--- layout: pattern title: Game Loop folder: game-loop permalink: /patterns/game-loop/ categories: Behavioral language: en tags: - Game programming --- ## Intent A game loop runs continuously during gameplay. Each turn of the loop, it processes user input without blocking, updates the game state, and renders the game. It tracks the passage of time to control the rate of gameplay. This pattern decouples progression of game time from user input and processor speed. ## Applicability This pattern is used in every game engine. ## Explanation Real world example > Game loop is the main process of all the game rendering threads. It's present in all modern games. > It drives input process, internal status update, rendering, AI and all the other processes. In plain words > Game Loop pattern ensures that game time progresses in equal speed in all different hardware > setups. Wikipedia says > The central component of any game, from a programming standpoint, is the game loop. The game loop > allows the game to run smoothly regardless of a user's input, or lack thereof. **Programmatic Example** Let's start with something simple. Here's `Bullet` class. Bullets will move in our game. For demonstration purposes it's enough that it has 1-dimensional position. ```java public class Bullet { private float position; public Bullet() { position = 0.0f; } public float getPosition() { return position; } public void setPosition(float position) { this.position = position; } } ``` `GameController` is responsible for moving objects in the game, including the aforementioned bullet. ```java public class GameController { protected final Bullet bullet; public GameController() { bullet = new Bullet(); } public void moveBullet(float offset) { var currentPosition = bullet.getPosition(); bullet.setPosition(currentPosition + offset); } public float getBulletPosition() { return bullet.getPosition(); } } ``` Now we introduce the game loop. Or actually in this demo we have 3 different game loops. Let's see the base class `GameLoop` first. ```java public enum GameStatus { RUNNING, STOPPED } public abstract class GameLoop { protected final Logger logger = LoggerFactory.getLogger(this.getClass()); protected volatile GameStatus status; protected GameController controller; private Thread gameThread; public GameLoop() { controller = new GameController(); status = GameStatus.STOPPED; } public void run() { status = GameStatus.RUNNING; gameThread = new Thread(this::processGameLoop); gameThread.start(); } public void stop() { status = GameStatus.STOPPED; } public boolean isGameRunning() { return status == GameStatus.RUNNING; } protected void processInput() { try { var lag = new Random().nextInt(200) + 50; Thread.sleep(lag); } catch (InterruptedException e) { logger.error(e.getMessage()); } } protected void render() { var position = controller.getBulletPosition(); logger.info("Current bullet position: " + position); } protected abstract void processGameLoop(); } ``` Here's the first game loop implementation, `FrameBasedGameLoop`: ```java public class FrameBasedGameLoop extends GameLoop { @Override protected void processGameLoop() { while (isGameRunning()) { processInput(); update(); render(); } } protected void update() { controller.moveBullet(0.5f); } } ``` Finally, we show all the game loops in action. ```java try { LOGGER.info("Start frame-based game loop:"); var frameBasedGameLoop = new FrameBasedGameLoop(); frameBasedGameLoop.run(); Thread.sleep(GAME_LOOP_DURATION_TIME); frameBasedGameLoop.stop(); LOGGER.info("Stop frame-based game loop."); LOGGER.info("Start variable-step game loop:"); var variableStepGameLoop = new VariableStepGameLoop(); variableStepGameLoop.run(); Thread.sleep(GAME_LOOP_DURATION_TIME); variableStepGameLoop.stop(); LOGGER.info("Stop variable-step game loop."); LOGGER.info("Start fixed-step game loop:"); var fixedStepGameLoop = new FixedStepGameLoop(); fixedStepGameLoop.run(); Thread.sleep(GAME_LOOP_DURATION_TIME); fixedStepGameLoop.stop(); LOGGER.info("Stop variable-step game loop."); } catch (InterruptedException e) { LOGGER.error(e.getMessage()); } ``` Program output: ```java Start frame-based game loop: Current bullet position: 0.5 Current bullet position: 1.0 Current bullet position: 1.5 Current bullet position: 2.0 Current bullet position: 2.5 Current bullet position: 3.0 Current bullet position: 3.5 Current bullet position: 4.0 Current bullet position: 4.5 Current bullet position: 5.0 Current bullet position: 5.5 Current bullet position: 6.0 Stop frame-based game loop. Start variable-step game loop: Current bullet position: 6.5 Current bullet position: 0.038 Current bullet position: 0.084 Current bullet position: 0.145 Current bullet position: 0.1805 Current bullet position: 0.28 Current bullet position: 0.32 Current bullet position: 0.42549998 Current bullet position: 0.52849996 Current bullet position: 0.57799995 Current bullet position: 0.63199997 Current bullet position: 0.672 Current bullet position: 0.778 Current bullet position: 0.848 Current bullet position: 0.8955 Current bullet position: 0.9635 Stop variable-step game loop. Start fixed-step game loop: Current bullet position: 0.0 Current bullet position: 1.086 Current bullet position: 0.059999995 Current bullet position: 0.12999998 Current bullet position: 0.24000004 Current bullet position: 0.33999994 Current bullet position: 0.36999992 Current bullet position: 0.43999985 Current bullet position: 0.5399998 Current bullet position: 0.65999967 Current bullet position: 0.68999964 Current bullet position: 0.7299996 Current bullet position: 0.79999954 Current bullet position: 0.89999944 Current bullet position: 0.98999935 Stop variable-step game loop. ``` ## Class diagram ![alt text](./etc/game-loop.urm.png "Game Loop pattern class diagram") ## Credits * [Game Programming Patterns - Game Loop](http://gameprogrammingpatterns.com/game-loop.html) * [Game Programming Patterns](https://www.amazon.com/gp/product/0990582906/ref=as_li_qf_asin_il_tl?ie=UTF8&tag=javadesignpat-20&creative=9325&linkCode=as2&creativeASIN=0990582906&linkId=1289749a703b3fe0e24cd8d604d7c40b) * [Game Engine Architecture, Third Edition](https://www.amazon.com/gp/product/1138035459/ref=as_li_qf_asin_il_tl?ie=UTF8&tag=javadesignpat-20&creative=9325&linkCode=as2&creativeASIN=1138035459&linkId=94502746617211bc40e0ef49d29333ac)