DI 프레임워크 없이 계층 연결하기

@main
struct MyApp: App {
  // All wiring happens here
  private let container = AppContainer()
  var body: some Scene {
    WindowGroup { container.makeRootView() }
  }
}

final class AppContainer {
  let db = SQLiteDatabase()
  lazy var userRepo: UserRepository = RemoteUserRepository(db: db)
  lazy var fetchUser = FetchUserUseCase(repo: userRepo)
  lazy var profileVM = ProfileViewModel(useCase: fetchUser)
  func makeRootView() -> some View { ProfileView(viewModel: profileVM) }
}

final class Container {
  lazy var network: NetworkClient = URLSessionClient()
  lazy var api: APIService = APIService(client: network)  // created after network
  lazy var repo: ItemRepository = RemoteItemRepository(api: api)
}

extension AppContainer {
  func makeOrderFlow() -> OrderFlowView {
    let repo = RemoteOrderRepository(api: api)
    let useCase = PlaceOrderUseCase(repo: repo)
    let vm = OrderViewModel(useCase: useCase)
    return OrderFlowView(viewModel: vm)
  }
}

private struct FetchUserKey: EnvironmentKey {
  static let defaultValue: FetchingUser = FetchUserUseCase(repo: NullUserRepo())
}
extension EnvironmentValues {
  var fetchUser: FetchingUser {
    get { self[FetchUserKey.self] } set { self[FetchUserKey.self] = newValue }
  }
}

ContentView()
  .environment(\.fetchUser, container.fetchUser)

// In a deep view:
@Environment(\.fetchUser) private var fetchUser

final class CheckoutContainer {
  let cart: CartRepository
  let payment: PaymentService
  lazy var placeOrder = PlaceOrderUseCase(cart: cart, payment: payment)
  init(parent: AppContainer) {
    cart = parent.cartRepo
    payment = parent.paymentService
  }
}

func testContainerBuildsProfileView() {
  let container = AppContainer(db: InMemoryDatabase())
  let view = container.makeRootView()
  XCTAssertTrue(type(of: view) == ProfileView.self)
}

// Circular:
// UserService → NotificationService → UserService
// Fix: UserService → NotificationInput (protocol) ← NotificationService

// Manual: fine for <20 top-level dependencies
// Framework: helps with large teams, 50+ services, code-generation for factories

final class AppContainer {
  private let db = CoreDataStack.shared
  lazy var userRepo: UserRepository = LocalUserRepository(db: db)
  lazy var fetchUser = FetchUserUseCase(repo: userRepo)
  func makeContentView() -> ContentView {
    ContentView(viewModel: ProfileViewModel(useCase: fetchUser))
  }
}