/// A button that shows a popover menu when clicked.
///
/// Due to technical limitations, the minimum supported OS's for menu buttons in
/// UIKitBackend are iOS 14 and tvOS 17.
public struct Menu {
    /// The menu's label.
    public var label: String
    /// The menu's items.
    public var items: [MenuItem]

    var buttonWidth: Int?

    /// Creates a menu.
    ///
    /// - Parameters:
    ///   - label: The menu's label.
    ///   - items: The menu's items.
    @MainActor
    public init(_ label: String, @ViewBuilder items: () -> some View) {
        self.label = label
        self.items = items()._asMenuItems
    }

    /// Resolves the menu to a representation used by backends.
    @MainActor
    func resolve() -> ResolvedMenu.Submenu {
        ResolvedMenu.Submenu(
            label: label,
            content: Self.resolve(items: items)
        )
    }

    @MainActor
    static func resolve(item: MenuItem) -> ResolvedMenu.Item {
        switch item {
            case .button(let button):
                .button(button.body.view0.view0.string, button.action)
            case .text(let text):
                .button(text.string, nil)
            case .toggle(let toggle):
                .toggle(
                    toggle.label,
                    toggle.active.wrappedValue,
                    onChange: { toggle.active.wrappedValue = $0 }
                )
            case .separator:
                .separator
            case .submenu(let submenu):
                .submenu(submenu.resolve())
            case .modifiedEnvironment(let item, let modification):
                .modifiedEnvironment(resolve(item: item()), modification())
        }
    }

    /// Resolves the menu's items to a representation used by backends.
    @MainActor
    static func resolve(items: [MenuItem]) -> ResolvedMenu {
        ResolvedMenu(items: items.map(resolve(item:)))
    }
}

@available(iOS 14, macCatalyst 14, tvOS 17, *)
extension Menu: TypeSafeView {
    public var body: EmptyView { return EmptyView() }

    public var _asMenuItems: [MenuItem] { [.submenu(self)] }

    func children<Backend: BaseAppBackend>(
        backend: Backend,
        snapshots: [ViewGraphSnapshotter.NodeSnapshot]?,
        environment: EnvironmentValues
    ) -> Children {
        MenuStorage()
    }

    func asWidget<Backend: BaseAppBackend>(
        _ children: MenuStorage,
        backend: Backend
    ) -> Backend.Widget {
        return backend.createSimpleButton()
    }

    func layoutableChildren<Backend: BaseAppBackend>(
        backend: Backend,
        children: MenuStorage
    ) -> [LayoutSystem.LayoutableChild] {
        []
    }

    func computeLayout<Backend: BaseAppBackend>(
        _ widget: Backend.Widget,
        children: MenuStorage,
        proposedSize: ProposedViewSize,
        environment: EnvironmentValues,
        backend: Backend
    ) -> ViewLayoutResult {
        func castBackend_inner<NewBackend: BaseAppBackend & BackendFeatures.MenuButtons>(_ backend: NewBackend) -> ViewLayoutResult {
            let widget = widget as! NewBackend.Widget
            // TODO: Look into ways to predict a button's natural size without
            //   updating its content so that computeLayout can be a bit more of
            //   a pure function.

            // Update the button before measuring its natural size
            switch backend.menuImplementationStyle {
            case .dynamicPopover(let backend):
            // Our menu button action implementation needs to know the size
            // of the button, but we don't have that yet, so just update it
            // with an empty action and fix it in commit.
            backend.updateSimpleButton(
            widget,
            label: label,
            environment: environment,
            action: {}
            )
            case .menuButton(let backend):
            let menu =
            children.menu.flatMap { $0 as? NewBackend.Menu }
            ?? backend.createPopoverMenu()
            children.menu = menu
            backend.updateButton(
            widget,
            label: label,
            menu: menu,
            environment: environment
            )
            }

            var size = backend.naturalSize(of: widget)
            size.x = buttonWidth ?? size.x
            return ViewLayoutResult.leafView(size: ViewSize(size))
        }
        guard let castedBackend = backend as? any (BaseAppBackend & BackendFeatures.MenuButtons) else {
            fatalError("Backend does not implement BackendFeatures.MenuButtons")
        }
        return castBackend_inner(castedBackend)
    }

    func commit<Backend: BaseAppBackend>(
        _ widget: Backend.Widget,
        children: MenuStorage,
        layout: ViewLayoutResult,
        environment: EnvironmentValues,
        backend: Backend
    ) {
        func castBackend_inner<NewBackend: BaseAppBackend & BackendFeatures.MenuButtons>(_ backend: NewBackend) {
            let widget = widget as! NewBackend.Widget
            let size = layout.size
            backend.setSize(of: widget, to: size.vector)

            switch backend.menuImplementationStyle {
            case .dynamicPopover(let backend):
            backend.updateSimpleButton(
            widget,
            label: label,
            environment: environment,
            action: {
            let content = resolve().content
            let menu = backend.createPopoverMenu()
            children.menu = menu
            backend.updatePopoverMenu(
            menu,
            content: content,
            environment: environment
            )
            backend.showPopoverMenu(
            menu,
            at: SIMD2(0, LayoutSystem.roundSize(size.height) + 2),
            relativeTo: widget
            ) {
            children.menu = nil
            }
            }
            )

            if let menu = children.menu {
            let content = resolve().content
            backend.updatePopoverMenu(
            menu as! NewBackend.Menu,
            content: content,
            environment: environment
            )
            }
            case .menuButton(let backend):
            // We can assume that computeLayout has already run, so children.menu
            // will already be correctly initialized.
            let content = resolve().content
            let menu = children.menu! as! NewBackend.Menu
            backend.updatePopoverMenu(
            menu,
            content: content,
            environment: environment
            )

            // Even though we update the button in computeLayout (in order
            // for naturalSize to work), we appear to have to update it again
            // in commit; otherwise UIKitBackend users get menu buttons that
            // aren't poppable until the second time that the view gets updated.
            // They also get menu button menus with toggles that only toggle every
            // second time. I'm not sure why any of that happens.
            // TODO: Investigate why the following is needed. It may point us to
            //   some layout system/state management issues.
            backend.updateButton(
            widget,
            label: label,
            menu: menu,
            environment: environment
            )
            }
        }
        guard let castedBackend = backend as? any (BaseAppBackend & BackendFeatures.MenuButtons) else {
            fatalError("Backend does not implement BackendFeatures.MenuButtons")
        }
castBackend_inner(castedBackend)
    }

    /// A temporary button width solution until arbitrary labels are supported.
    public func _buttonWidth(_ width: Int?) -> Menu {
        var menu = self
        menu.buttonWidth = width
        return menu
    }
}

class MenuStorage: ViewGraphNodeChildren {
    var menu: Any?

    var widgets: [AnyWidget] = []
    var erasedNodes: [ErasedViewGraphNode] = []

    init() {}
}