คำถามสัมภาษณ์ SwiftUI NavigationStack: รูปแบบการนำทาง 2026

// ❌ Old pattern with NavigationView (deprecated)
struct OldNavigation: View {
    var body: some View {
        NavigationView {
            NavigationLink("Details", destination: DetailView())
        }
    }
}

// ✅ New pattern with NavigationStack
struct ModernNavigation: View {
    // Navigation stack is explicit and controllable
    @State private var path = NavigationPath()

    var body: some View {
        NavigationStack(path: $path) {
            List {
                // NavigationLink with typed value
                NavigationLink("User 1", value: User(id: 1, name: "Alice"))
                NavigationLink("User 2", value: User(id: 2, name: "Bob"))
            }
            // Destination defined by value type
            .navigationDestination(for: User.self) { user in
                UserDetailView(user: user)
            }
        }
    }
}

struct ContentView: View {
    // NavigationPath can contain different Hashable types
    @State private var path = NavigationPath()

    var body: some View {
        NavigationStack(path: $path) {
            VStack(spacing: 20) {
                Button("View user profile") {
                    // Adds a User to the stack
                    path.append(User(id: 1, name: "Alice"))
                }

                Button("View settings") {
                    // Adds a Settings enum to the stack
                    path.append(SettingsRoute.notifications)
                }

                Button("Back to root") {
                    // Clears the entire stack
                    path.removeLast(path.count)
                }
            }
            .navigationDestination(for: User.self) { user in
                UserDetailView(user: user)
            }
            .navigationDestination(for: SettingsRoute.self) { route in
                SettingsView(route: route)
            }
        }
    }
}

// Types must be Hashable
struct User: Hashable {
    let id: Int
    let name: String
}

enum SettingsRoute: Hashable {
    case notifications
    case privacy
    case account
}

// Centralized router to manage navigation
@Observable
class NavigationRouter {
    var path = NavigationPath()

    // Navigate to a specific screen
    func navigateTo(_ destination: AppRoute) {
        path.append(destination)
    }

    // Go back one level
    func goBack() {
        guard !path.isEmpty else { return }
        path.removeLast()
    }

    // Return to root
    func popToRoot() {
        path.removeLast(path.count)
    }

    // Navigate to a complete stack (deep link)
    func navigateToPath(_ routes: [AppRoute]) {
        popToRoot()
        for route in routes {
            path.append(route)
        }
    }
}

// Enum defining all app routes
enum AppRoute: Hashable {
    case userList
    case userDetail(userId: Int)
    case userEdit(userId: Int)
    case settings
    case settingsDetail(SettingsSection)
}

enum SettingsSection: String, Hashable {
    case notifications, privacy, account
}

// Usage in main view
struct MainView: View {
    @State private var router = NavigationRouter()

    var body: some View {
        NavigationStack(path: $router.path) {
            HomeView()
                .navigationDestination(for: AppRoute.self) { route in
                    destinationView(for: route)
                }
        }
        .environment(router)
    }

    @ViewBuilder
    private func destinationView(for route: AppRoute) -> some View {
        switch route {
        case .userList:
            UserListView()
        case .userDetail(let userId):
            UserDetailView(userId: userId)
        case .userEdit(let userId):
            UserEditView(userId: userId)
        case .settings:
            SettingsView()
        case .settingsDetail(let section):
            SettingsDetailView(section: section)
        }
    }
}

@Observable
class DeepLinkHandler {
    var router: NavigationRouter

    init(router: NavigationRouter) {
        self.router = router
    }

    // Parse a URL and navigate to destination
    func handle(url: URL) {
        guard let components = URLComponents(url: url, resolvingAgainstBaseURL: true),
              let host = components.host else {
            return
        }

        // Build navigation stack according to URL
        let routes = parseRoutes(host: host, path: components.path, queryItems: components.queryItems)
        router.navigateToPath(routes)
    }

    private func parseRoutes(host: String, path: String, queryItems: [URLQueryItem]?) -> [AppRoute] {
        // myapp://users/42/edit → [.userList, .userDetail(42), .userEdit(42)]
        switch host {
        case "users":
            return parseUserPath(path)
        case "settings":
            return parseSettingsPath(path)
        default:
            return []
        }
    }

    private func parseUserPath(_ path: String) -> [AppRoute] {
        let segments = path.split(separator: "/").map(String.init)
        var routes: [AppRoute] = [.userList]

        if let userIdString = segments.first, let userId = Int(userIdString) {
            routes.append(.userDetail(userId: userId))

            // /users/42/edit
            if segments.count > 1 && segments[1] == "edit" {
                routes.append(.userEdit(userId: userId))
            }
        }

        return routes
    }

    private func parseSettingsPath(_ path: String) -> [AppRoute] {
        var routes: [AppRoute] = [.settings]

        let segments = path.split(separator: "/").map(String.init)
        if let sectionString = segments.first,
           let section = SettingsSection(rawValue: sectionString) {
            routes.append(.settingsDetail(section))
        }

        return routes
    }
}

// In the main App
@main
struct MyApp: App {
    @State private var router = NavigationRouter()
    @State private var deepLinkHandler: DeepLinkHandler?

    var body: some Scene {
        WindowGroup {
            MainView()
                .environment(router)
                .onOpenURL { url in
                    // Handle deep links
                    deepLinkHandler?.handle(url: url)
                }
                .onAppear {
                    deepLinkHandler = DeepLinkHandler(router: router)
                }
        }
    }
}

// Extension to make NavigationPath persistable
extension NavigationPath {
    // Encode path to Data
    func encoded() -> Data? {
        guard let representation = self.codable else { return nil }
        return try? JSONEncoder().encode(representation)
    }

    // Decode from Data
    static func decoded(from data: Data) -> NavigationPath? {
        guard let representation = try? JSONDecoder().decode(
            NavigationPath.CodableRepresentation.self,
            from: data
        ) else {
            return nil
        }
        return NavigationPath(representation)
    }
}

// Router with persistence
@Observable
class PersistentNavigationRouter {
    var path: NavigationPath {
        didSet {
            saveState()
        }
    }

    private let storageKey = "navigation_path"

    init() {
        // Restore state at startup
        if let data = UserDefaults.standard.data(forKey: storageKey),
           let restored = NavigationPath.decoded(from: data) {
            self.path = restored
        } else {
            self.path = NavigationPath()
        }
    }

    private func saveState() {
        if let data = path.encoded() {
            UserDefaults.standard.set(data, forKey: storageKey)
        }
    }

    func clearPersistedState() {
        UserDefaults.standard.removeObject(forKey: storageKey)
    }
}

enum AuthState {
    case unauthenticated
    case authenticated(User)
}

@Observable
class AuthManager {
    var state: AuthState = .unauthenticated
    var pendingDeepLink: URL?

    func login(email: String, password: String) async throws {
        // Simulated authentication
        let user = try await AuthService.shared.login(email: email, password: password)
        state = .authenticated(user)
    }

    func logout() {
        state = .unauthenticated
    }
}

struct RootView: View {
    @State private var authManager = AuthManager()
    @State private var router = NavigationRouter()

    var body: some View {
        Group {
            switch authManager.state {
            case .unauthenticated:
                // Separate navigation stack for auth
                AuthNavigationStack(authManager: authManager)
            case .authenticated:
                // Main app stack
                MainNavigationStack(router: router, authManager: authManager)
            }
        }
        .onChange(of: authManager.state) { oldState, newState in
            handleAuthStateChange(from: oldState, to: newState)
        }
    }

    private func handleAuthStateChange(from oldState: AuthState, to newState: AuthState) {
        switch (oldState, newState) {
        case (.unauthenticated, .authenticated):
            // Login successful: process pending deep link
            if let pendingURL = authManager.pendingDeepLink {
                DeepLinkHandler(router: router).handle(url: pendingURL)
                authManager.pendingDeepLink = nil
            }
        case (.authenticated, .unauthenticated):
            // Logout: reset navigation
            router.popToRoot()
        default:
            break
        }
    }
}

struct AuthNavigationStack: View {
    let authManager: AuthManager
    @State private var authPath = NavigationPath()

    var body: some View {
        NavigationStack(path: $authPath) {
            LoginView(authManager: authManager)
                .navigationDestination(for: AuthRoute.self) { route in
                    switch route {
                    case .register:
                        RegisterView(authManager: authManager)
                    case .forgotPassword:
                        ForgotPasswordView()
                    }
                }
        }
    }
}

enum AuthRoute: Hashable {
    case register
    case forgotPassword
}

struct ParentView: View {
    @State private var mainPath = NavigationPath()
    @State private var showSettings = false
    @State private var showUserProfile: User?

    var body: some View {
        NavigationStack(path: $mainPath) {
            ContentView()
                .toolbar {
                    Button("Settings") {
                        showSettings = true
                    }
                }
                .navigationDestination(for: MainRoute.self) { route in
                    MainRouteView(route: route)
                }
        }
        // Sheet with its own NavigationStack
        .sheet(isPresented: $showSettings) {
            SettingsSheet()
        }
        // Conditional sheet based on item
        .sheet(item: $showUserProfile) { user in
            UserProfileSheet(user: user)
        }
    }
}

// Each sheet has its own NavigationStack
struct SettingsSheet: View {
    @Environment(\.dismiss) private var dismiss
    @State private var settingsPath = NavigationPath()

    var body: some View {
        NavigationStack(path: $settingsPath) {
            SettingsListView()
                .navigationTitle("Settings")
                .toolbar {
                    ToolbarItem(placement: .cancellationAction) {
                        Button("Close") {
                            dismiss()
                        }
                    }
                }
                .navigationDestination(for: SettingsSection.self) { section in
                    SettingsDetailView(section: section)
                }
        }
    }
}

// Extension to make User identifiable for sheet(item:)
extension User: Identifiable {}

// Protocol to abstract the router
protocol NavigationRouterProtocol {
    var pathCount: Int { get }
    func navigateTo(_ destination: AppRoute)
    func goBack()
    func popToRoot()
}

// Concrete implementation
@Observable
class AppNavigationRouter: NavigationRouterProtocol {
    var path = NavigationPath()

    var pathCount: Int {
        path.count
    }

    func navigateTo(_ destination: AppRoute) {
        path.append(destination)
    }

    func goBack() {
        guard !path.isEmpty else { return }
        path.removeLast()
    }

    func popToRoot() {
        path.removeLast(path.count)
    }
}

// Unit tests
import XCTest

final class NavigationRouterTests: XCTestCase {
    var router: AppNavigationRouter!

    override func setUp() {
        router = AppNavigationRouter()
    }

    func testNavigateToAddsToPath() {
        // Given
        XCTAssertEqual(router.pathCount, 0)

        // When
        router.navigateTo(.userDetail(userId: 42))

        // Then
        XCTAssertEqual(router.pathCount, 1)
    }

    func testGoBackRemovesLastItem() {
        // Given
        router.navigateTo(.userList)
        router.navigateTo(.userDetail(userId: 1))
        XCTAssertEqual(router.pathCount, 2)

        // When
        router.goBack()

        // Then
        XCTAssertEqual(router.pathCount, 1)
    }

    func testPopToRootClearsPath() {
        // Given
        router.navigateTo(.userList)
        router.navigateTo(.userDetail(userId: 1))
        router.navigateTo(.userEdit(userId: 1))
        XCTAssertEqual(router.pathCount, 3)

        // When
        router.popToRoot()

        // Then
        XCTAssertEqual(router.pathCount, 0)
    }

    func testGoBackOnEmptyPathDoesNothing() {
        // Given
        XCTAssertEqual(router.pathCount, 0)

        // When
        router.goBack()

        // Then
        XCTAssertEqual(router.pathCount, 0) // No crash
    }
}

// Navigation state for each tab
@Observable
class TabNavigationState {
    var homePath = NavigationPath()
    var searchPath = NavigationPath()
    var profilePath = NavigationPath()
    var selectedTab: Tab = .home

    enum Tab: Hashable {
        case home, search, profile
    }

    func resetCurrentTab() {
        switch selectedTab {
        case .home:
            homePath.removeLast(homePath.count)
        case .search:
            searchPath.removeLast(searchPath.count)
        case .profile:
            profilePath.removeLast(profilePath.count)
        }
    }

    func resetAllTabs() {
        homePath.removeLast(homePath.count)
        searchPath.removeLast(searchPath.count)
        profilePath.removeLast(profilePath.count)
    }
}

struct TabRootView: View {
    @State private var tabState = TabNavigationState()

    var body: some View {
        TabView(selection: $tabState.selectedTab) {
            // Home tab
            NavigationStack(path: $tabState.homePath) {
                HomeView()
                    .navigationDestination(for: HomeRoute.self) { route in
                        HomeRouteView(route: route)
                    }
            }
            .tabItem { Label("Home", systemImage: "house") }
            .tag(TabNavigationState.Tab.home)

            // Search tab
            NavigationStack(path: $tabState.searchPath) {
                SearchView()
                    .navigationDestination(for: SearchRoute.self) { route in
                        SearchRouteView(route: route)
                    }
            }
            .tabItem { Label("Search", systemImage: "magnifyingglass") }
            .tag(TabNavigationState.Tab.search)

            // Profile tab
            NavigationStack(path: $tabState.profilePath) {
                ProfileView()
                    .navigationDestination(for: ProfileRoute.self) { route in
                        ProfileRouteView(route: route)
                    }
            }
            .tabItem { Label("Profile", systemImage: "person") }
            .tag(TabNavigationState.Tab.profile)
        }
        .environment(tabState)
    }
}

struct OptimizedNavigationStack: View {
    @State private var path = NavigationPath()

    var body: some View {
        NavigationStack(path: $path) {
            LazyContentView()
                // ✅ Lazy-loaded destinations
                .navigationDestination(for: HeavyRoute.self) { route in
                    // View only created on navigation
                    HeavyDetailView(route: route)
                }
        }
    }
}

// ✅ View with lazy content loading
struct LazyContentView: View {
    @State private var items: [Item] = []

    var body: some View {
        // LazyVStack only creates visible views
        ScrollView {
            LazyVStack(spacing: 12) {
                ForEach(items) { item in
                    NavigationLink(value: HeavyRoute.detail(item.id)) {
                        ItemRow(item: item)
                    }
                }
            }
        }
        .task {
            items = await loadItems()
        }
    }
}

// ✅ Detail with progressive loading
struct HeavyDetailView: View {
    let route: HeavyRoute
    @State private var data: DetailData?

    var body: some View {
        Group {
            if let data {
                DetailContent(data: data)
            } else {
                ProgressView()
            }
        }
        .task {
            // Load data only when view appears
            data = await loadDetailData(for: route)
        }
    }
}

// ❌ Avoid: eager creation of heavy views
struct BadNavigationStack: View {
    let allItems: [Item]

    var body: some View {
        NavigationStack {
            List(allItems) { item in
                // Creates all destinations immediately
                NavigationLink {
                    HeavyDetailView(item: item) // ❌ Created upfront
                } label: {
                    ItemRow(item: item)
                }
            }
        }
    }
}