fix: navBar initialization-Android & iOS (backport #1275)

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

@@ -26,13 +26,12 @@
 
 namespace Uno.Toolkit.UI
 {
-	public partial class NativeNavigationBarPresenter : ContentPresenter, INavigationBarPresenter
+	public partial class NativeNavigationBarPresenter
 	{
 		private SerialDisposable _mainCommandClickHandler = new SerialDisposable();
 
 		public NativeNavigationBarPresenter()
 		{
-			Loaded += OnLoaded;
 			Unloaded += OnUnloaded;
 		}
 
@@ -41,29 +40,24 @@ private void OnUnloaded(object sender, RoutedEventArgs e)
 			_mainCommandClickHandler.Disposable = null;
 		}
 
-		public void SetOwner(NavigationBar navigationBar)
+		private void OnMainCommandClicked(object sender, RoutedEventArgs e)
 		{
-			//Owner is accessed through TemplatedParent on Uno platforms
+			var navBar = GetNavBar();
+			navBar?.TryPerformMainCommand();
 		}
 
-		private void OnLoaded(object sender, RoutedEventArgs e)
+		partial void OnOwnerChanged()
 		{
-			var navBar = TemplatedParent as NavigationBar;
-			if (navBar is { })
+			_mainCommandClickHandler.Disposable = null;
+
+			if (GetNavBar() is { } navBar)
 			{
 				Content = navBar.GetOrAddDefaultRenderer().Native;
 				ContentTemplate = null; // normally, the ContentTemplate is inherited from the NavigationBar, but in this case, we don't want it to. We want to use the renderer directly as the child.
 				navBar.MainCommand.Click += OnMainCommandClicked;
-				_mainCommandClickHandler.Disposable = null;
 				_mainCommandClickHandler.Disposable = Disposable.Create(() => navBar.MainCommand.Click -= OnMainCommandClicked);
 			}
 		}
-
-		private void OnMainCommandClicked(object sender, RoutedEventArgs e)
-		{
-			var navBar = TemplatedParent as NavigationBar;
-			navBar?.TryPerformMainCommand();
-		}
 	}
 }
 #endif

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

@@ -0,0 +1,60 @@
+#if __ANDROID__ || __IOS__
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using Uno.Disposables;
+
+#if IS_WINUI
+using Microsoft.UI.Xaml;
+using Microsoft.UI.Xaml.Controls;
+using Microsoft.UI.Xaml.Controls.Primitives;
+using Microsoft.UI.Xaml.Data;
+using Microsoft.UI.Xaml.Input;
+using Microsoft.UI.Xaml.Media;
+using Microsoft.UI.Xaml.Navigation;
+#else
+using Windows.UI.Xaml;
+using Windows.UI.Xaml.Controls;
+using Windows.UI.Xaml.Media;
+using Windows.UI.Xaml.Navigation;
+#endif
+
+namespace Uno.Toolkit.UI
+{
+	partial class NativeNavigationBarPresenter : ContentPresenter, INavigationBarPresenter
+	{
+		private WeakReference<NavigationBar?>? _weakNavBar;
+
+		public void SetOwner(NavigationBar? navigationBar)
+		{
+			if (GetNavBar() == navigationBar)
+			{
+				return;
+			}
+			_weakNavBar = new WeakReference<NavigationBar?>(navigationBar);
+
+			OnOwnerChanged();
+		}
+
+		private NavigationBar? GetNavBar()
+		{
+			if (_weakNavBar == null)
+			{
+				return null;
+			}
+
+			NavigationBar? targetNavBar = null;
+			if (_weakNavBar.TryGetTarget(out targetNavBar))
+			{
+				return targetNavBar;
+			}
+
+			return null;
+		}
+
+		partial void OnOwnerChanged();
+	}
+}
+#endif

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

@@ -31,51 +31,36 @@
 
 namespace Uno.Toolkit.UI
 {
-	public partial class NativeNavigationBarPresenter : ContentPresenter, INavigationBarPresenter
+	public partial class NativeNavigationBarPresenter
 	{
 		private readonly SerialDisposable _statusBarSubscription = new SerialDisposable();
 		private readonly SerialDisposable _orientationSubscription = new SerialDisposable();
 		private SerialDisposable _mainCommandClickHandler = new SerialDisposable();
-		private WeakReference<NavigationBar?>? _navBarRef;
 
 		private readonly bool _isPhone = UIDevice.CurrentDevice.UserInterfaceIdiom == UIUserInterfaceIdiom.Phone;
 
 		public NativeNavigationBarPresenter()
 		{
-			Loaded += OnLoaded;
 			Unloaded += OnUnloaded;
 		}
 
-		public void SetOwner(NavigationBar navigationBar)
-		{
-			//Owner is accessed through TemplatedParent on Uno platforms
-		}
-
 		private void OnUnloaded(object sender, RoutedEventArgs e)
 		{
 			_statusBarSubscription.Disposable = null;
 			_orientationSubscription.Disposable = null;
 			_mainCommandClickHandler.Disposable = null;
 		}
 
-		private void OnLoaded(object sender, RoutedEventArgs e)
+		partial void OnOwnerChanged()
 		{
 			// TODO: Find a proper way to decide whether a NavigationBar exists on canvas (within Page), or is mapped to the UINavigationController's NavigationBar.
 
-			NavigationBar? navBar = null;
-
-			_navBarRef?.TryGetTarget(out navBar);
-
-			if (navBar == null)
-			{
-				navBar = TemplatedParent as NavigationBar;
-				_navBarRef = new WeakReference<NavigationBar?>(navBar);
-			}
+			_mainCommandClickHandler.Disposable = null;
 
-			if (navBar is { })
+			if (GetNavBar() is { } navBar)
 			{
 				navBar.MainCommand.Click += OnMainCommandClick;
-				_mainCommandClickHandler.Disposable = null;
+
 				_mainCommandClickHandler.Disposable = Disposable.Create(() => navBar.MainCommand.Click -= OnMainCommandClick);
 
 				LayoutNativeNavBar(navBar);
@@ -122,11 +107,8 @@ void OnStatusBarChanged(XamlStatusBar sender, object args)
 
 		private void OnMainCommandClick(object sender, RoutedEventArgs e)
 		{
-			NavigationBar? navBar = null;
-			if (_navBarRef?.TryGetTarget(out navBar) ?? false)
-			{
-				navBar?.TryPerformMainCommand();
-			}
+			var navBar = GetNavBar();
+			navBar?.TryPerformMainCommand();
 		}
 
 		protected override Size MeasureOverride(Size size)