AppDelegate와 SceneDelegate 역할 차이

// AppDelegate: 앱 전체 레벨
func application(
  _ application: UIApplication,
  didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?
) -> Bool {
  // 글로벌 초기화
  return true
}

// SceneDelegate: scene(창) 레벨
func scene(
  _ scene: UIScene,
  willConnectTo session: UISceneSession,
  options connectionOptions: UIScene.ConnectionOptions
) {
  guard let windowScene = scene as? UIWindowScene else { return }
  let window = UIWindow(windowScene: windowScene)
  window.rootViewController = RootViewController()
  self.window = window
  window.makeKeyAndVisible()
}

// 1) AppDelegate: 이벤트를 전역으로 수집
func application(_ application: UIApplication,
                 didReceiveRemoteNotification userInfo: [AnyHashable: Any]) async
  -> UIBackgroundFetchResult {
  AppEventBus.shared.publish(.push(userInfo))
  return .noData
}

// 2) SceneDelegate: 활성 scene에서만 라우팅 실행
func sceneDidBecomeActive(_ scene: UIScene) {
  Task { @MainActor in
    for await event in AppEventBus.shared.events {
      guard let windowScene = scene as? UIWindowScene else { continue }
      SceneRouter(windowScene: windowScene).handle(event)
    }
  }
}

UIApplication (앱 당 1개, 싱글턴)
    │
    ├── connectedScenes: Set<UIScene>
    ├── openSessions: Set<UISceneSession>
    │
    └── UISceneSession ←──1:1──→ UIWindowScene
                                      ↑
                                      │ windowScene (strong 참조)
                                      │
                                UIWindow ── rootViewController
                                UIWindow (keyboard)
                                UIWindow (alert)

// UIWindow가 UIWindowScene을 참조 (strong)
class UIWindow {
    var windowScene: UIWindowScene?  // Window → Scene
}

// UIWindowScene은 연결된 Window들을 조회 (weak 추정)
class UIWindowScene {
    var windows: [UIWindow] { get }  // Scene → Windows (역방향 조회)
}

// 관계: 1:N
// - Window 1개는 Scene 1개만 참조
// - Scene 1개에 Window 여러 개 연결 가능 (main, keyboard, alert 등)

class SceneDelegate: UIResponder, UIWindowSceneDelegate {
    var window: UIWindow?

    // 1️⃣ Scene 연결 (최초 생성)
    func scene(_ scene: UIScene, willConnectTo session: UISceneSession, ...) {
        guard let windowScene = scene as? UIWindowScene else { return }
        window = UIWindow(windowScene: windowScene)
        window?.rootViewController = MainViewController()
        window?.makeKeyAndVisible()
    }

    // 2️⃣ Scene 연결 해제
    func sceneDidDisconnect(_ scene: UIScene) { }

    // 3️⃣ Active (사용자 상호작용 가능)
    func sceneDidBecomeActive(_ scene: UIScene) { }

    // 4️⃣ Inactive (상호작용 불가, 아직 보임)
    func sceneWillResignActive(_ scene: UIScene) { }

    // 5️⃣ Foreground 진입 예정
    func sceneWillEnterForeground(_ scene: UIScene) { }

    // 6️⃣ Background 진입
    func sceneDidEnterBackground(_ scene: UIScene) { }
}

class SceneDelegate: UIWindowSceneDelegate {
    var window: UIWindow?           // SceneDelegate가 Window 소유
    var coordinator: AppCoordinator?

    func scene(_ scene: UIScene, willConnectTo session: UISceneSession, ...) {
        guard let windowScene = scene as? UIWindowScene else { return }

        let window = UIWindow(windowScene: windowScene)
        self.window = window

        // Coordinator에게 화면 구성 위임
        coordinator = AppCoordinator(window: window)
        coordinator?.start()  // rootVC 설정 + 화면 흐름 관리
    }
}

class AppCoordinator {
    private let window: UIWindow  // 참조만 (소유 X)

    func start() {
        window.rootViewController = UINavigationController(rootViewController: HomeVC())
        window.makeKeyAndVisible()
    }
}