รูปแบบกรณีใช้งานและคลังข้อมูล

struct PlaceOrderUseCase {
  let cart: CartRepository
  let payment: PaymentService
  let orders: OrderRepository
  func execute(userId: Int) async throws -> Order {
    let items = try await cart.items(for: userId)
    let receipt = try await payment.charge(items: items)
    return try await orders.create(receipt: receipt)
  }
}

// One use case per business action:
struct FetchUserUseCase { ... }
struct UpdateUserUseCase { ... }
struct DeleteUserUseCase { ... }

protocol PlacingOrder {
  func execute(userId: Int) async throws -> Order
}
struct PlaceOrderUseCase: PlacingOrder { ... }

protocol OrderRepository {
  func create(receipt: PaymentReceipt) async throws -> Order
  func list(userId: Int) async throws -> [Order]
  func cancel(orderId: Int) async throws
}

struct RemoteOrderRepository: OrderRepository {
  func create(receipt: PaymentReceipt) async throws -> Order {
    let dto = receipt.toDTO()
    let data = try await api.post("/orders", body: dto)
    return try JSONDecoder().decode(OrderDTO.self, from: data).toDomain()
  }
  func list(userId: Int) async throws -> [Order] { ... }
  func cancel(orderId: Int) async throws { ... }
}

final class InMemoryOrderRepo: OrderRepository {
  var orders: [Order] = []
  func create(receipt: PaymentReceipt) async throws -> Order {
    let order = Order(id: orders.count + 1, items: receipt.items)
    orders.append(order)
    return order
  }
  func list(userId: Int) async throws -> [Order] { orders }
  func cancel(orderId: Int) async throws { orders.removeAll { $0.id == orderId } }
}

struct CheckoutUseCase {
  let validateCart: ValidateCartUseCase
  let placeOrder: PlaceOrderUseCase
  let notifyUser: NotifyUserUseCase
  func execute(userId: Int) async throws -> Order {
    try await validateCart.execute(userId: userId)
    let order = try await placeOrder.execute(userId: userId)
    try await notifyUser.execute(order: order)
    return order
  }
}

// Repository: single entity
protocol UserRepository { func getUser(id: Int) async throws -> User }
// Service: cross-entity operation
protocol AuthService { func login(email: String, password: String) async throws -> Session }

struct CachedUserRepository: UserRepository {
  private var cache: [Int: User] = [:]
  private let remote: UserRepository
  func getUser(id: Int) async throws -> User {
    if let cached = cache[id] { return cached }
    let user = try await remote.getUser(id: id)
    cache[id] = user
    return user
  }
  func saveUser(_ user: User) async throws { try await remote.saveUser(user) }
}

let repo = InMemoryOrderRepo()
let sut = PlaceOrderUseCase(orders: repo, ...)
let order = try await sut.execute(userId: 1)
XCTAssertEqual(repo.orders.count, 1)

func getUser(id: Int) async throws -> User {
  do {
    let data = try await api.get("/users/\(id)")
    return try decode(data)
  } catch let e as URLError where e.code == .notConnectedToInternet {
    throw DomainError.offline
  }
}