UIKitTabView. SwiftUI tab bar view that respects navigation stacks when tabs are switched (unlike the TabView implementation)

UIKitTabView.swift

/// An iOS style TabView that doesn't reset it's childrens navigation stacks when tabs are switched.
public struct UIKitTabView: View {
    private var viewControllers: [UIHostingController<AnyView>]
    private var selectedIndex: Binding<Int>?
    @State private var fallbackSelectedIndex: Int = 0
    
    public init(selectedIndex: Binding<Int>? = nil, @TabBuilder _ views: () -> [Tab]) {
        self.viewControllers = views().map {
            let host = UIHostingController(rootView: $0.view)
            host.tabBarItem = $0.barItem
            return host
        }
        self.selectedIndex = selectedIndex
    }
    
    public var body: some View {
        TabBarController(controllers: viewControllers, selectedIndex: selectedIndex ?? $fallbackSelectedIndex)
            .edgesIgnoringSafeArea(.all)
    }
    
    public struct Tab {
        var view: AnyView
        var barItem: UITabBarItem
    }
}

@_functionBuilder
public struct TabBuilder {
    public static func buildBlock(_ items: UIKitTabView.Tab...) -> [UIKitTabView.Tab] {
        items
    }
}

extension View {
    public func tab(title: String, image: String? = nil, selectedImage: String? = nil, badgeValue: String? = nil) -> UIKitTabView.Tab {
        func imageOrSystemImage(named: String?) -> UIImage? {
            guard let name = named else { return nil }
            return UIImage(named: name) ?? UIImage(systemName: name)
        }
        
        let image = imageOrSystemImage(named: image)
        let selectedImage = imageOrSystemImage(named: selectedImage)
        let barItem = UITabBarItem(title: title, image: image, selectedImage: selectedImage)
        
        barItem.badgeValue = badgeValue
        
        return UIKitTabView.Tab(view: AnyView(self), barItem: barItem)
    }
}

fileprivate struct TabBarController: UIViewControllerRepresentable {
    var controllers: [UIViewController]
    @Binding var selectedIndex: Int

    func makeUIViewController(context: Context) -> UITabBarController {
        let tabBarController = UITabBarController()
        tabBarController.viewControllers = controllers
        tabBarController.delegate = context.coordinator
        tabBarController.selectedIndex = 0
        return tabBarController
    }

    func updateUIViewController(_ tabBarController: UITabBarController, context: Context) {
        tabBarController.selectedIndex = selectedIndex
    }
    
    func makeCoordinator() -> Coordinator {
        Coordinator(self)
    }
    
    class Coordinator: NSObject, UITabBarControllerDelegate {
        var parent: TabBarController

        init(_ tabBarController: TabBarController) {
            self.parent = tabBarController
        }
        
        func tabBarController(_ tabBarController: UITabBarController, didSelect viewController: UIViewController) {
            if parent.selectedIndex == tabBarController.selectedIndex {
                popToRootOrScrollUp(on: viewController)
            }
            
            parent.selectedIndex = tabBarController.selectedIndex
        }
        
        private func popToRootOrScrollUp(on viewController: UIViewController) {
            let nvc = navigationController(for: viewController)
            let popped = nvc?.popToRootViewController(animated: true)
            
            if (popped ?? []).isEmpty {
                let rootViewController = nvc?.viewControllers.first ?? viewController
                if let scrollView = firstScrollView(in: rootViewController.view ?? UIView()) {
                    let preservedX = scrollView.contentOffset.x
                    let y = -scrollView.adjustedContentInset.top
                    scrollView.setContentOffset(CGPoint(x: preservedX, y: y), animated: true)
                }
            }
        }
        
        private func navigationController(for viewController: UIViewController) -> UINavigationController? {
            for child in viewController.children {
                if let nvc = viewController as? UINavigationController {
                    return nvc
                } else if let nvc = navigationController(for: child) {
                    return nvc
                }
            }
            return nil
        }
        
        public func firstScrollView(in view: UIView) -> UIScrollView? {
            for subview in view.subviews {
                if let scrollView = view as? UIScrollView {
                    return scrollView
                } else if let scrollView = firstScrollView(in: subview) {
                    return scrollView
                }
            }
            
            return nil
        }
    }
}

zExample.swift

struct ExampleView: View {
    @State var text: String = ""
    
    var body: some View {
        UIKitTabView {
            NavView().tab(title: "First", badgeValue: "3")
            Text("Second View").tab(title: "Second")
        }
    }
}

struct NavView: View {
    var body: some View {
        NavigationView {
            VStack {
                NavigationLink(destination: Text("This page stays when you switch back and forth between tabs (as expected on iOS)")) {
                    Text("Go to detail")
                }
            }
        }
    }
}

This is still a problem in iOS 14.5.. I'm hoping WWDC 2021 fixes the broken TabView. Screens drop state in simple scenarios which is a major problem since the tab view houses the entire app. This workaround works really well, thank you!!

I'm adding my variation below too but cuts out some options for my needs so less maintenance for the temp fix, and made the API a little more similar to native so it can be seamlessly swapped out to the SwiftUI version when finally fixed.

Usage:

UITabView(selection: $selectedTab) {
    NavigationView {
        ContentView1()
    }
    .tabItem("Tab 1", image: UIImage(named: "blah1"))
    NavigationView {
        ContentView2()
    }
    .tabItem("Tab 2", image: UIImage(named: "blah2"))
    NavigationView {
        ContentView3()
    }
    .tabItem("Tab 3", image: UIImage(named: "blah3"))
}

Code:

private struct UITabView: View {
    private let viewControllers: [UIHostingController<AnyView>]
    private let tabBarItems: [TabBarItem]
    @Binding private var selectedIndex: Int

    init(selection: Binding<Int>, @TabBuilder _ content: () -> [TabBarItem]) {
        _selectedIndex = selection

        (viewControllers, tabBarItems) = content().reduce(into: ([], [])) { result, next in
            let tabController = UIHostingController(rootView: next.view)
            tabController.tabBarItem = next.barItem
            result.0.append(tabController)
            result.1.append(next)
        }
    }

    var body: some View {
        TabBarController(
            controllers: viewControllers,
            tabBarItems: tabBarItems,
            selectedIndex: $selectedIndex
        )
        .ignoresSafeArea()
    }
}

private extension UITabView {
    struct TabBarItem {
        let view: AnyView
        let barItem: UITabBarItem
        let badgeValue: String?

        init<T>(
            title: String,
            image: UIImage?,
            selectedImage: UIImage? = nil,
            badgeValue: String? = nil,
            content: T
        ) where T: View {
            self.view = AnyView(content)
            self.barItem = UITabBarItem(title: title, image: image, selectedImage: selectedImage)
            self.badgeValue = badgeValue
        }
    }

    struct TabBarController: UIViewControllerRepresentable {
        let controllers: [UIViewController]
        let tabBarItems: [TabBarItem]
        @Binding var selectedIndex: Int

        func makeUIViewController(context: Context) -> UITabBarController {
            let tabBarController = UITabBarController()
            tabBarController.viewControllers = controllers
            tabBarController.delegate = context.coordinator
            tabBarController.selectedIndex = selectedIndex
            return tabBarController
        }

        func updateUIViewController(_ tabBarController: UITabBarController, context: Context) {
            tabBarController.selectedIndex = selectedIndex

            tabBarItems.forEach { tab in
                guard let index = tabBarItems.firstIndex(where: { $0.barItem == tab.barItem }),
                      let controllers = tabBarController.viewControllers
                else {
                    return
                }

                guard controllers.indices.contains(index) else { return }
                controllers[index].tabBarItem.badgeValue = tab.badgeValue
            }
        }

        func makeCoordinator() -> TabBarCoordinator {
            TabBarCoordinator(self)
        }
    }

    class TabBarCoordinator: NSObject, UITabBarControllerDelegate {
        private static let inlineTitleRect = CGRect(x: 0, y: 0, width: 1, height: 1)
        private var parent: TabBarController

        init(_ tabBarController: TabBarController) {
            self.parent = tabBarController
        }

        func tabBarController(_ tabBarController: UITabBarController, didSelect viewController: UIViewController) {
            guard parent.selectedIndex == tabBarController.selectedIndex else {
                parent.selectedIndex = tabBarController.selectedIndex
                return
            }

            guard let navigationController = navigationController(in: viewController)  else {
                scrollToTop(in: viewController)
                return
            }

            guard navigationController.visibleViewController == navigationController.viewControllers.first else {
                navigationController.popToRootViewController(animated: true)
                return
            }

            scrollToTop(in: navigationController, selectedIndex: tabBarController.selectedIndex)
        }

        func scrollToTop(in navigationController: UINavigationController, selectedIndex: Int) {
            let views = navigationController.viewControllers
                .map(\.view.subviews)
                .reduce([], +) // swiftlint:disable:this reduce_into

            guard let scrollView = scrollView(in: views) else { return }
            scrollView.scrollRectToVisible(Self.inlineTitleRect, animated: true)
        }

        func scrollToTop(in viewController: UIViewController) {
            let views = viewController.view.subviews

            guard let scrollView = scrollView(in: views) else { return }
            scrollView.scrollRectToVisible(Self.inlineTitleRect, animated: true)
        }

        func scrollView(in views: [UIView]) -> UIScrollView? {
            var view: UIScrollView?

            views.forEach {
                guard let scrollView = $0 as? UIScrollView else {
                    view = scrollView(in: $0.subviews)
                    return
                }

                view = scrollView
            }

            return view
        }

        func navigationController(in viewController: UIViewController) -> UINavigationController? {
            var controller: UINavigationController?

            if let navigationController = viewController as? UINavigationController {
                return navigationController
            }

            viewController.children.forEach {
                guard let navigationController = $0 as? UINavigationController else {
                    controller = navigationController(in: $0)
                    return
                }

                controller = navigationController
            }

            return controller
        }
    }
}

private extension View {
    func tabItem(
        _ title: String,
        image: UIImage?,
        selectedImage: UIImage? = nil,
        badgeValue: String? = nil
    ) -> UITabView.TabBarItem {
        UITabView.TabBarItem(
            title: title,
            image: image,
            selectedImage: selectedImage,
            badgeValue: badgeValue,
            content: self
        )
    }
}

@resultBuilder
private struct TabBuilder {
    static func buildBlock(_ elements: UITabView.TabBarItem...) -> [UITabView.TabBarItem] {
        elements
    }
}

Thank you to @Amzd for the inspiration and taking the charge, and to @NicholasBellucci for evolving it. cc @bitwit @gkye

Noticed a caveat that will cause a crash.. inject environment variables and objects don't work so well when doing it through a UIKit entry point. Instead of doing this:

UITabView(selection: $selectedTab) {
    NavigationView {
        ContentView1()
    }
    .tabItem("Tab 1", image: UIImage(named: "blah1"))
    NavigationView {
        ContentView2()
    }
    .tabItem("Tab 2", image: UIImage(named: "blah2"))
    NavigationView {
        ContentView3()
    }
    .tabItem("Tab 3", image: UIImage(named: "blah3"))
}
.environmentObject(someModel)
.environment(\.someThing, someThing)

In some scenarios, this caused a crash: Thread 1: Fatal error: No observable object of type ..Model.Type found. A View.environmentObject(_:) for ..Model.Type may be missing as an ancestor of this view.

So I ended up having to inject the environment instances into each tab instead which resolved the problem:

UITabView(selection: $selectedTab) {
    NavigationView {
        ContentView1()
    }
    .environmentObject(someModel)
    .environment(\.someThing, someThing)
    .tabItem("Tab 1", image: UIImage(named: "blah1"))
    NavigationView {
        ContentView2()
    }
    .environmentObject(someModel)
    .environment(\.someThing, someThing)
    .tabItem("Tab 2", image: UIImage(named: "blah2"))
    NavigationView {
        ContentView3()
    }
    .environmentObject(someModel)
    .environment(\.someThing, someThing)
    .tabItem("Tab 3", image: UIImage(named: "blah3"))
}

Injecting it from a native SwiftUI tab also worked properly, so something to be aware of something isn't right when injecting environments from UIKit it seems.

@basememara Thanks! It works perfectly. The only thing I am missing is to hide the tabbar when a new view is pushed (with a navigation. link with SwiftUI)

Do you know if that is possible? I've tried to set

tabBarController.hidesBottomBarWhenPushed = true

But it's not working for me.

@pabloecab because the NavigatinView does not work together with the UITabBarController. Jusr hide it manually when something is pushed.

@pabloecab because the NavigatinView does not work together with the UITabBarController. Jusr hide it manually when something is pushed.

@Amzd Could you please help out? How exactly would you hide it manually? Thanks!

@pakenas Add new @Binding property on TabBarController and in updateView do: tabBarController.tabBar.hidden = newProperty

If you want custom animations and stuff I would advice just use a SwiftUI view instead and keep the tabBar hidden at all times.

Even better: don't ever use SwiftUI's NavigationView or .sheet and use a custom Coordinator system that handles all navigation using UINavigationControllers which then embed the SwiftUI views in UIHostingController and forward the necessary environment objects