Method update

entityx/System_test.cc:62–70

class CounterSystem : public System<CounterSystem> {
public:
  void update(EntityManager &es, EventManager &events, TimeDelta) override {
    EntityManager::View entities =
        es.entities_with_components<Counter>();
    Counter::Handle counter;
    for (auto entity : entities) {
      entity.unpack<Counter>(counter);
      counter->counter++;
    }
  }
};

class EntitiesFixture : public EntityX {