fix: avoid creating disconnected native navbars on load

src/Uno.Toolkit.RuntimeTests/Tests/NavigationBarTests.cs

@@ -260,6 +260,32 @@ public async Task NavigationBar_Renders_With_Invalid_AppBarButton_IconElement(Ty
 			AssertNavigationBar(frame);
 		}
 
+		[TestMethod]
+		[RequiresFullWindow]
+		public async Task Can_Navigate_Forward_And_Backwards()
+		{
+			var frame = new Frame() { Width = 400, Height = 400 };
+			var content = new Grid { Children = { frame } };
+
+			await UnitTestUIContentHelperEx.SetContentAndWait(content);
+
+			await UnitTestsUIContentHelper.WaitForIdle();
+
+			var firstNavBar = await frame.NavigateAndGetNavBar<NavBarFirstPage>();
+
+			await UnitTestsUIContentHelper.WaitForLoaded(firstNavBar!);
+
+			var secondNavBar = await frame.NavigateAndGetNavBar<NavBarSecondPage>();
+
+			await UnitTestsUIContentHelper.WaitForLoaded(secondNavBar!);
+
+			await Task.Delay(1000);
+
+			frame.GoBack();
+
+			await UnitTestsUIContentHelper.WaitForLoaded(firstNavBar!);
+		}
+
 
 #if __ANDROID__
 		private static void AssertNavigationBar(Frame frame)
@@ -534,17 +560,14 @@ public static class NavigationBarTestHelper
 	{
 #if __IOS__
 		public static UINavigationBar? GetNativeNavBar(this NavigationBar? navBar) => navBar
-			?.TryGetRenderer<NavigationBar, NavigationBarRenderer>()
-			?.Native;
+			?.TryGetNative<NavigationBar, NavigationBarRenderer, UINavigationBar>(out var native) ?? false ? native : null;
 
 		public static UINavigationItem? GetNativeNavItem(this NavigationBar? navBar) => navBar
-			?.TryGetRenderer<NavigationBar, NavigationBarNavigationItemRenderer>()
-			?.Native;
+			?.TryGetNative<NavigationBar, NavigationBarNavigationItemRenderer, UINavigationItem>(out var native) ?? false ? native : null;
 
 #elif __ANDROID__
 		public static AndroidX.AppCompat.Widget.Toolbar? GetNativeNavBar(this NavigationBar? navBar) => navBar
-			?.TryGetRenderer<NavigationBar, NavigationBarRenderer>()
-			?.Native;
+			?.TryGetNative<NavigationBar, NavigationBarRenderer, AndroidX.AppCompat.Widget.Toolbar>(out var native) ?? false ? native : null;
 #endif
 		public static Task<NavigationBar?> NavigateAndGetNavBar<TPage>(this Frame frame) where TPage : Page
 		{

src/Uno.Toolkit.UI/Controls/NavigationBar/NativeNavigationBarPresenter.iOS.cs

@@ -38,7 +38,6 @@ public partial class NativeNavigationBarPresenter : ContentPresenter, INavigatio
 		private SerialDisposable _mainCommandClickHandler = new SerialDisposable();
 		private WeakReference<NavigationBar?>? _navBarRef;
 
-		private UINavigationBar? _navigationBar;
 		private readonly bool _isPhone = UIDevice.CurrentDevice.UserInterfaceIdiom == UIUserInterfaceIdiom.Phone;
 
 		public NativeNavigationBarPresenter()
@@ -73,52 +72,54 @@ private void OnLoaded(object sender, RoutedEventArgs e)
 				_navBarRef = new WeakReference<NavigationBar?>(navBar);
 			}
 
-			_navigationBar = navBar?.GetOrAddDefaultRenderer().Native;
-
 			if (navBar is { })
 			{
 				navBar.MainCommand.Click += OnMainCommandClick;
 				_mainCommandClickHandler.Disposable = null;
 				_mainCommandClickHandler.Disposable = Disposable.Create(() => navBar.MainCommand.Click -= OnMainCommandClick);
-			}
 
-			if (_navigationBar == null)
-			{
-				throw new InvalidOperationException("No NavigationBar from renderer");
+				LayoutNativeNavBar(navBar);
 			}
+		}
 
-			_navigationBar.SetNeedsLayout();
+		private void LayoutNativeNavBar(NavigationBar navBar)
+		{
+			if (navBar.TryGetNative<NavigationBar, NavigationBarRenderer, UINavigationBar>(out var nativeBar)
+				&& nativeBar is { })
+			{
+				nativeBar.SetNeedsLayout();
 
-			var navigationBarSuperview = _navigationBar?.Superview;
+				var navigationBarSuperview = nativeBar.Superview;
 
-			// Allows the UINavigationController's NavigationBar instance to be moved to the Page. This feature
-			// is used in the context of the sample application to test NavigationBars outside of a NativeFramePresenter for 
-			// UI Testing. In general cases, this should not happen as the bar may be moved back to to this presenter while
-			// another page is already visible, making this bar overlay on top of another.			
-			if (FeatureConfiguration.CommandBar.AllowNativePresenterContent && (navigationBarSuperview == null || navigationBarSuperview is NativeNavigationBarPresenter))
-			{
-				Content = _navigationBar;
-			}
+				// Allows the UINavigationController's NavigationBar instance to be moved to the Page. This feature
+				// is used in the context of the sample application to test NavigationBars outside of a NativeFramePresenter for 
+				// UI Testing. In general cases, this should not happen as the bar may be moved back to to this presenter while
+				// another page is already visible, making this bar overlay on top of another.			
+				if (FeatureConfiguration.CommandBar.AllowNativePresenterContent && (navigationBarSuperview == null || navigationBarSuperview is NativeNavigationBarPresenter))
+				{
+					Content = nativeBar;
+				}
 
-			var statusBar = XamlStatusBar.GetForCurrentView();
+				var statusBar = XamlStatusBar.GetForCurrentView();
 
-			statusBar.Showing += OnStatusBarChanged;
-			statusBar.Hiding += OnStatusBarChanged;
+				statusBar.Showing += OnStatusBarChanged;
+				statusBar.Hiding += OnStatusBarChanged;
 
-			_statusBarSubscription.Disposable = Disposable.Create(() =>
-			{
-				statusBar.Showing -= OnStatusBarChanged;
-				statusBar.Hiding -= OnStatusBarChanged;
-			});
+				_statusBarSubscription.Disposable = Disposable.Create(() =>
+				{
+					statusBar.Showing -= OnStatusBarChanged;
+					statusBar.Hiding -= OnStatusBarChanged;
+				});
 
-			// iOS doesn't automatically update the navigation bar position when the status bar visibility changes.
-			void OnStatusBarChanged(XamlStatusBar sender, object args)
-			{
-				_navigationBar!.SetNeedsLayout();
-				_navigationBar!.Superview.SetNeedsLayout();
+				// iOS doesn't automatically update the navigation bar position when the status bar visibility changes.
+				void OnStatusBarChanged(XamlStatusBar sender, object args)
+				{
+					nativeBar.SetNeedsLayout();
+					nativeBar.Superview.SetNeedsLayout();
+				}
 			}
 		}
-
+		
 		private void OnMainCommandClick(object sender, RoutedEventArgs e)
 		{
 			NavigationBar? navBar = null;

src/Uno.Toolkit.UI/Controls/NavigationBar/NavigationBarNavigationItemRenderer.iOS.cs

@@ -50,7 +50,7 @@ internal partial class NavigationBarNavigationItemRenderer : Renderer<Navigation
 
 		public NavigationBarNavigationItemRenderer(NavigationBar element) : base(element) { }
 
-		protected override UINavigationItem CreateNativeInstance() => new UINavigationItem();
+		protected override UINavigationItem CreateNativeInstance() => throw new NotSupportedException("The Native instance must be provided.");
 
 		protected override IEnumerable<IDisposable> Initialize()
 		{

src/Uno.Toolkit.UI/Controls/NavigationBar/NavigationBarRenderer.iOS.cs

@@ -36,17 +36,7 @@ internal partial class NavigationBarRenderer : Renderer<NavigationBar, UINavigat
 	{
 		public NavigationBarRenderer(NavigationBar element) : base(element) { }
 
-		protected override UINavigationBar CreateNativeInstance()
-		{
-			var navigationBar = new UINavigationBar();
-			if (Element is { } element)
-			{
-				var renderer = element.GetOrAddRenderer(navBar => new NavigationBarNavigationItemRenderer(navBar));
-				navigationBar.PushNavigationItem(renderer.Native, false);
-			}
-
-			return navigationBar;
-		}
+		protected override UINavigationBar CreateNativeInstance() => throw new NotSupportedException("The Native instance must be provided.");
 
 		protected override IEnumerable<IDisposable> Initialize()
 		{

src/Uno.Toolkit.UI/Controls/NavigationBar/Renderer.cs

@@ -91,12 +91,14 @@ public TNative Native
 			}
 		}
 
+		public bool HasNative => _native != null;
+
 		private void OnNativeChanged()
 		{
 			// We remove subscriptions to the previous pair of element and native 
 			_subscriptions.Dispose();
 
-			if (_native != null)
+			if (HasNative)
 			{
 				_subscriptions = new CompositeDisposable(Initialize());
 			}
@@ -109,7 +111,7 @@ private void OnNativeChanged()
 		public void Invalidate()
 		{
 			// We don't render anything if there's no rendering target
-			if (_native != null
+			if (HasNative
 				// Prevent Render() being called reentrantly - this can happen when the Element's parent changes within the Render() method
 				&& !_isRendering)
 			{
@@ -152,6 +154,25 @@ internal static class RendererHelper
 			return renderer;
 		}
 
+		public static bool TryGetNative<TElement, TRenderer, TNative>(this TElement element, out TNative? native)
+			where TElement :
+#if HAS_UNO
+			class,
+#endif
+			DependencyObject
+			where TRenderer : Renderer<TElement, TNative>
+			where TNative : class
+		{
+			native = null;
+			if (TryGetRenderer<TElement, TRenderer>(element) is { } renderer && renderer.HasNative)
+			{
+				native = renderer.Native;
+				return true;
+			}
+
+			return false;
+		}
+
 		public static void SetRenderer<TElement, TRenderer>(this TElement element, TRenderer? renderer)
 			where TElement : DependencyObject
 		{