fix: avoid creating disconnected native commandbars on load

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml_Controls/CommandBarPages/CommandBarFirstPage.xaml

@@ -0,0 +1,21 @@
+<Page
+    x:Class="Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml_Controls.CommandBarPages.CommandBarFirstPage"
+    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+    xmlns:local="using:Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml_Controls.CommandBarPages"
+    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+    mc:Ignorable="d"
+    Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
+
+	<Grid>
+		<Grid.RowDefinitions>
+			<RowDefinition Height="Auto" />
+			<RowDefinition Height="100" />
+		</Grid.RowDefinitions>
+		<CommandBar Content="CommandBarFirstPage"
+						   Width="100" />
+		<TextBlock Grid.Row="1"
+				   Text="Hello from CommandBarFirstPage" />
+	</Grid>
+</Page>

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml_Controls/CommandBarPages/CommandBarFirstPage.xaml.cs

@@ -0,0 +1,29 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Runtime.InteropServices.WindowsRuntime;
+using Uno.UI.Samples;
+using Windows.Foundation;
+using Windows.Foundation.Collections;
+using Windows.UI.Xaml;
+using Windows.UI.Xaml.Controls;
+using Windows.UI.Xaml.Controls.Primitives;
+using Windows.UI.Xaml.Data;
+using Windows.UI.Xaml.Input;
+using Windows.UI.Xaml.Media;
+using Windows.UI.Xaml.Navigation;
+
+// The Blank Page item template is documented at https://go.microsoft.com/fwlink/?LinkId=234238
+
+namespace Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml_Controls.CommandBarPages;
+/// <summary>
+/// An empty page that can be used on its own or navigated to within a Frame.
+/// </summary>
+public sealed partial class CommandBarFirstPage : Page
+{
+	public CommandBarFirstPage()
+	{
+		this.InitializeComponent();
+	}
+}

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml_Controls/CommandBarPages/CommandBarSecondPage.xaml

@@ -0,0 +1,20 @@
+<Page
+    x:Class="Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml_Controls.CommandBarPages.CommandBarSecondPage"
+    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+    xmlns:local="using:Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml_Controls.CommandBarPages"
+    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+    mc:Ignorable="d"
+    Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
+
+	<Grid>
+		<Grid.RowDefinitions>
+			<RowDefinition Height="Auto" />
+			<RowDefinition Height="100" />
+		</Grid.RowDefinitions>
+		<CommandBar Content="CommandBarSecondPage" />
+		<TextBlock Grid.Row="1"
+				   Text="Hello from CommandBarSecondPage" />
+	</Grid>
+</Page>

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml_Controls/CommandBarPages/CommandBarSecondPage.xaml.cs

@@ -0,0 +1,29 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Runtime.InteropServices.WindowsRuntime;
+using Uno.UI.Samples;
+using Windows.Foundation;
+using Windows.Foundation.Collections;
+using Windows.UI.Xaml;
+using Windows.UI.Xaml.Controls;
+using Windows.UI.Xaml.Controls.Primitives;
+using Windows.UI.Xaml.Data;
+using Windows.UI.Xaml.Input;
+using Windows.UI.Xaml.Media;
+using Windows.UI.Xaml.Navigation;
+
+// The Blank Page item template is documented at https://go.microsoft.com/fwlink/?LinkId=234238
+
+namespace Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml_Controls.CommandBarPages;
+/// <summary>
+/// An empty page that can be used on its own or navigated to within a Frame.
+/// </summary>
+public sealed partial class CommandBarSecondPage : Page
+{
+	public CommandBarSecondPage()
+	{
+		this.InitializeComponent();
+	}
+}

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml_Controls/Given_CommandBar.cs

@@ -7,6 +7,14 @@
 using Windows.UI.Xaml.Media;
 using static Private.Infrastructure.TestServices;
 
+using Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml_Controls.CommandBarPages;
+
+#if __IOS__
+using Uno.UI.Controls;
+using Uno.UI.Helpers.WinUI;
+using UIKit;
+#endif
+
 namespace Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml_Controls
 {
 	[TestClass]
@@ -57,5 +65,61 @@ public async Task When_Popup_Open_Then_Click_Outside()
 
 			Assert.AreEqual(0, VisualTreeHelper.GetOpenPopupsForXamlRoot(SUT.XamlRoot).Count);
 		}
+
+#if __IOS__
+		[TestMethod]
+		[RunsOnUIThread]
+		[RequiresFullWindow]
+
+		public async Task Can_Navigate_Forward_And_Backwards()
+		{
+			var frame = new Frame() { Width = 400, Height = 400 };
+			var content = new Grid { Children = { frame } };
+
+			WindowHelper.WindowContent = content;
+			await WindowHelper.WaitForIdle();
+
+			var firstNavBar = await frame.NavigateAndGetNavBar<CommandBarFirstPage>();
+
+			await WindowHelper.WaitForLoaded(firstNavBar);
+
+			var secondNavBar = await frame.NavigateAndGetNavBar<CommandBarSecondPage>();
+
+			await WindowHelper.WaitForLoaded(secondNavBar);
+
+			await Task.Delay(1000);
+
+			frame.GoBack();
+
+			await WindowHelper.WaitForLoaded(firstNavBar);
+		}
+#endif
+	}
+
+#if __IOS__
+	public static class NavigationBarTestHelper
+	{
+		public static UINavigationBar GetNativeNavBar(this CommandBar navBar) => navBar
+			?.TryGetNative<CommandBar, CommandBarRenderer, UINavigationBar>(out var native) ?? false ? native : null;
+
+		public static UINavigationItem GetNativeNavItem(this CommandBar navBar) => navBar
+			?.TryGetNative<CommandBar, CommandBarNavigationItemRenderer, UINavigationItem>(out var native) ?? false ? native : null;
+
+
+		public static Task<CommandBar> NavigateAndGetNavBar<TPage>(this Frame frame) where TPage : Page
+		{
+			return frame.NavigateAndGetNavBar(typeof(TPage));
+		}
+
+		public static async Task<CommandBar> NavigateAndGetNavBar(this Frame frame, Type pageType)
+		{
+			frame.Navigate(pageType);
+			await WindowHelper.WaitForIdle();
+
+			var page = frame.Content as Page;
+			await WindowHelper.WaitForLoaded(page!);
+			return SharedHelpers.FindInVisualTreeByType<CommandBar>(page);
+		}
 	}
+#endif
 }

src/Uno.UI/Controls/CommandBar/CommandBarNavigationItemRenderer.iOS.cs

@@ -27,7 +27,7 @@ internal partial class CommandBarNavigationItemRenderer : Renderer<CommandBar, U
 
 		public CommandBarNavigationItemRenderer(CommandBar 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.UI/Controls/CommandBar/CommandBarRenderer.iOS.cs

@@ -22,14 +22,7 @@ internal partial class CommandBarRenderer : Renderer<CommandBar, UINavigationBar
 
 		public CommandBarRenderer(CommandBar element) : base(element) { }
 
-		protected override UINavigationBar CreateNativeInstance()
-		{
-			var navigationBar = new UINavigationBar();
-			var navigationItem = Element.GetRenderer(() => new CommandBarNavigationItemRenderer(Element)).Native;
-			navigationBar.PushNavigationItem(navigationItem, false);
-
-			return navigationBar;
-		}
+		protected override UINavigationBar CreateNativeInstance() => throw new NotSupportedException("The Native instance must be provided.");
 
 		protected override IEnumerable<IDisposable> Initialize()
 		{

src/Uno.UI/Controls/CommandBar/NativeCommandBarPresenter.iOS.cs

@@ -24,7 +24,6 @@ public partial class NativeCommandBarPresenter : ContentPresenter
 		private readonly SerialDisposable _orientationSubscription = new SerialDisposable();
 		private WeakReference<CommandBar?>? _commandBar;
 
-		private UINavigationBar? _navigationBar;
 		private readonly bool _isPhone = UIDevice.CurrentDevice.UserInterfaceIdiom == UIUserInterfaceIdiom.Phone;
 
 		private protected override void OnLoaded()
@@ -40,31 +39,48 @@ private protected override void OnLoaded()
 			{
 				commandBar = TemplatedParent as CommandBar;
 				_commandBar = new WeakReference<CommandBar?>(commandBar);
-
-				_navigationBar = commandBar?.GetRenderer(RendererFactory).Native;
-
 			}
-			else
+
+			if (commandBar?.TryGetRenderer<CommandBar, CommandBarRenderer>() is { } commandBarRenderer)
 			{
-				_navigationBar = commandBar?.ResetRenderer(RendererFactory).Native;
+				if (commandBarRenderer.Native is { } nativeBar)
+				{
+					LayoutNativeBar(nativeBar);
+				}
 			}
-
-			if (_navigationBar == null)
+			else if (commandBar is { } && FeatureConfiguration.CommandBar.AllowNativePresenterContent)
 			{
-				throw new InvalidOperationException("No NavigationBar from renderer");
+				commandBarRenderer = new CommandBarRenderer(commandBar);
+				commandBar.SetRenderer(commandBarRenderer);
+
+
+				var commandBarItemRenderer = new CommandBarNavigationItemRenderer(commandBar);
+				commandBar.SetRenderer(commandBarItemRenderer);
+
+				var navigationItem = new UINavigationItem();
+				var nativeBar = new UINavigationBar();
+				nativeBar.PushNavigationItem(navigationItem, false);
+
+				commandBarItemRenderer.Native = navigationItem;
+				commandBarRenderer.Native = nativeBar;
+
+				LayoutNativeBar(nativeBar);
 			}
+		}
 
-			_navigationBar.SetNeedsLayout();
+		private void LayoutNativeBar(UINavigationBar nativeBar)
+		{
+			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 NativeCommandBarPresenter))
 			{
-				Content = _navigationBar;
+				Content = nativeBar;
 			}
 
 			var statusBar = StatusBar.GetForCurrentView();
@@ -81,18 +97,10 @@ private protected override void OnLoaded()
 			// iOS doesn't automatically update the navigation bar position when the status bar visibility changes.
 			void OnStatusBarChanged(StatusBar sender, object args)
 			{
-				_navigationBar!.SetNeedsLayout();
-				_navigationBar!.Superview.SetNeedsLayout();
+				nativeBar.SetNeedsLayout();
+				nativeBar.Superview.SetNeedsLayout();
 			}
 		}
-
-		CommandBarRenderer RendererFactory()
-		{
-			CommandBar? target = null;
-			_commandBar?.TryGetTarget(out target);
-			return new CommandBarRenderer(target);
-		}
-
 		protected override Size MeasureOverride(Size size)
 		{
 			var measuredSize = base.MeasureOverride(size);

src/Uno.UI/Controls/Renderer.cs

@@ -11,6 +11,8 @@
 
 namespace Uno.UI.Controls
 {
+	public delegate void NativeChangedHandler(object sender, object native);
+
 	internal abstract class Renderer<TElement, TNative> : IDisposable
 		where TElement : DependencyObject
 		where TNative : class
@@ -20,6 +22,8 @@ internal abstract class Renderer<TElement, TNative> : IDisposable
 		private TNative _native;
 		private bool _isRendering;
 
+		public event NativeChangedHandler NativeChanged;
+
 		public Renderer(TElement element)
 		{
 			if (element == null)
@@ -56,25 +60,29 @@ 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());
 			}
 
 			Invalidate();
+
+			NativeChanged?.Invoke(this, _native);
 		}
 
 		protected abstract TNative CreateNativeInstance();
 
 		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)
 			{
@@ -104,6 +112,38 @@ internal static class RendererHelper
 	{
 		private static readonly WeakAttachedDictionary<DependencyObject, Type> _renderers = new WeakAttachedDictionary<DependencyObject, Type>();
 
+		public static TRenderer TryGetRenderer<TElement, TRenderer>(this TElement element)
+			where TElement : DependencyObject
+			where TRenderer : class
+		{
+			TRenderer renderer = null;
+			if (_renderers.GetValue<TRenderer>(element, typeof(TRenderer)) is { } existingRenderer)
+			{
+				renderer = existingRenderer;
+			}
+
+			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 TRenderer GetRenderer<TElement, TRenderer>(this TElement element, Func<TRenderer> rendererFactory)
 			where TElement : DependencyObject
 		{
@@ -114,5 +154,11 @@ public static TRenderer ResetRenderer<TElement, TRenderer>(this TElement element
 		{
 			return _renderers.GetValue(element, typeof(TRenderer), rendererFactory);
 		}
+
+		public static void SetRenderer<TElement, TRenderer>(this TElement element, TRenderer renderer)
+			where TElement : DependencyObject
+		{
+			_renderers.SetValue(element, typeof(TRenderer), renderer);
+		}
 	}
 }