Systems

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Systems look for certain combinations of components in entities and modify the data of said components in each frame. Systems determine the behaviour of entities.

There are a total of 4 different types of systems:

Order	Type	Description
1	Initialisation systems	Initialize entities when they are first added to the world and when they are re-activated.
2	Input systems	Read user input and modify data of components in each frame.
3	Update systems	Modify the data of components in each frame.
4	Render systems	Turn components into visuals that are rendered to the screen.

Initialisation systems are also executed when an inactive entity is re-activated. Input, update and render systems are executed in each frame in the order specified above.

Creating systems

Below is a list of 3 different ways of creating systems, orderered from most to least simple. All examples are update systems, but other types of systems can be created in the same way. The examples are systems that require a Transform component.

Option 1: Extending system base class

This option is best used for systems that need to perform certain logic once per frame, before or after processing all entities.

public class TransformInitializer extends InitSystemBase {

  public TransformInitializer() {
    super(Transform.class);
  }

  @Override
  public void init() {
    // ...

    forEach(chunk -> {
      Transform transform = chunk.getComponent(Transform.class);

      // ...
    });
  }

}

public class TransformInputHandler extends InputSystemBase {

  public TransformInputHandler() {
    super(Transform.class);
  }

  @Override
  public void input(float deltaTime) {
    // ...

    forEach(chunk -> {
      Transform transform = chunk.getComponent(Transform.class);

      // ...
    });
  }

}

public class TransformUpdater extends UpdateSystemBase {

  public TransformUpdater() {
    super(Transform.class);
  }

  @Override
  public void update(float deltaTime) {
    // ...

    forEach(chunk -> {
      Transform transform = chunk.getComponent(Transform.class);

      // ...
    });
  }

}

public class TransformRenderSystem extends RenderSystemBase {

  public TransformRenderSystem() {
    super(Transform.class);
  }

  @Override
  public void render(Renderer renderer) {
    // ...

    forEach(chunk -> {
      Transform transform = chunk.getComponent(Transform.class);

      // ...
    });
  }

}

Option 2: Extending system class

public class TransformInitializer extends UpdateSystem {

  public TransformInitializer() {
    super(Transform.class);
  }

  @Override
  public void process(EntityChunk chunk) {
    Transform transform = chunk.getComponent(Transform.class);

    // ...
  }

}

public class TransformInputHandler extends UpdateSystem {

  public TransformInputHandler() {
    super(Transform.class);
  }

  @Override
  public void process(EntityChunk chunk, Input input, float deltaTime) {
    Transform transform = chunk.getComponent(Transform.class);

    // ...
  }

}

public class TransformUpdater extends UpdateSystem {

  public TransformUpdater() {
    super(Transform.class);
  }

  @Override
  public void process(EntityChunk chunk, float deltaTime) {
    Transform transform = chunk.getComponent(Transform.class);

    // ...
  }

}

public class TransformRenderSystem extends UpdateSystem {

  public TransformRenderSystem() {
    super(Transform.class);
  }

  @Override
  public void process(EntityChunk chunk, Renderer renderer) {
    Transform transform = chunk.getComponent(Transform.class);

    // ...
  }

}

Option 3: Using system builder

InitSystemBuilder initSystemBuilder = new InitSystemBuilder(Transform.class);
initSystemBuilder.setCallback(chunk -> {
  Transform transform = chunk.getComponent(Transform.class);

  // ...
});

InitSystem transformInitializer = initSystemBuilder.build();

InputSystemBuilder inputSystemBuilder = new InputSystemBuilder(Transform.class);
inputSystemBuilder.setCallback(chunk, input, deltaTime -> {
  Transform transform = chunk.getComponent(Transform.class);

  // ...
});

InputSystem transformInputHandler = inputSystemBuilder.build();

UpdateSystemBuilder updateSystemBuilder = new UpdateSystemBuilder(Transform.class);
updateSystemBuilder.setCallback(chunk, deltaTime -> {
  Transform transform = chunk.getComponent(Transform.class);

  // ...
});

UpdateSystem transformUpdater = updateSystemBuilder.build();

RenderSystemBuilder renderSystemBuilder = new RenderSystemBuilder(Transform.class);
renderSystemBuilder.setCallback(chunk, renderer -> {
  Transform transform = chunk.getComponent(Transform.class);

  // ...
});

RenderSystem transformRenderSystem = renderSystemBuilder.build();

Adding systems to the world

Systems should be added to the world during scene loading, before any entities are added to the scene.

world.addSystem(transformUpdater);