[Shell] Propagate Page bindings to TitleView and Shell Binding to Flyout (#5934) fixes #5650 fixes #5501

* propagate bindingcontext

* - add exception message and fix poorly named xaml file

* add ui test automation

* - fix unit test to represent new code

* - changed from ui test to unit test

* - propagate visual, parent, bc to titleview

* - style fixes

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/TestPages/TestPages.cs

@@ -581,6 +581,30 @@ namespace Xamarin.Forms.Controls
 #endif
 		}
 
+		public ContentPage CreateContentPage()
+		{
+			ContentPage page = new ContentPage();
+			ShellItem item = new ShellItem()
+			{
+				Items =
+				{
+					new ShellSection()
+					{
+						Items =
+						{
+							new ShellContent()
+							{
+								Content = page
+							}
+						}
+					}
+				}
+			};
+
+			Items.Add(item);
+			return page;
+
+		}
 #if UITEST
 		[SetUp]
 		public void Setup()

Xamarin.Forms.Controls/ShellContentTest.xaml

@@ -5,7 +5,7 @@
 			 Routing.Route="shellcontent"
 			 Shell.SetPaddingInsets="true"
 			 Shell.TabBarIsVisible="false"
-             x:Class="Xamarin.Forms.Controls.ShellContent">
+             x:Class="Xamarin.Forms.Controls.ShellContentTest">
 	<Page.ToolbarItems>
 		<ToolbarItem Text="Search" Icon="bank.png" />
 	</Page.ToolbarItems>

Xamarin.Forms.Controls/ShellContentTest.xaml.cs

@@ -13,7 +13,7 @@ namespace Xamarin.Forms.Controls
 	[Preserve]
 	[QueryProperty("Text", "welcome")]
 	[XamlCompilation(XamlCompilationOptions.Compile)]
-	public partial class ShellContent : ContentPage
+	public partial class ShellContentTest : ContentPage
 	{
 		private class MySearchHandler : SearchHandler
 		{
@@ -51,7 +51,7 @@ namespace Xamarin.Forms.Controls
 
 		private string _text;
 
-		public ShellContent()
+		public ShellContentTest()
 		{
 			InitializeComponent();
 
@@ -89,7 +89,7 @@ namespace Xamarin.Forms.Controls
 
 		private void InsertClicked(object sender, EventArgs e)
 		{
-			Navigation.InsertPageBefore(new ShellContent(), this);
+			Navigation.InsertPageBefore(new ShellContentTest(), this);
 		}
 
 		private void ToggleClicked(object sender, EventArgs e)
@@ -122,7 +122,7 @@ namespace Xamarin.Forms.Controls
 
 		private async void PushClicked(object sender, EventArgs e)
 		{
-			await Navigation.PushAsync(new ShellContent()
+			await Navigation.PushAsync(new ShellContentTest()
 			{
 				Text = Text + "1"
 			});

Xamarin.Forms.Controls/Xamarin.Forms.Controls.csproj

@@ -50,6 +50,9 @@
     <Compile Update="GalleryPages\VisualStateManagerGalleries\OnPlatformExample.xaml.cs">
       <DependentUpon>OnPlatformExample.xaml</DependentUpon>
     </Compile>
+    <Compile Update="ShellContentTest.xaml.cs">
+      <DependentUpon>ShellContentTest.xaml</DependentUpon>
+    </Compile>
     <EmbeddedResource Update="GalleryPages\BindableLayoutGalleryPage.xaml">
       <Generator>MSBuild:UpdateDesignTimeXaml</Generator>
     </EmbeddedResource>

Xamarin.Forms.Core.UnitTests/ShellTests.cs

@@ -303,6 +303,9 @@ namespace Xamarin.Forms.Core.UnitTests
 
 			var label = new Label();
 
+			var viewModel = new Object();
+			shell.BindingContext = viewModel;
+
 			shell.FlyoutHeader = label;
 
 			Assert.AreEqual(((IShellController)shell).FlyoutHeader, label);
@@ -315,7 +318,7 @@ namespace Xamarin.Forms.Core.UnitTests
 			});
 
 			Assert.AreEqual(((IShellController)shell).FlyoutHeader, label2);
-			Assert.AreEqual(((IShellController)shell).FlyoutHeader.BindingContext, label);
+			Assert.AreEqual(((IShellController)shell).FlyoutHeader.BindingContext, viewModel);
 
 			shell.FlyoutHeaderTemplate = null;
 
@@ -370,5 +373,110 @@ namespace Xamarin.Forms.Core.UnitTests
 			shell.GoToAsync("//rootlevelcontent1");
 			Assert.AreEqual(shell.CurrentItem, item1);
 		}
+
+		[Test]
+		public async Task TitleViewBindingContext()
+		{
+			Shell shell = new Shell();
+			ContentPage page = new ContentPage();
+			shell.Items.Add(CreateShellItem(page));
+			page.BindingContext = new { Text = "Binding" };
+
+			// setup title view
+			StackLayout layout = new StackLayout() { BackgroundColor = Color.White };
+			Label label = new Label();
+			label.SetBinding(Label.TextProperty, "Text");
+			layout.Children.Add(label);
+			Shell.SetTitleView(page, layout);
+
+			Assert.AreEqual("Binding", label.Text);
+			page.BindingContext = new { Text = "Binding Changed" };
+			Assert.AreEqual("Binding Changed", label.Text);
+		}
+
+		[Test]
+		public async Task VisualPropagationPageLevel()
+		{
+			Shell shell = new Shell();
+			ContentPage page = new ContentPage();
+			shell.Items.Add(CreateShellItem(page));
+
+			// setup title view
+			StackLayout titleView = new StackLayout() { BackgroundColor = Color.White };
+			Button button = new Button();
+			titleView.Children.Add(button);
+			Shell.SetTitleView(page, titleView);
+			IVisualController visualController = button as IVisualController;
+
+
+			Assert.AreEqual(page, titleView.Parent);
+
+			Assert.AreEqual(VisualMarker.Default, ((IVisualController)button).EffectiveVisual);
+			page.Visual = VisualMarker.Material;
+			Assert.AreEqual(VisualMarker.Material, ((IVisualController)button).EffectiveVisual);
+		}
+
+		[Test]
+		public async Task VisualPropagationShellLevel()
+		{
+			Shell shell = new Shell();
+			ContentPage page = new ContentPage();
+			shell.Items.Add(CreateShellItem(page));
+
+			// setup title view
+			StackLayout titleView = new StackLayout() { BackgroundColor = Color.White };
+			Button button = new Button();
+			titleView.Children.Add(button);
+			Shell.SetTitleView(page, titleView);
+			IVisualController visualController = button as IVisualController;
+
+
+			Assert.AreEqual(page, titleView.Parent);
+			Assert.AreEqual(VisualMarker.Default, ((IVisualController)button).EffectiveVisual);
+			shell.Visual = VisualMarker.Material;
+			Assert.AreEqual(VisualMarker.Material, ((IVisualController)button).EffectiveVisual);
+		}
+
+		[Test]
+		public async Task FlyoutViewVisualPropagation()
+		{
+			Shell shell = new Shell();
+			ContentPage page = new ContentPage();
+			shell.Items.Add(CreateShellItem(page));
+
+			
+			// setup title view
+			StackLayout flyoutView = new StackLayout() { BackgroundColor = Color.White };
+			Button button = new Button();
+			flyoutView.Children.Add(button);
+			shell.SetValue(Shell.FlyoutHeaderProperty, flyoutView);
+
+			IVisualController visualController = button as IVisualController;
+			Assert.AreEqual(VisualMarker.Default, visualController.EffectiveVisual);
+			shell.Visual = VisualMarker.Material;
+			Assert.AreEqual(VisualMarker.Material, visualController.EffectiveVisual);
+		}
+
+		[Test]
+		public async Task FlyoutViewBindingContext()
+		{
+			Shell shell = new Shell();
+			ContentPage page = new ContentPage();
+			shell.Items.Add(CreateShellItem(page));
+			shell.BindingContext = new { Text = "Binding" };
+
+			// setup title view
+			StackLayout flyoutView = new StackLayout() { BackgroundColor = Color.White };
+			Label label = new Label();
+			label.SetBinding(Label.TextProperty, "Text");
+			flyoutView.Children.Add(label);
+			shell.SetValue(Shell.FlyoutHeaderProperty, flyoutView);
+
+			Assert.AreEqual("Binding", label.Text);
+			shell.BindingContext = new { Text = "Binding Changed" };
+			Assert.AreEqual("Binding Changed", label.Text);
+			shell.SetValue(Shell.FlyoutHeaderProperty, new ContentView());
+			Assert.AreEqual(null, flyoutView.BindingContext);
+		}
 	}
 }

Xamarin.Forms.Core/BindableObject.cs

@@ -202,6 +202,9 @@ namespace Xamarin.Forms
 
 			if (Shell.GetSearchHandler(this) is SearchHandler searchHandler)
 				SetInheritedBindingContext(searchHandler, BindingContext);
+
+			if (Shell.GetTitleView(this) is View titleView)
+				SetInheritedBindingContext(titleView, BindingContext);
 		}
 
 		protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)

Xamarin.Forms.Core/Cells/Cell.cs

@@ -49,6 +49,9 @@ namespace Xamarin.Forms
 			get { return _effectiveVisual; }
 			set
 			{
+				if (value == _effectiveVisual)
+					return;
+
 				_effectiveVisual = value;
 				OnPropertyChanged(VisualElement.VisualProperty.PropertyName);
 			}

Xamarin.Forms.Core/Element.cs

@@ -338,6 +338,10 @@ namespace Xamarin.Forms
 		{
 			base.OnPropertyChanged(propertyName);
 
+			IPropertyPropagationController titleView = Shell.GetTitleView(this) ?? NavigationPage.GetTitleView(this);
+			if(titleView != null)
+				PropertyPropagationExtensions.PropagatePropertyChanged(propertyName, this, new[] { titleView });
+
 			if (_effects == null || _effects.Count == 0)
 				return;
 

Xamarin.Forms.Core/Shell/BaseShellItem.cs

@@ -67,6 +67,9 @@ namespace Xamarin.Forms
 			get { return _effectiveVisual; }
 			set
 			{
+				if (value == _effectiveVisual)
+					return;
+
 				_effectiveVisual = value;
 				OnPropertyChanged(VisualElement.VisualProperty.PropertyName);
 			}

Xamarin.Forms.Core/Shell/IShellController.cs

@@ -16,8 +16,6 @@ namespace Xamarin.Forms
 
 	public interface IShellController : IPageController
 	{
-		event EventHandler HeaderChanged;
-
 		event EventHandler StructureChanged;
 
 		View FlyoutHeader { get; }

Xamarin.Forms.Core/Shell/Shell.cs

@@ -187,19 +187,12 @@ namespace Xamarin.Forms
 		List<(IAppearanceObserver Observer, Element Pivot)> _appearanceObservers = new List<(IAppearanceObserver Observer, Element Pivot)>();
 		List<IFlyoutBehaviorObserver> _flyoutBehaviorObservers = new List<IFlyoutBehaviorObserver>();
 
-		event EventHandler IShellController.HeaderChanged
-		{
-			add { _headerChanged += value; }
-			remove { _headerChanged -= value; }
-		}
-
 		event EventHandler IShellController.StructureChanged
 		{
 			add { _structureChanged += value; }
 			remove { _structureChanged -= value; }
 		}
 
-		event EventHandler _headerChanged;
 		event EventHandler _structureChanged;
 
 		View IShellController.FlyoutHeader => FlyoutHeaderView;
@@ -708,10 +701,16 @@ namespace Xamarin.Forms
 				_flyoutHeaderView = value;
 				if (_flyoutHeaderView != null)
 					OnChildAdded(_flyoutHeaderView);
-				_headerChanged?.Invoke(this, EventArgs.Empty);
 			}
 		}
 
+		protected override void OnBindingContextChanged()
+		{
+			base.OnBindingContextChanged();
+			if (FlyoutHeaderView != null)
+				SetInheritedBindingContext(FlyoutHeaderView, BindingContext);
+		}
+
 		List<List<Element>> IShellController.GenerateFlyoutGrouping()
 		{
 			// The idea here is to create grouping such that the Flyout would
@@ -1033,10 +1032,6 @@ namespace Xamarin.Forms
 				else
 					FlyoutHeaderView = null;
 			}
-			else
-			{
-				FlyoutHeaderView.BindingContext = newVal;
-			}
 		}
 
 		void OnFlyoutHeaderTemplateChanged(DataTemplate oldValue, DataTemplate newValue)
@@ -1051,7 +1046,6 @@ namespace Xamarin.Forms
 			else
 			{
 				var newHeaderView = (View)newValue.CreateContent(FlyoutHeader, this);
-				newHeaderView.BindingContext = FlyoutHeader;
 				FlyoutHeaderView = newHeaderView;
 			}
 		}

Xamarin.Forms.Core/Shell/ShellContent.cs

@@ -89,7 +89,8 @@ namespace Xamarin.Forms
 
 		internal override ReadOnlyCollection<Element> LogicalChildrenInternal => _logicalChildrenReadOnly ?? (_logicalChildrenReadOnly = new ReadOnlyCollection<Element>(_logicalChildren));
 
-		Page ContentCache {
+		Page ContentCache
+		{
 			get { return _contentCache; }
 			set
 			{

Xamarin.Forms.Platform.Android/Renderers/ShellItemRenderer.cs

@@ -64,6 +64,9 @@ namespace Xamarin.Forms.Platform.Android
 			_bottomView.SetBackgroundColor(Color.White.ToAndroid());
 			_bottomView.SetOnNavigationItemSelectedListener(this);
 
+			if(ShellItem == null)
+				throw new ArgumentException("Active Shell Item not set. Have you added any Shell Items to your Shell?", nameof(ShellItem));
+
 			HookEvents(ShellItem);
 			SetupMenu();
 

Xamarin.Forms.Platform.Android/Renderers/ShellToolbarTracker.cs

@@ -386,8 +386,6 @@ namespace Xamarin.Forms.Platform.Android
 			}
 			else
 			{
-				// FIXME
-				titleView.Parent = _shellContext.Shell;
 				_titleViewContainer = new ContainerView(context, titleView);
 				_titleViewContainer.MatchHeight = _titleViewContainer.MatchWidth = true;
 				_titleViewContainer.LayoutParameters = new Toolbar.LayoutParams(LP.MatchParent, LP.MatchParent)