fix: Fix spurious DataContext propagation in ContentControl

Author: davidjohnoliver

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml_Controls/ContentControlPages/BoundContentApplicator.cs

@@ -0,0 +1,27 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+using Windows.UI.Xaml;
+using Windows.UI.Xaml.Controls;
+
+namespace Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml_Controls.ContentControlPages
+{
+	public partial class BoundContentApplicator : Grid
+	{
+		public object PseudoContent
+		{
+			get { return (object)GetValue(PseudoContentProperty); }
+			set { SetValue(PseudoContentProperty, value); }
+		}
+
+		public static readonly DependencyProperty PseudoContentProperty =
+			DependencyProperty.Register("PseudoContent", typeof(object), typeof(BoundContentApplicator), new PropertyMetadata(null));
+
+		public void SpawnButton()
+		{
+			var button = new Button(); // Since we create the Button from code, its default style and its template will only be applied when it's loaded into the visual tree
+			button.Content = PseudoContent;
+			Children.Add(button);
+		}
+	}
+}

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml_Controls/ContentControlPages/When_Template_Applied_On_Loading_DataContext_Propagation_Page.xaml

@@ -0,0 +1,22 @@
+<Page
+    x:Class="Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml_Controls.ContentControlPages.When_Template_Applied_On_Loading_DataContext_Propagation_Page"
+    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.ContentControlPages"
+    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+    mc:Ignorable="d"
+    Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
+	<StackPanel Margin="0,50,0,0"
+				Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
+		<Button x:Name="AddButtonButton"
+				Content="Add test button"
+				Click="AddBoundContentButton" />
+		<local:BoundContentApplicator x:Name="SpawnedButtonHost" x:FieldModifier="public">
+			<local:BoundContentApplicator.PseudoContent>
+				<ComboBox ItemsSource="{Binding MyItemsSource}"
+						  SelectedItem="{Binding MyText, Mode=TwoWay}" />
+			</local:BoundContentApplicator.PseudoContent>
+		</local:BoundContentApplicator>
+	</StackPanel>
+</Page>

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml_Controls/ContentControlPages/When_Template_Applied_On_Loading_DataContext_Propagation_Page.xaml.cs

@@ -0,0 +1,57 @@
+using System;
+using System.Collections.Generic;
+using System.ComponentModel;
+using System.IO;
+using System.Linq;
+using System.Runtime.InteropServices.WindowsRuntime;
+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.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.ContentControlPages
+{
+	/// <summary>
+	/// An empty page that can be used on its own or navigated to within a Frame.
+	/// </summary>
+	public sealed partial class When_Template_Applied_On_Loading_DataContext_Propagation_Page : Page
+	{
+		public When_Template_Applied_On_Loading_DataContext_Propagation_Page()
+		{
+			this.InitializeComponent();
+			DataContext = new ViewModel() { MyItemsSource = new[] { "Froot", "Veg" }, MyText = "Froot" };
+		}
+
+		private void AddBoundContentButton(object sender, object args)
+		{
+			SpawnedButtonHost.SpawnButton();
+		}
+
+		private class ViewModel : INotifyPropertyChanged
+		{
+			public event PropertyChangedEventHandler PropertyChanged;
+
+			public IEnumerable<string> MyItemsSource { get; set; }
+
+			public string MyText
+			{
+				get { return _myText; }
+				set
+				{
+					if (_myText != value)
+					{
+						_myText = value;
+						PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(MyText)));
+					}
+				}
+			}
+			string _myText;
+		}
+	}
+}

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

@@ -12,6 +12,7 @@
 using System.Linq;
 using static Private.Infrastructure.TestServices;
 using Uno.UI.RuntimeTests.Helpers;
+using Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml_Controls.ContentControlPages;
 #if NETFX_CORE
 using Uno.UI.Extensions;
 #elif __IOS__
@@ -132,6 +133,28 @@ public async Task When_ContentTemplateSelector_And_Default_Style_And_Fluent()
 			}
 		}
 
+		[TestMethod]
+		public async Task When_Template_Applied_On_Loading_DataContext_Propagation()
+		{
+			var page = new When_Template_Applied_On_Loading_DataContext_Propagation_Page();
+			WindowHelper.WindowContent = page;
+			await WindowHelper.WaitForLoaded(page);
+			var comboBox = page.SpawnedButtonHost.PseudoContent as ComboBox;
+
+			var dataContextChangedCounter = 0;
+			var itemsSourceChangedCounter = 0;
+			comboBox.DataContextChanged += (_, __) => dataContextChangedCounter++;
+			comboBox.RegisterPropertyChangedCallback(ItemsControl.ItemsSourceProperty, (_, __) => itemsSourceChangedCounter++);
+
+			page.SpawnedButtonHost.SpawnButton();
+			Assert.IsNotNull(comboBox);
+
+			await WindowHelper.WaitForLoaded(comboBox);
+			Assert.AreEqual("Froot", comboBox.SelectedItem);
+			Assert.AreEqual(1, dataContextChangedCounter);
+			Assert.AreEqual(1, itemsSourceChangedCounter);
+		}
+
 		private class SignInViewModel
 		{
 			public string UserName { get; set; } = "Steve";

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

@@ -119,6 +119,12 @@ public virtual object Content
 				typeof(ContentControl),
 				new FrameworkPropertyMetadata(
 					defaultValue: null,
+					// Don't propagate DataContext to Content qua Content, only propagate it via the visual tree. Prevents spurious
+					// propagation in case that default style and template is only applied once the control enters the visual tree
+					// (ie if created in code by new SomeControl())
+					// NOTE: There's a case we currently don't support: if the Content is a DependencyObject but *not* a FrameworkElement, then
+					// the DataContext won't get propagated and any bindings won't get updated.
+					FrameworkPropertyMetadataOptions.ValueDoesNotInheritDataContext,
 					propertyChangedCallback: (s, e) => ((ContentControl)s)?.OnContentChanged(e.OldValue, e.NewValue)
 				)
 			);