class AddToShoppingCartComponent implements Component {}
class RemoveFromShoppingCartComponent implements Component {}
class SwitchCurrencyComponent implements UniqueComponent {}

class AddItemToShoppingCartSystem extends ReactiveSystem {
  @override
  GroupChangeEvent get event => GroupChangeEvent.addedOrUpdated;
  @override
  EntityMatcher get matcher => EntityMatcher(all: [AddToShoppingCartComponent]);

  @override
  executeWith(List<Entity> entities) {
    for (var e in entities) {
      var newCount = (e.get<CountComponent>()?.value ?? 0) + 1;
      e += CountComponent(newCount);
    }
  }
}

class RemoveItemFromShoppingCartSystem extends ReactiveSystem {
  @override
  GroupChangeEvent get event => GroupChangeEvent.addedOrUpdated;
  @override
  EntityMatcher get matcher => EntityMatcher(all: [RemoveFromShoppingCartComponent]);

  @override
  executeWith(List<Entity> entities) {
    for (var e in entities) {
      final newCount = (e.get<CountComponent>()?.value ?? 0) - 1;
      if (newCount > 0) {
        e += CountComponent(newCount);
      } else {
        e -= CountComponent;
      }
    }
  }
}

class SwitchCurrencySystem extends ReactiveSystem {
  @override
  GroupChangeEvent get event => GroupChangeEvent.addedOrUpdated;
  @override
  EntityMatcher get matcher => EntityMatcher(all: [SwitchCurrencyComponent]);

  @override
  executeWith(List<Entity> entities) {
    final currency = entityManager.getUnique<SelectedCurrencyComponent>().value;
    if ( currency == null) return;
    if (currency == Currency.usd) {
      entityManager.setUnique(SelectedCurrencyComponent(Currency.euro));
    } else {
      entityManager.setUnique(SelectedCurrencyComponent(Currency.usd));
    }
  }
}