Built in Entity Systems

Ashley comes with a few Entity Systems that will spare you from typing boilerplate code.

1. IteratingSystem
2. IntervalSystem
3. IntervalIteratingSystem
4. SortedIteratingSystem

IteratingSystem

Most of the time, you will only need to iterate over a family of entities. In that case you can just extend the IteratingSystem class and override its processEntity() method instead.

public class MovementSystem extends IteratingSystem {
	private ComponentMapper<PositionComponent> pm = ComponentMapper.getFor(PositionComponent.class);
	private ComponentMapper<VelocityComponent> vm = ComponentMapper.getFor(VelocityComponent.class);

	public MovementSystem() {
		super(Family.getFor(PositionComponent.class, VelocityComponent.class));
	}

	public void processEntity(Entity entity, float deltaTime) {
		PositionComponent position = pm.get(entity);
		VelocityComponent velocity = vm.get(entity);
		
		position.x += velocity.x * deltaTime;
		position.y += velocity.y * deltaTime;
	}
}

IntervalSystem

Sometimes you might want to make a system not run every tick, but at constant intervals. Rather than messing about with setProcessing() you can extend the IntervalSystem class and implement its updateInterval(float) method.

public class MovementSystem extends IntervalSystem {
	private ComponentMapper<PositionComponent> pm = ComponentMapper.getFor(PositionComponent.class);
	private ComponentMapper<VelocityComponent> vm = ComponentMapper.getFor(VelocityComponent.class);
	ImmutableArray<Entity> entities;

	@Override
	public void addedToEngine(Engine engine) {
		entities = engine.getEntitiesFor(Family.getFor(PositionComponent.class, VelocityComponent.class));
	}

	protected void updateInterval() {
		PositionComponent position = pm.get(entity);
		VelocityComponent velocity = vm.get(entity);
		
		position.x += velocity.x * deltaTime;
		position.y += velocity.y * deltaTime;
	}
}

IntervalIteratingSystem

It is also possible to have the best of both worlds (IteratingSystem and IntervalSystem) thanks to IntervalIteratingSystem. In this case, you will need to implement its processEntity(Entity) method.

SortedIteratingSystem

Whenever you need to iterate over a family of entities in a specific order, SortedIteratingSystem will be your friend. You simply need to specify the family a comparator and implement the processEntity() method.

Let's say we're writing a RenderingSystem that processes entities with a RenderableComponent and a PositionComponent. The z value of the position will determine the order in which entities are processed, the system will sort them for you automatically.

public class RenderingSystem extends SortedIteratingSystem {
	private ComponentMapper<RenderableComponent> rm;
	private ComponentMapper<PositionComponent> pm;

	public RenderingSystem() {
		super(Family.getFor(RenderableComponent.class, PositionComponent.class), new ZComparator());

		rm = ComponentMapper.getFor(RenderableComponent.class);
		pm = ComponentMapper.getFor(PositionComponent.class);
	}

	protected void processEntity(Entity entity, float deltaTime) {
		// Render the entity
	}

	private static class ZComparator implements Comparator<Entity> {
		@Override
		public int compare(Entity e1, Entity e2) {
			return (int)Math.signum(pm.get(e1).z - pm.get(e2).z);
		}
	}
}