High performance component retrieval from entities. Use this wherever you need to retrieve components from entities often and fast.

class ComponentMapper<A extends Component> {
 ComponentType _type;
 Bag<Component> _components;

 ComponentMapper(Type componentType, World world) {
   this._type = ComponentTypeManager.getTypeFor(componentType);
   _components = world.componentManager.getComponentsByType(this._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.
  */
 A get(Entity e) => _components[e.id] as A;

 /**
  * Fast and safe retrieval of a component for this entity.
  * If the entity does not have this component then null is returned.
  */
 A getSafe(Entity e) {
   if(_components.isIndexWithinBounds(e.id)) {
     return _components[e.id] as A;
   }
   return null;
 }

 /**
  * Checks if the entity has this type of component.
  */
 bool has(Entity e) => getSafe(e) != null;
}