feat: Adding handlers for frame and page

src/Uno.UI/UI/Xaml/Controls/Frame/Frame.HotReload.cs

@@ -0,0 +1,42 @@
+using System;
+using System.Reflection.Metadata;
+using Uno.UI.Helpers;
+
+[assembly: ElementMetadataUpdateHandlerAttribute(typeof(Windows.UI.Xaml.Controls.Frame), typeof(Windows.UI.Xaml.Controls.FrameElementMetadataUpdateHandler))]
+
+namespace Windows.UI.Xaml.Controls
+{
+	public partial class FrameElementMetadataUpdateHandler
+	{
+		public static void ElementUpdate(FrameworkElement element, Type[] updatedTypes)
+		{
+			var frame = element as Frame;
+			if (frame is null)
+			{
+				return;
+			}
+
+			foreach (var entry in frame.BackStack)
+			{
+				var expectedType = entry.SourcePageType.GetReplacementType();
+				if (entry.Instance is not null &&
+					entry.Instance.GetType() != expectedType)
+				{
+					var dc = entry.Instance.DataContext;
+					entry.Instance = Activator.CreateInstance(expectedType) as Page;
+					entry.Instance.Frame = frame;
+					if (entry.Instance is not null)
+					{
+						entry.Instance.DataContext = dc;
+					}
+				}
+
+				if (entry.SourcePageType is not null &&
+					entry.SourcePageType != expectedType)
+				{
+					entry.SourcePageType = expectedType;
+				}
+			}
+		}
+	}
+}

src/Uno.UI/UI/Xaml/Controls/Frame/Frame.cs

@@ -14,6 +14,7 @@
 using System.Runtime.CompilerServices;
 using Windows.UI.Xaml.Input;
 using Uno.UI.Xaml.Core;
+using Uno.UI.Helpers;
 
 namespace Windows.UI.Xaml.Controls
 {
@@ -55,6 +56,13 @@ protected override void OnContentChanged(object oldValue, object newValue)
 			{
 				CurrentEntry = null;
 			}
+			else if (CurrentEntry is not null)
+			{
+				// This is to support hot reload scenarios - the PageStackEntry 
+				// is used when navigating back to this page as it's maintained in the BackStack
+				CurrentEntry.Instance = newValue as Page;
+				CurrentEntry.SourcePageType = newValue.GetType();
+			}
 		}
 
 		#region BackStackDepth DependencyProperty
@@ -272,7 +280,7 @@ public void GoForward()
 
 		public bool Navigate(Type sourcePageType, object parameter, NavigationTransitionInfo infoOverride)
 		{
-			var entry = new PageStackEntry(sourcePageType, parameter, infoOverride);
+			var entry = new PageStackEntry(sourcePageType.GetReplacementType(), parameter, infoOverride);
 			return InnerNavigate(entry, NavigationMode.New);
 		}
 
@@ -432,7 +440,8 @@ private void ReleasePages(IList<PageStackEntry> pageStackEntries)
 
 		internal Page EnsurePageInitialized(PageStackEntry entry)
 		{
-			if (entry is { Instance: null } && CreatePageInstanceCached(entry.SourcePageType) is { } page)
+			if (entry is { Instance: null } &&
+				CreatePageInstanceCached(entry.SourcePageType) is { } page)
 			{
 				page.Frame = this;
 				entry.Instance = page;

src/Uno.UI/UI/Xaml/Controls/Page/Page.HotReload.cs

@@ -0,0 +1,25 @@
+using System;
+using System.Reflection.Metadata;
+using Uno.UI.Helpers;
+
+[assembly: ElementMetadataUpdateHandlerAttribute(typeof(Windows.UI.Xaml.Controls.Page), typeof(Windows.UI.Xaml.Controls.PageElementMetadataUpdateHandler))]
+
+namespace Windows.UI.Xaml.Controls
+{
+	public partial class PageElementMetadataUpdateHandler
+	{
+		public static void AfterElementReplaced(FrameworkElement oldView, FrameworkElement newView, Type[] updatedTypes)
+		{
+			if (oldView is Page oldPage &&
+				newView is Page newPage)
+			{
+				newPage.Frame = oldPage.Frame;
+
+				// If we've replaced the Page in its frame, we may need to
+				// swap the content property as well. If may be required
+				// if the frame is handled by a (native) FramePresenter.
+				newPage.Frame.Content = newPage;
+			}
+		}
+	}
+}