import UIKit

@main
final class AppDelegate: UIResponder, UIApplicationDelegate {
    var window: UIWindow?
    private var context: AppContext?

    func application(
        _ application: UIApplication,
        didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]? = nil
    ) -> Bool {
        let window = UIWindow(frame: UIScreen.main.bounds)
        let context = AppContext()
        self.context = context
        window.rootViewController = context.makeRootController()
        window.makeKeyAndVisible()
        self.window = window
        return true
    }
}

@MainActor
private final class AppContext: NSObject, UITabBarControllerDelegate {
    private let tabs = UITabBarController()
    private var navigationControllers: [UINavigationController] = []
    private var restoreTask: Task<Void, Never>?

    func makeRootController() -> UIViewController {
        let shell = mobileShellFixture(state: .loading)
        navigationControllers = shell.pages.map { page in
            let root = ShellViewController(page: page)
            let navigation = UINavigationController(rootViewController: root)
            navigation.navigationBar.prefersLargeTitles = true
            navigation.tabBarItem = UITabBarItem(
                title: page.title,
                image: UIImage(systemName: page.systemImage),
                selectedImage: UIImage(systemName: page.selectedSystemImage)
            )
            return navigation
        }
        tabs.viewControllers = navigationControllers
        tabs.delegate = self
        restoreSelectedTab()
        return tabs
    }

    func tabBarController(
        _ tabBarController: UITabBarController,
        didSelect viewController: UIViewController
    ) {
        guard
            let navigation = viewController as? UINavigationController,
            let root = navigation.viewControllers.first as? ShellViewController
        else {
            return
        }
        do {
            try setSelectedMobileTab(tab: root.shellTab)
        } catch {
            let alert = UIAlertController(
                title: "Selection Was Not Saved",
                message: error.localizedDescription,
                preferredStyle: .alert
            )
            alert.addAction(UIAlertAction(title: "OK", style: .default))
            tabs.present(alert, animated: true)
        }
    }

    private func restoreSelectedTab() {
        restoreTask?.cancel()
        restoreTask = Task { [weak self] in
            let shell = await Task.detached(priority: .userInitiated) { mobileShell() }.value
            guard !Task.isCancelled, let self else { return }
            if let index = shell.pages.firstIndex(where: { $0.tab == shell.selectedTab }) {
                tabs.selectedIndex = index
            }
        }
    }
}

@MainActor
private final class ShellViewController: UITableViewController {
    fileprivate let shellTab: MobileTab
    private var page: MobilePage
    private var loadTask: Task<Void, Never>?
    private var loadGeneration = 0

    init(page: MobilePage) {
        shellTab = page.tab
        self.page = page
        super.init(style: .insetGrouped)
        title = page.title
        navigationItem.largeTitleDisplayMode = .always
        refreshControl = UIRefreshControl()
        refreshControl?.addTarget(self, action: #selector(refreshRequested), for: .valueChanged)
    }

    @available(*, unavailable)
    required init?(coder: NSCoder) {
        fatalError("init(coder:) is not supported")
    }

    deinit {
        loadTask?.cancel()
    }

    override func viewDidLoad() {
        super.viewDidLoad()
        apply(page)
    }

    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)
        reloadShell()
    }

    override func viewDidDisappear(_ animated: Bool) {
        super.viewDidDisappear(animated)
        loadTask?.cancel()
    }

    override func numberOfSections(in tableView: UITableView) -> Int {
        page.state == .ready ? 1 : 0
    }

    override func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        1
    }

    override func tableView(
        _ tableView: UITableView,
        cellForRowAt indexPath: IndexPath
    ) -> UITableViewCell {
        let cell = UITableViewCell(style: .subtitle, reuseIdentifier: nil)
        var content = cell.defaultContentConfiguration()
        content.image = UIImage(systemName: page.systemImage)
        content.text = page.stateTitle
        content.secondaryText = page.stateDetail
        content.secondaryTextProperties.numberOfLines = 0
        cell.contentConfiguration = content
        cell.selectionStyle = .none
        cell.accessibilityLabel = "\(page.stateTitle). \(page.stateDetail)"
        return cell
    }

    @objc private func refreshRequested() {
        reloadShell()
    }

    private func reloadShell() {
        loadGeneration += 1
        let generation = loadGeneration
        loadTask?.cancel()
        loadTask = Task { [weak self] in
            let shell = await Task.detached(priority: .userInitiated) { mobileShell() }.value
            guard
                !Task.isCancelled,
                let self,
                generation == loadGeneration,
                let page = shell.pages.first(where: { $0.tab == self.shellTab })
            else { return }
            apply(page)
        }
    }

    private func apply(_ page: MobilePage) {
        self.page = page
        title = page.title
        refreshControl?.endRefreshing()
        tableView.reloadData()

        guard page.state != .ready else {
            contentUnavailableConfiguration = nil
            return
        }
        var configuration = page.state == .loading
            ? UIContentUnavailableConfiguration.loading()
            : UIContentUnavailableConfiguration.empty()
        configuration.text = page.stateTitle
        configuration.secondaryText = page.stateDetail
        configuration.image = UIImage(systemName: stateImage(page.state))
        contentUnavailableConfiguration = configuration
    }

    private func stateImage(_ state: MobileShellState) -> String {
        switch state {
        case .loading: "hourglass"
        case .empty: "lock.shield"
        case .ready: page.systemImage
        case .locked: "lock.fill"
        case .error: "exclamationmark.triangle"
        }
    }
}