UIKit App Modernization: Adaptive Layout & Bar APIs

import UIKit

class ModernTabBarController: UITabBarController {

    override func viewDidLoad() {
        super.viewDidLoad()

        // Build three tabs
        let feedVC = makeTab(
            title: "Feed",
            systemImage: "house.fill",
            identifier: "feed"
        )
        let searchVC = makeTab(
            title: "Search",
            systemImage: "magnifyingglass",
            identifier: "search"
        )
        let profileVC = makeTab(
            title: "Profile",
            systemImage: "person.fill",
            identifier: "profile"
        )

        setViewControllers([feedVC, searchVC, profileVC], animated: false)

        // iOS 27: Opt iPhone into sidebar layout when space allows
        sidebar.preferredPlacement = .sidebar

        // iOS 27: Keep "Feed" tab always visible even when tab bar collapses
        prominentTabIdentifier = "feed"

        // Show or hide sidebar-specific UI based on availability
        updateSidebarAvailability()
    }

    private func makeTab(title: String, systemImage: String, identifier: String) -> UINavigationController {
        let content = ContentViewController(title: title)
        let nav = UINavigationController(rootViewController: content)

        let tabItem = UITabBarItem(
            title: title,
            image: UIImage(systemName: systemImage),
            tag: 0
        )
        nav.tabBarItem = tabItem

        // iOS 27: Configure navigation bar minimization
        content.navigationItem.barMinimizationBehavior = .always
        content.navigationItem.barMinimizationSafeAreaAdjustment = .automatic

        return nav
    }

    private func updateSidebarAvailability() {
        // iOS 27: sidebar.isAvailable tells you if sidebar is currently shown
        if sidebar.isAvailable {
            print("Sidebar is active — surface nested tabs in sidebar")
        } else {
            print("No sidebar — ensure nested tab content is reachable elsewhere")
        }
    }
}

class ContentViewController: UIViewController {
    init(title: String) {
        super.init(nibName: nil, bundle: nil)
        self.title = title
    }
    required init?(coder: NSCoder) { fatalError() }

    override func viewDidLoad() {
        super.viewDidLoad()
        view.backgroundColor = .systemBackground

        // Use trait collection instead of UIScreen.main.scale (iOS 27 best practice)
        let scale = traitCollection.displayScale
        print("Display scale from trait collection: \(scale)")
    }

    override func traitCollectionDidChange(_ previousTraitCollection: UITraitCollection?) {
        super.traitCollectionDidChange(previousTraitCollection)
        // Use size class, not interfaceOrientation or userInterfaceIdiom
        let isRegular = traitCollection.horizontalSizeClass == .regular
        print("Horizontal size class regular: \(isRegular)")
    }
}