
                    switch (systemManager.stage.displayState) {
                        case StageDisplayState.FULL_SCREEN:
                            /* If already in full screen mode, switch to normal mode. */
                            systemManager.stage.displayState = StageDisplayState.NORMAL;
                            break;
                        default:
                            /* If not in full screen mode, switch to full screen mode. */
                            systemManager.stage.displayState = StageDisplayState.FULL_SCREEN;
                            break;
                    }
                } catch (err:SecurityError) {
                    // ignore
                }
            }
        ]]>
    </mx:Script>

    <mx:String id="dispState" />

    <mx:Label text="width={Application.application.width}" />
    <mx:Label text="height={Application.application.height}" />
    <mx:Label text="displayState={dispState}" />

    <mx:Button label="Toggle fullscreen" click="toggleFullScreen()" />

</mx:Application>