Artur Gurgul - Scenes, multiwindow + state restoration

ios-platform · memo

In one line: Since iOS 13 one process drives many UI instances: each window is a UIWindowScene with its own lifecycle, backed by a persistent UISceneSession. The system may disconnect a background scene to save memory and reconnect it later; restoration hands the scene back its own NSUserActivity so it rebuilds UI state (ids, not model data) — unless a deep link says where to go.

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Scenes, multiwindow + state restoration — figure 1

How it works — scenes and windows

AppDelegate = process: didFinishLaunching, push token, services, configurationForConnecting, didDiscardSceneSessions. SceneDelegate = one window: willConnectTo, sceneDidBecomeActive/WillResignActive/ WillEnterForeground/DidEnterBackground, sceneDidDisconnect, openURLContexts, continue (links while running). With scenes, the app delegate’s applicationDidBecomeActive family is not called.
Info.plist UIApplicationSceneManifest: UIApplicationSupportsMultipleScenes = YES or the app stays single-window on iPad. Open a window with UIApplication.shared.requestScene SessionActivation(_:userActivity:options: errorHandler:): nil session = new window; an existing one = bring it forward — prefer that to spawning duplicates.
Disconnect ≠ discard: disconnect frees a background scene’s UI (session kept, may reconnect via willConnectTo); discard = the user closed it — clean up per-window data (may arrive at the next launch).
keyWindow / windows are ambiguous with several scenes: use view.window?.windowScene or connectedScenes.

How it works — iPad multitasking

Split View, Slide Over, Stage Manager (iPadOS 16), external display: the system sizes your window, live. Adapt to size classes (horizontalSizeClass is .compact in a narrow pane on the biggest iPad), never to userInterfaceIdiom.
React to traits: registerForTraitChanges (iOS 17; replaces traitCollectionDidChange), viewWillTransition(to:with:); UISplitViewController / NavigationSplitView collapse to a stack in compact width.
Two windows share the process: a global “current document” breaks. Shared model at app level, per-window state in the scene.

Likely questions

Where do you restore? — willConnectTo, after checking connection options.
Save when? — stateRestorationActivity(for:), on entering background.
Open item X in a new iPad window? — activity with X’s id → requestSceneSessionActivation; SwiftUI openWindow(value:).

Example — restore, but let links win

func scene(_ scene: UIScene,
           willConnectTo session: UISceneSession,
           options: UIScene.ConnectionOptions) {
  guard let ws = scene as? UIWindowScene else { return }
  window = UIWindow(windowScene: ws)
  if let url = options.urlContexts.first?.url { router.open(url) }
  else if let ua = options.userActivities.first {
    router.continue(ua) }                      // UL / Handoff
  else if let id = session.stateRestorationActivity?
            .userInfo?["itemID"] as? String, store.exists(id) {
    router.show(id) }                          // validated
  window?.rootViewController = router.root
  window?.makeKeyAndVisible() }
// asked on entering the background: keep it cheap
func stateRestorationActivity(for scene: UIScene)
    -> NSUserActivity? {
  let ua = NSUserActivity(activityType: "com.you.viewing")
  ua.addUserInfoEntries(from: ["itemID": router.currentID])
  return ua }

Restoration + SwiftUI

userInfo = property-list values: ids, offsets, tab, draft text. One NSUserActivity can also drive Handoff (isEligibleForHandoff) and Spotlight.
Restores after a system kill. Discarded after the user closes the window / swipes the app away, or a crash during restore. Test: background the app, then Stop in Xcode, relaunch.
Legacy: restorationIdentifier + encodeRestorableState(with:), opt-in shouldSaveSecureApplicationState.
SwiftUI: WindowGroup (multi-instance on iPadOS/macOS); @Environment(\.openWindow) → openWindow(id:) / openWindow(value:). @SceneStorage("k") = small per-scene values, restored automatically; @AppStorage = one UserDefaults for all windows. scenePhase is per scene (combined when read in App).

Interview traps

Archiving model objects into restoration — store ids and re-fetch; the record may be gone.
@AppStorage for per-window UI state: two windows clobber each other. @StateObject inside WindowGroup = one per window.
NavigationPath is not Codable itself — persist path.codable (a CodableRepresentation) as Data.

Remember

Session persists, scene comes and goes · link > restore > default · ids, not models.