class ComputeTotalSumSystem extends ReactiveSystem {
  @override
  GroupChangeEvent get event => GroupChangeEvent.any;

  @override
  executeWith(List<Entity> entities) {
    final sum = entityManager.group(all: [CountComponent, AmountInSelectedCurrencyComponent]).entities.fold(0.0, (sum, e) => sum
        + (e.get<AmountInSelectedCurrencyComponent>()?.value ?? 0.0)
            * (e.get<CountComponent>()?.value ?? 0)
    );
    entityManager.setUnique(TotalAmountComponent(sum));
  }

  @override
  EntityMatcher get matcher => EntityMatcher(all:[CountComponent, AmountInSelectedCurrencyComponent]);

}

class ComputeTotalSumLabelSystem extends ReactiveSystem {
  @override
  GroupChangeEvent get event => GroupChangeEvent.addedOrUpdated;

  @override
  executeWith(List<Entity> entities) {
    final currency = entityManager.getUnique<SelectedCurrencyComponent>()?.value;
    if (currency == null) return;
    final sum = entityManager.getUnique<TotalAmountComponent>()?.value;
    if (sum == null) return;
    entityManager.setUnique(TotalAmountLabelComponent(priceLabel(currency, sum)));
  }

  @override
  EntityMatcher get matcher => EntityMatcher(all:[TotalAmountComponent]);

}