Simplify Flow Between Screens with the “Scene System”

package com.michaeljameswilliams.gamedev.gamescenes 
{
 
	/**
	 * Based on http://www.rivermanmedia.com/programming/6-object-oriented-game-programming-the-scene-system
	 * More info: http://gamedev.michaeljameswilliams.com/2009/04/13/as3-scene-system/
	 * @author MichaelJWilliams
	 */
 
	public interface IGameScene 
	{
		/**
		 * Update logic for this game scene.
		 */
		function update():void;
		/**
		 * Redraw everything on screen.
		 */
		function redraw():void;
		/**
		 * Load all of the data and graphics that this scene needs to function.
		 */
		function load():void;
		/**
		 * Unload everything that the garbage collector won't unload itself, including graphics.
		 */
		function unload():void;
	}
 
}
 
 
 
 
 
package com.michaeljameswilliams.gamedev.gamescenes 
{
	import flash.display.DisplayObject;
	import flash.display.DisplayObjectContainer;
 
	/**
	 * Based on http://www.rivermanmedia.com/programming/6-object-oriented-game-programming-the-scene-system
	 * More info: http://gamedev.michaeljameswilliams.com/2009/04/13/as3-scene-system/
	 * Singleton template from here: http://www.gskinner.com/blog/archives/2006/07/as3_singletons.html
	 * @author MichaelJWilliams
	 */
 
	public class GameSceneManager {
		private static var _instance:GameSceneManager;
		private static var _allowInstantiation:Boolean;
		private static var _currentScene:IGameScene;
		private static var _sceneContainer:DisplayObjectContainer;
 
		public static function getInstance( p_sceneContainer:DisplayObjectContainer = null ):GameSceneManager 
		{
			if ( _instance == null ) 
			{
				_allowInstantiation = true;
				_instance = new GameSceneManager();
				_allowInstantiation = false;
			}
 
			if ( p_sceneContainer )
			{
				setSceneContainer( p_sceneContainer );
			}
 
			return _instance;
		}
 
		/**
		 * 
		 * @param	p_sceneContainer	Object that is parent of each scene -- for example, the document class
		 */
		public function GameSceneManager():void 
		{
			if ( !_allowInstantiation ) 
			{
				throw new Error("Error: Instantiation failed: Use GameSceneManager.getInstance() instead of new.");
			}
		}
 
		public static function setSceneContainer( p_sceneContainer:DisplayObjectContainer ):void
		{
			_sceneContainer = p_sceneContainer;
		}
 
		/**
		 * Update logic for the current scene.
		 */
		public static function update():void
		{
			_currentScene.update();
		}
 
		/**
		 * Redraw everything on screen.
		 */
		public static function redraw():void
		{
			_currentScene.redraw();
		}
 
		/**
		 * A simplified way of loading and unloading scenes
		 * Notice that the method is static so that anyone can call it.
		 * @param	p_newScene	The scene to switch to.
		 */
		public static function changeScene( p_newScene:IGameScene ):void
		{
			if ( ( p_newScene is DisplayObject ) && ( _sceneContainer ) )
			{
				if ( _currentScene != null )
				{
					_sceneContainer.removeChild( _currentScene as DisplayObject );
					_currentScene.unload();
				}
				p_newScene.load();
				_currentScene = p_newScene;
				_sceneContainer.addChild( _currentScene as DisplayObject );
			}
			else
			{
				throw new Error("Error: game scenes must be of type DisplayObject and scene container must be set.");
			}
		}
	}
}