Fixed content update issue when changing the ControlTemplate at runtime for a ContentPage (#28216)

* Fixed the ControlTemplate runtime issue on ContentPage

* Update Issue15649.xaml

Changes added.

Author: sheiksyedm

src/Controls/src/Core/TemplatedPage.cs

@@ -59,6 +59,11 @@ void IControlTemplated.OnApplyTemplate()
 
 		protected virtual void OnApplyTemplate()
 		{
+			OnApplyTemplateImpl();
+		}
+		void OnApplyTemplateImpl()
+		{
+			Handler?.UpdateValue(nameof(IContentView.Content));
 		}
 
 		protected override void OnChildRemoved(Element child, int oldLogicalIndex)

src/Controls/tests/TestCases.HostApp/Issues/Issue15649.xaml

@@ -0,0 +1,48 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<ContentPage xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
+             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+             x:Class="Controls.TestCases.HostApp.Issues.Issue15649"
+             xmlns:local="clr-namespace:Controls.TestCases.HostApp.Issues"
+             x:Name="HomePage"
+             Title="Issue15649">
+    <ContentPage.Resources>
+        <ResourceDictionary>
+            <ControlTemplate x:Key="Page1">
+                <StackLayout BackgroundColor="Red">
+                    <Button Text="Change Value"
+                            AutomationId="Button"
+                            Clicked="Button_Clicked"/>
+                    <Label Text="First Page"
+                            AutomationId="Page1Label"
+                            HorizontalOptions="Center"
+                            VerticalOptions="Center"></Label>
+
+                </StackLayout>
+            </ControlTemplate>
+            <ControlTemplate x:Key="Page2">
+                <StackLayout BackgroundColor="Blue">
+                    <Label Text="Second Page"
+                            AutomationId="Page2Label"
+                            HorizontalOptions="Center"
+                            VerticalOptions="Center"></Label>
+                </StackLayout>
+
+            </ControlTemplate>
+        </ResourceDictionary>
+
+    </ContentPage.Resources>
+    <ContentPage.Triggers>
+        <DataTrigger TargetType="ContentPage"
+                     Binding="{Binding PositionSelected, Source={x:Reference HomePage},x:DataType=local:Issue15649}"
+                     Value="1">
+            <Setter Property="ControlTemplate"
+                    Value="{DynamicResource Page1}"/>
+        </DataTrigger>
+        <DataTrigger TargetType="ContentPage"
+                     Binding="{Binding PositionSelected, Source={x:Reference HomePage},x:DataType=local:Issue15649}"
+                     Value="2">
+            <Setter Property="ControlTemplate"
+                    Value="{DynamicResource Page2}"/>
+        </DataTrigger>
+    </ContentPage.Triggers>
+</ContentPage>

src/Controls/tests/TestCases.HostApp/Issues/Issue15649.xaml.cs

@@ -0,0 +1,32 @@
+namespace Controls.TestCases.HostApp.Issues;
+
+[Issue(IssueTracker.Github, 15649, "Updating a ControlTemplate at runtime for a Content Page is not working.", PlatformAffected.All)]
+public partial class Issue15649 : ContentPage
+{
+	public int _positionSelected = 1;
+
+		public int PositionSelected
+		{
+			set
+			{
+				if (_positionSelected != value)
+				{
+					_positionSelected = value;
+
+					OnPropertyChanged();
+				}
+			}
+			get => _positionSelected;
+		}
+
+	public Issue15649()
+	{
+		InitializeComponent();
+		this.BindingContext = this;
+	}
+	private void Button_Clicked(object sender, EventArgs e)
+	{
+		PositionSelected = 2;
+	}
+
+}

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Issue15649.cs

@@ -0,0 +1,19 @@
+using NUnit.Framework;
+using UITest.Appium;
+using UITest.Core;
+
+namespace Microsoft.Maui.TestCases.Tests.Issues;
+
+public class Issue15649(TestDevice testDevice) : _IssuesUITest(testDevice)
+{
+    public override string Issue => "Updating a ControlTemplate at runtime for a Content Page is not working.";
+
+    [Test]
+    [Category(UITestCategories.Page)]
+    public void DynamicallyUpdatingContentPage()
+    {
+        App.WaitForElement("Page1Label");
+        App.Tap("Button");
+        App.WaitForElement("Page2Label");
+    }
+}