Use-case: nə vaxt lazımdır, nə vaxt zərərlidir

// fayl: lib/domain/usecases/calculate_cart_total.dart
import 'package:my_app/domain/models/cart.dart';
import 'package:my_app/domain/models/cart_total.dart';
import 'package:my_app/domain/repositories/cart_repository.dart';
import 'package:my_app/domain/repositories/coupon_repository.dart';
import 'package:my_app/domain/repositories/delivery_repository.dart';

class CalculateCartTotal {
  const CalculateCartTotal({
    required CartRepository cartRepository,
    required CouponRepository couponRepository,
    required DeliveryRepository deliveryRepository,
  })  : _cart = cartRepository,
        _coupons = couponRepository,
        _delivery = deliveryRepository;

  final CartRepository _cart;
  final CouponRepository _coupons;
  final DeliveryRepository _delivery;

  /// Üç mənbəni birləşdirir — buna görə use-case-dir.
  Future<CartTotal> call({String? couponCode, required String cityId}) async {
    final cart = await _cart.fetchCurrent();

    final subtotal = cart.items.fold<double>(
      0,
      (sum, item) => sum + item.product.finalPrice * item.quantity,
    );

    // Kupon: yoxdursa, ya da etibarsızdırsa endirim sıfırdır.
    final coupon = couponCode == null
        ? null
        : await _coupons.findValid(couponCode, subtotal: subtotal);
    final discount = coupon?.discountFor(subtotal) ?? 0;

    // Çatdırılma: pulsuz həddi keçildikdə sıfırlanır.
    final tariff = await _delivery.tariffFor(cityId);
    final shipping =
        subtotal - discount >= tariff.freeFrom ? 0.0 : tariff.price;

    return CartTotal(
      subtotal: subtotal,
      discount: discount,
      shipping: shipping,
      total: subtotal - discount + shipping,
      appliedCoupon: coupon,
    );
  }
}

// ── Riverpod-da bağlanması ──
@riverpod
CalculateCartTotal calculateCartTotal(Ref ref) => CalculateCartTotal(
      cartRepository: ref.watch(cartRepositoryProvider),
      couponRepository: ref.watch(couponRepositoryProvider),
      deliveryRepository: ref.watch(deliveryRepositoryProvider),
    );

// ── Notifier sadə qalır: bir çağırış ──
@riverpod
class CartSummary extends _$CartSummary {
  @override
  Future<CartTotal> build({String? couponCode, required String cityId}) =>
      ref.watch(calculateCartTotalProvider)(
        couponCode: couponCode,
        cityId: cityId,
      );
}