UIKit App Modernization for Resizable Environments

 import UIKit
 
−// OLD: App Delegate lifecycle (no longer acceptable when building with iOS 27 SDK)
−@main
−class AppDelegate: UIResponder, UIApplicationDelegate {
+// MARK: - Scene Delegate (required in iOS 27; apps without this won't launch)
+class SceneDelegate: UIResponder, UIWindowSceneDelegate {
     var window: UIWindow?
 
−    func application(
−        _ application: UIApplication,
−        didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?
−    ) -> Bool {
−        // OLD: Using UIScreen.main — breaks in iPhone Mirroring & external displays
−        let screenBounds = UIScreen.main.bounds
−        let scale = UIScreen.main.scale
−        print("Screen bounds: \(screenBounds), scale: \(scale)")
+    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 = makeRootViewController()
+        self.window = window
+        window.makeKeyAndVisible()
+    }
 
−        window = UIWindow(frame: screenBounds)
−        window?.rootViewController = makeLegacyRootViewController()
−        window?.makeKeyAndVisible()
−        return true
+    // React to size changes instead of using UIScreen.main.bounds
+    func windowScene(
+        _ windowScene: UIWindowScene,
+        didUpdate previousCoordinateSpace: UICoordinateSpace,
+        interfaceOrientation previousInterfaceOrientation: UIInterfaceOrientation,
+        traitCollection previousTraitCollection: UITraitCollection
+    ) {
+        // Use effectiveGeometry for available space — never UIScreen.main
+        let availableSize = windowScene.effectiveGeometry.systemMinimumLayoutMargins
+        print("Scene geometry updated: \(availableSize)")
     }
 
−    private func makeLegacyRootViewController() -> UIViewController {
−        let feedVC = FeedViewControllerLegacy()
+    private func makeRootViewController() -> UIViewController {
+        // Tab 1: Feed
+        let feedVC = FeedViewController()
         let feedNav = UINavigationController(rootViewController: feedVC)
−        feedNav.tabBarItem = UITabBarItem(title: "Feed", image: UIImage(systemName: "house"), tag: 0)
+        let feedTab = UITabBarItem(title: "Feed", image: UIImage(systemName: "house"), tag: 0)
+        feedNav.tabBarItem = feedTab
 
+        // Tab 2: Search
+        let searchVC = UIViewController()
+        searchVC.view.backgroundColor = .systemBackground
+        searchVC.title = "Search"
+        let searchNav = UINavigationController(rootViewController: searchVC)
+        searchNav.tabBarItem = UITabBarItem(title: "Search", image: UIImage(systemName: "magnifyingglass"), tag: 1)
+
         let tabController = UITabBarController()
−        tabController.viewControllers = [feedNav]
−        // No prominentTabIdentifier, no sidebar support
+        tabController.viewControllers = [feedNav, searchNav]
+
+        // iOS 27: Pin a prominent tab that stays visible even when bar collapses
+        tabController.prominentTabIdentifier = feedTab.identifier
+
+        // iOS 27: Opt iPhone app into sidebar layout when horizontal size class is regular
+        tabController.sidebar.preferredPlacement = .sidebar
+
         return tabController
     }
 }
 
−class FeedViewControllerLegacy: UIViewController {
+// MARK: - Feed View Controller demonstrating barMinimizationBehavior
+class FeedViewController: UIViewController {
+    private let tableView = UITableView(frame: .zero, style: .plain)
+
     override func viewDidLoad() {
         super.viewDidLoad()
         title = "Feed"
−        // No barMinimizationBehavior — navigation bar never slides away
+        view.backgroundColor = .systemBackground
+
+        // iOS 27: Force navigation bar to slide away on scroll
+        navigationItem.barMinimizationBehavior = .always
+        // If you manage safe area insets yourself, suppress automatic adjustment:
+        // navigationItem.barMinimizationSafeAreaAdjustment = .never
+
+        tableView.dataSource = self
+        tableView.register(UITableViewCell.self, forCellReuseIdentifier: "cell")
+        tableView.translatesAutoresizingMaskIntoConstraints = false
+        view.addSubview(tableView)
+        NSLayoutConstraint.activate([
+            tableView.topAnchor.constraint(equalTo: view.topAnchor),
+            tableView.leadingAnchor.constraint(equalTo: view.leadingAnchor),
+            tableView.trailingAnchor.constraint(equalTo: view.trailingAnchor),
+            tableView.bottomAnchor.constraint(equalTo: view.bottomAnchor)
+        ])
     }
 
−    override func viewWillTransition(
−        to size: CGSize,
−        with coordinator: UIViewControllerTransitionCoordinator
−    ) {
−        super.viewWillTransition(to: size, with: coordinator)
−        // OLD: Checking interface orientation for layout — broken in resizable envs
−        let orientation = UIApplication.shared.statusBarOrientation
−        if orientation.isLandscape {
−            print("Landscape layout")
−        } else {
−            print("Portrait layout")
−        }
+    // iOS 27: Use trait collection for scale — never UIScreen.main.scale
+    override func layoutSubviews() {
+        // traitCollection.displayScale is automatically tracked;
+        // layout is re-triggered when it changes (e.g. moving to external display)
+        let scale = traitCollection.displayScale
+        print("Display scale from trait collection: \(scale)")
     }
+}
+
+extension FeedViewController: UITableViewDataSource {
+    func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int { 40 }
+    func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
+        let cell = tableView.dequeueReusableCell(withIdentifier: "cell", for: indexPath)
+        cell.textLabel?.text = "Item \(indexPath.row + 1)"
+        return cell
+    }
 }