Game Loop Pattern (#1083)

* Add game loop module

* Add game loop module

* Fix merge issue

* Implement game loop module

* Implement game loop module

* Implement time based game loop

* implement VariableStepGameLoop

* Implement FixedStepGameLoop

* Add UT

* Add Unit tests

* Fix checkstyle issues

* Add README.md

* Fix code review issues

* Fix code review issues

* update README.md

game-loop/README.md

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+
+---  
+layout: pattern  
+title: Game Loop 
+folder:  game-loop  
+permalink: /patterns/game-loop/  
+categories: Other  
+tags:  
+ - Java  
+ - Difficulty-Beginner  
+---  
+
+## 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 decouple the progression of game time from user input and processor speed.
+
+## Applicability  
+This pattern is used in every game engine. 
+
+## Explanation
+Game loop is the main process of all the game rendering threads. It drives input process, internal status update, rendering, AI and all the other processes. 
+
+There are a lot of implementations of game loop:
+
+- Frame-based game loop
+
+Frame-based game loop is the easiest implementation. The loop always keeps spinning for the following three processes: processInput, update and render. The problem with it is you have no control over how fast the game runs. On a fast machine, that loop will spin so fast users won’t be able to see what’s going on. On a slow machine, the game will crawl. If you have a part of the game that’s content-heavy or does more AI or physics, the game will actually play slower there.
+
+- Variable-step game loop
+
+The variable-step game loop chooses a time step to advance based on how much real time passed since the last frame. The longer the frame takes, the bigger steps the game takes. It always keeps up with real time because it will take bigger and bigger steps to get there.
+
+- Fixed-step game loop
+
+For fixed-step game loop, a certain amount of real time has elapsed since the last turn of the game loop. This is how much game time need to be simulated for the game’s “now” to catch up with the player’s.
+
+
+## Credits  
+
+* [Game Programming Patterns - Game Loop](http://gameprogrammingpatterns.com/game-loop.html)

game-loop/pom.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+
+    The MIT License
+    Copyright © 2014-2019 Ilkka Seppälä
+
+    Permission is hereby granted, free of charge, to any person obtaining a copy
+    of this software and associated documentation files (the "Software"), to deal
+    in the Software without restriction, including without limitation the rights
+    to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+    copies of the Software, and to permit persons to whom the Software is
+    furnished to do so, subject to the following conditions:
+
+    The above copyright notice and this permission notice shall be included in
+    all copies or substantial portions of the Software.
+
+    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+    IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+    FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+    AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+    LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+    OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+    THE SOFTWARE.
+
+-->
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <parent>
+        <artifactId>java-design-patterns</artifactId>
+        <groupId>com.iluwatar</groupId>
+        <version>1.22.0-SNAPSHOT</version>
+    </parent>
+    <modelVersion>4.0.0</modelVersion>
+
+    <artifactId>game-loop</artifactId>
+    <dependencies>
+        <dependency>
+            <groupId>junit</groupId>
+            <artifactId>junit</artifactId>
+        </dependency>
+    </dependencies>
+
+
+</project>

game-loop/src/main/java/com/iluwatar/gameloop/App.java

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+/*
+ * The MIT License
+ * Copyright © 2014-2019 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+package com.iluwatar.gameloop;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * 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.
+ */
+public class App {
+
+  private static final Logger LOGGER = LoggerFactory.getLogger(App.class);
+
+  /**
+   * Each type of game loop will run for 2 seconds.
+   */
+  private static final int GAME_LOOP_DURATION_TIME = 2000;
+
+  /**
+   * Program entry point.
+   * @param args runtime arguments
+   */
+  public static void main(String[] args) {
+
+    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());
+    }
+  }
+
+}

game-loop/src/main/java/com/iluwatar/gameloop/Bullet.java

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+/*
+ * The MIT License
+ * Copyright © 2014-2019 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+package com.iluwatar.gameloop;
+
+/**
+ * Bullet object class.
+ */
+public class Bullet {
+
+  private float position;
+
+  public Bullet() {
+    position = 0.0f;
+  }
+
+  public float getPosition() {
+    return position;
+  }
+
+  public void setPosition(float position) {
+    this.position = position;
+  }
+}

game-loop/src/main/java/com/iluwatar/gameloop/FixedStepGameLoop.java

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+/*
+ * The MIT License
+ * Copyright © 2014-2019 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+package com.iluwatar.gameloop;
+
+/**
+ * For fixed-step game loop, a certain amount of real time has elapsed since the
+ * last turn of the game loop. This is how much game time need to be simulated for
+ * the game’s “now” to catch up with the player’s.
+ */
+public class FixedStepGameLoop extends GameLoop {
+
+  /**
+   * 20 ms per frame = 50 FPS.
+   */
+  private static final long MS_PER_FRAME = 20;
+
+  @Override
+  protected void processGameLoop() {
+    var previousTime = System.currentTimeMillis();
+    var lag = 0L;
+    while (isGameRunning()) {
+      var currentTime = System.currentTimeMillis();
+      var elapsedTime = currentTime - previousTime;
+      previousTime = currentTime;
+      lag += elapsedTime;
+
+      processInput();
+
+      while (lag >= MS_PER_FRAME) {
+        update();
+        lag -= MS_PER_FRAME;
+      }
+
+      render();
+    }
+  }
+
+  protected void update() {
+    controller.moveBullet(0.5f * MS_PER_FRAME / 1000);
+  }
+}

game-loop/src/main/java/com/iluwatar/gameloop/FrameBasedGameLoop.java

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+/*
+ * The MIT License
+ * Copyright © 2014-2019 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+package com.iluwatar.gameloop;
+
+/**
+ * Frame-based game loop is the easiest implementation. The loop always keeps spinning
+ * for the following three processes: processInput, update and render. The problem with
+ * it is you have no control over how fast the game runs. On a fast machine, that loop
+ * will spin so fast users won’t be able to see what’s going on. On a slow machine, the
+ * game will crawl. If you have a part of the game that’s content-heavy or does more AI
+ * or physics, the game will actually play slower there.
+ */
+public class FrameBasedGameLoop extends GameLoop {
+
+  @Override
+  protected void processGameLoop() {
+    while (isGameRunning()) {
+      processInput();
+      update();
+      render();
+    }
+  }
+
+  /**
+   * Each time when update() is invoked, a new frame is created, and the bullet will be
+   * moved 0.5f away from the current position.
+   */
+  protected void update() {
+    controller.moveBullet(0.5f);
+  }
+
+}

game-loop/src/main/java/com/iluwatar/gameloop/GameController.java

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+/*
+ * The MIT License
+ * Copyright © 2014-2019 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+package com.iluwatar.gameloop;
+
+/**
+ * Update and render objects in the game. Here we add a Bullet object to the
+ * game system to show how the game loop works.
+ */
+public class GameController {
+
+  protected final Bullet bullet;
+
+  /**
+   * Initialize Bullet instance.
+   */
+  public GameController() {
+    bullet = new Bullet();
+  }
+
+  /**
+   * Move bullet position by the provided offset.
+   *
+   * @param offset moving offset
+   */
+  public void moveBullet(float offset) {
+    var currentPosition = bullet.getPosition();
+    bullet.setPosition(currentPosition + offset);
+  }
+
+  /**
+   * Get current position of the bullet.
+   *
+   * @return position of bullet
+   */
+  public float getBulletPosition() {
+    return bullet.getPosition();
+  }
+
+}
+