[Handlers] Make sure to update Content on handlers when the ControlTemplate changes

src/Controls/src/Core/ContentView.cs

@@ -5,7 +5,7 @@ namespace Microsoft.Maui.Controls
 {
 	/// <include file="../../docs/Microsoft.Maui.Controls/ContentView.xml" path="Type[@FullName='Microsoft.Maui.Controls.ContentView']/Docs" />
 	[ContentProperty("Content")]
-	public class ContentView : TemplatedView, IContentView
+	public partial class ContentView : TemplatedView
 	{
 		/// <include file="../../docs/Microsoft.Maui.Controls/ContentView.xml" path="//Member[@MemberName='ContentProperty']/Docs" />
 		public static readonly BindableProperty ContentProperty = BindableProperty.Create(nameof(Content), typeof(View), typeof(ContentView), null, propertyChanged: TemplateUtilities.OnContentChanged);
@@ -17,9 +17,6 @@ public View Content
 			set { SetValue(ContentProperty, value); }
 		}
 
-		object IContentView.Content => Content;
-		IView IContentView.PresentedContent => ((this as IControlTemplated).TemplateRoot as IView) ?? Content;
-
 		protected override void OnBindingContextChanged()
 		{
 			base.OnBindingContextChanged();

src/Controls/src/Core/HandlerImpl/ContentView/ContentView.Impl.cs

@@ -0,0 +1,14 @@
+namespace Microsoft.Maui.Controls
+{
+	public partial class ContentView : IContentView
+	{
+		object IContentView.Content => Content;
+		IView IContentView.PresentedContent => ((this as IControlTemplated).TemplateRoot as IView) ?? Content;
+
+		protected override void OnApplyTemplate()
+		{
+			base.OnApplyTemplate();
+			Handler?.UpdateValue(nameof(Content));
+		}
+	}
+}

src/Controls/tests/DeviceTests/Elements/ContentView/ContentViewTests.Android.cs

@@ -0,0 +1,21 @@
+using Microsoft.Maui.Handlers;
+using Microsoft.Maui.Platform;
+
+namespace Microsoft.Maui.DeviceTests
+{
+	public partial class ContentViewTests
+	{
+		static int GetChildCount(ContentViewHandler contentViewHandler)
+		{
+			return contentViewHandler.PlatformView.ChildCount;
+		}
+
+		static int GetContentChildCount(ContentViewHandler contentViewHandler)
+		{
+			if (contentViewHandler.PlatformView.GetChildAt(0) is LayoutViewGroup childLayoutViewGroup)
+				return childLayoutViewGroup.ChildCount;
+			else 
+				return 0;
+		}
+	}
+}

src/Controls/tests/DeviceTests/Elements/ContentView/ContentViewTests.Windows.cs

@@ -0,0 +1,21 @@
+using Microsoft.Maui.Handlers;
+using Microsoft.Maui.Platform;
+
+namespace Microsoft.Maui.DeviceTests
+{
+	public partial class ContentViewTests
+	{
+		static int GetChildCount(ContentViewHandler contentViewHandler)
+		{
+			return contentViewHandler.PlatformView.Children.Count;
+		}
+
+		static int GetContentChildCount(ContentViewHandler contentViewHandler)
+		{
+			if (contentViewHandler.PlatformView.Children[0] is LayoutPanel childLayoutPanel)
+				return childLayoutPanel.Children.Count;
+			else
+				return 0;
+		}
+	}
+}

src/Controls/tests/DeviceTests/Elements/ContentView/ContentViewTests.cs

@@ -0,0 +1,55 @@
+using System;
+using System.Threading.Tasks;
+using Microsoft.Maui.Controls;
+using Microsoft.Maui.Handlers;
+using Microsoft.Maui.Hosting;
+using Microsoft.Maui.Platform;
+using Xunit;
+
+namespace Microsoft.Maui.DeviceTests
+{
+	[Category(TestCategory.ContentView)]
+	public partial class ContentViewTests : HandlerTestBase
+	{
+		void SetupBuilder()
+		{
+			EnsureHandlerCreated(builder =>
+			{
+				builder.ConfigureMauiHandlers(handlers =>
+				{
+					handlers.AddHandler<Label, LabelHandler>();
+					handlers.AddHandler<IContentView, ContentViewHandler>();
+					handlers.AddHandler<Grid, LayoutHandler>();
+				});
+			});
+		}
+
+		[Fact("ContentView updating it's ControlTemplate works")]
+		public async Task ControlTemplateUpdates()
+		{
+			SetupBuilder();
+			var child = new Label { Text = "Content 1" };
+			var contentView = new Microsoft.Maui.Controls.ContentView();
+			var header = new Label { Text = "Header" };
+			var footer = new Label { Text = "Footer" };
+			var presenter = new ContentPresenter();
+			var grid = new Grid();
+
+			var contentViewHandler = await CreateHandlerAsync<ContentViewHandler>(contentView);
+
+			await InvokeOnMainThreadAsync(() =>
+			{
+				contentView.Content = child;
+				Assert.True(GetChildCount(contentViewHandler) == 1);
+				Assert.True(GetContentChildCount(contentViewHandler) == 0);
+				grid.Children.Add(header);
+				grid.Children.Add(presenter);
+				grid.Children.Add(footer);
+				var dataTemplate = new ControlTemplate(() => grid);
+				contentView.ControlTemplate = dataTemplate;
+				Assert.True(GetChildCount(contentViewHandler) == 1);
+				Assert.True(GetContentChildCount(contentViewHandler) == 3);
+			});
+		}
+	}
+}

src/Controls/tests/DeviceTests/Elements/ContentView/ContentViewTests.iOS.cs

@@ -0,0 +1,17 @@
+using Microsoft.Maui.Handlers;
+
+namespace Microsoft.Maui.DeviceTests
+{
+	public partial class ContentViewTests
+	{
+		static int GetChildCount(ContentViewHandler contentViewHandler)
+		{
+			return contentViewHandler.PlatformView.Subviews.Length;
+		}
+
+		static int GetContentChildCount(ContentViewHandler contentViewHandler)
+		{
+			return contentViewHandler.PlatformView.Subviews[0].Subviews.Length;
+		}
+	}
+}

src/Controls/tests/DeviceTests/TestCategory.cs

@@ -4,6 +4,7 @@ public static class TestCategory
 	{
 		public const string Behavior = "Behavior";
 		public const string Button = "Button";
+		public const string ContentView = "ContentView";
 		public const string Dispatcher = "Dispatcher";
 		public const string Editor = "Editor";
 		public const string Element = "Element";