Customize the initialization of Starling in a Feathers MXML application (Starling version)

When you create a new Feathers application in MXML, Starling is initialized automatically by the compiler, behind the scenes. The default settings will work well for most Feathers apps to support common requirements — like stage and view port resizing, high DPI displays on desktop, and creating a theme. However, advanced developers may wish to customize some of these settings.

In your project's application, you can add [Frame] metadata to provide your own custom class that initializes Starling:

<f:Application xmlns:fx=""


A simple startup class might look something like this:

package com.example
	import flash.display.Sprite;

	import starling.core.Starling;

	public class CustomBootstrap extends Sprite
		public function CustomBootstrap()
			this.stage.scaleMode = StageScaleMode.NO_SCALE;
			this.stage.align = StageAlign.TOP_LEFT;

			// we need to instantiate the theme before the root is created,
			// so we pass null to the Starling constructor
			this._starling = new Starling( null, this.stage );

			// configure any other Starling properties here

			// let's wait for the Stage 3D context to be created
			this._starling.addEventListener( Event.CONTEXT3D_CREATE, starling_context3DCreateHandler );

		private var _starling:Starling;

		private function starling_context3DCreateHandler( event:Event ):void
			//this listener shouldn't be called again if context is lost
			this._starling.removeEventListener( Event.CONTEXT3D_CREATE, starling_context3DCreateHandler );

			// the context is ready, so the theme can create its textures
			new MetalWorksMobileTheme();

			// finally, pass Starling the root class and get things started!
			this._starling.rootClass = MyApp;

You'll notice that we're manually instantiating the theme in our custom bootstrap class, rather than specifying it using the theme property in the main MXML application. Since we're replacing the default bootstrap code with our own, it becomes our responsibility to configure Starling and the theme.

The convenience properties available on the Application class, including theme and context3DProfile, will not work with a custom bootstrap class.

Pre-loading Assets

One reason to use a custom bootstrap is to preload your assets before starting your app. Let's expand the previous example by loading some files using the Starling AssetManager:

private var _assets:AssetManager;

private function starling_context3DCreateHandler( event:Event ):void
	// the context is ready, so the AssetManager can create textures
	this._assets = new AssetManager();
	this._assets.enqueue( "file1.png" );
	this._assets.enqueue( "file2.xml" );
	this._assets.loadQueue( assets_onProgress );

Similar to before, we wait until the Stage 3D context is ready. This time, we create the AssetManager, enqueue our assets, and start loading. We'll wait for the assets to load before we start Starling.

At this point, we might consider displaying some kind of loading indicator on the native stage. A splash screen or a progress bar are both good options.

Next, let's implement the assets_onProgress function to track the loading progress of our assets:

private function assets_onProgress( ratio:Number ):void
	if( ratio < 1 )
		// you could update some kind of simple progress bar here

	this._starling.rootClass = MyApp;

Once the assets are fully loaded, we can start Starling. If the app has a theme, we can instantiate it immediately before setting the rootClass, just like before.

Any asynchronous task can be added to a custom bootstrap class, and it's easy to defer the instantiation of the root MXML application until everything is ready.