Navigation bar briefly flickering as large title before changing to inline - ios

Question

I'm using a UITabController in my SwiftUI app and it works well, except for one issue.
When I have a NavigationView that uses a large title, it changes to inline as I scroll, as expected. So far so good.
When I switch tabs, then switch back again, it should remember the scroll position and therefore be inline. Instead, it very briefly flickers as a large title before moving to inline.
This only seems to happen on device.
Screen recording of the issue
Is anyone able to help me with a workaround?
struct ContentView: View {
var body: some View {
CustomTabView([
Tab(view: FirstView(),
barItem: UITabBarItem(title: "First", image: UIImage(systemName: "1.circle"), selectedImage: nil)),
Tab(view: SecondView(),
barItem: UITabBarItem(title: "Second", image: UIImage(systemName: "2.circle"), selectedImage: nil))
])
}
}
struct FirstView: View {
var body: some View {
NavigationView {
List {
ForEach( 0...40, id: \.self ) {
Text("\($0)")
.padding()
}
}
.navigationTitle("First View")
}
}
}
struct SecondView: View {
var body: some View {
NavigationView {
List {
ForEach( 41...80, id: \.self ) {
Text("\($0)")
.padding()
}
}
.navigationTitle("Second View")
}
}
}
struct TabBarController: UIViewControllerRepresentable {
var controllers: [UIViewController]
func makeUIViewController(context: Context) -> UITabBarController {
let tabBarController = UITabBarController()
tabBarController.viewControllers = controllers
tabBarController.delegate = context.coordinator
return tabBarController
}
func updateUIViewController(_ uiViewController: UITabBarController, context: Context) {}
func makeCoordinator() -> Coordinator {
Coordinator()
}
class Coordinator: NSObject, UITabBarControllerDelegate {
func tabBarController(_ tabBarController: UITabBarController, shouldSelect viewController: UIViewController) -> Bool {
guard let fromView = tabBarController.selectedViewController?.view, let toView = viewController.view else {
return false
}
if fromView != toView {
fromView.superview!.addSubview(toView)
}
return true
}
}
}
struct CustomTabView: View {
var viewControllers: [UIHostingController<AnyView>]
init(_ tabs: [Tab]) {
self.viewControllers = tabs.map {
let host = UIHostingController(rootView: $0.view)
host.tabBarItem = $0.barItem
return host
}
}
var body: some View {
TabBarController(controllers: viewControllers)
.edgesIgnoringSafeArea(.all)
}
}
struct Tab {
var view: AnyView
var barItem: UITabBarItem
init<V: View>(view: V, barItem: UITabBarItem) {
self.view = AnyView(view)
self.barItem = barItem
}
}

Answer 1

I have found a workaround / fix.
I needed to use a custom UIHostingController, find the navigation controller, and add a line to the viewWillAppear function:
public class MyHostingController: UIHostingController<AnyView> {
override open func viewWillAppear(_ animated: Bool) {
if let navigationController = navigationController() {
navigationController.view.setNeedsUpdateConstraints()
}
super.viewWillAppear(animated)
}
}
private extension UIViewController {
func navigationController() -> UINavigationController? {
var controller: UINavigationController?
if let navigationController = self as? UINavigationController {
return navigationController
}
children.forEach {
if let navigationController = $0 as? UINavigationController {
controller = navigationController
} else {
controller = $0.navigationController()
}
}
return controller
}
}
And then obvious replaced the two usages of UIHostingController in my original code with MyHostingController.
The most important line is this though:
navigationController.view.setNeedsUpdateConstraints()
Hope this helps anyone else that finds a similar issue.