ComponentMapper.java

package com.artemis;

import com.artemis.utils.Bag;

/**
 * High performance component retrieval from entities. Use this wherever you
 * need to retrieve components from entities often and fast.
 * 
 * @author Arni Arent
 *
 * @param <A> the class type of the component
 */
public class ComponentMapper<A extends Component> {
	private ComponentType type;
	private Class<A> classType;
	private Bag<Component> components;

	private ComponentMapper(Class<A> type, World world) {
		this.type = ComponentType.getTypeFor(type);
		components = world.getComponentManager().getComponentsByType(this.type);
		this.classType = type;
	}

	/**
	 * Fast but unsafe retrieval of a component for this entity.
	 * No bounding checks, so this could throw an ArrayIndexOutOfBoundsExeption,
	 * however in most scenarios you already know the entity possesses this component.
	 * 
	 * @param e the entity that should possess the component
	 * @return the instance of the component
	 */
	public A get(Entity e) {
		return classType.cast(components.get(e.getId()));
	}

	/**
	 * Fast and safe retrieval of a component for this entity.
	 * If the entity does not have this component then null is returned.
	 * 
	 * @param e the entity that should possess the component
	 * @return the instance of the component
	 */
	public A getSafe(Entity e) {
		if(components.isIndexWithinBounds(e.getId())) {
			return classType.cast(components.get(e.getId()));
		}
		return null;
	}
	
	/**
	 * Checks if the entity has this type of component.
	 * @param e the entity to check
	 * @return true if the entity has this component type, false if it doesn't.
	 */
	public boolean has(Entity e) {
		return getSafe(e) != null;		
	}

	/**
	 * Returns a component mapper for this type of components.
	 * 
	 * @param type the type of components this mapper uses.
	 * @param world the world that this component mapper should use.
	 * @return a new mapper.
	 */
	public static <T extends Component> ComponentMapper<T> getFor(Class<T> type, World world) {
		return new ComponentMapper<T>(type, world);
	}

}