Method update

examples/example.cc:241–251

class ParticleSystem : public ex::System<ParticleSystem> {
public:
  void update(ex::EntityManager &es, ex::EventManager &events, ex::TimeDelta dt) override {
    ex::ComponentHandle<Particle> particle;
    for (ex::Entity entity : es.entities_with_components(particle)) {
      particle->alpha -= particle->d * dt;
      if (particle->alpha <= 0) {
        entity.destroy();
      } else {
        particle->colour.a = particle->alpha;
      }
    }
  }
};