[Shell] propagate BindingContext to SearchHandler (#5730)

- fixes #5705

Xamarin.Forms.Core/BindableObject.cs

@@ -199,6 +199,9 @@ namespace Xamarin.Forms
 			BindingContextChanged?.Invoke(this, EventArgs.Empty);
 			if (Shell.GetBackButtonBehavior(this) is BackButtonBehavior buttonBehavior)
 				SetInheritedBindingContext(buttonBehavior, BindingContext);
+
+			if (Shell.GetSearchHandler(this) is SearchHandler searchHandler)
+				SetInheritedBindingContext(searchHandler, BindingContext);
 		}
 
 		protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)

Xamarin.Forms.Core/Shell/Shell.cs

@@ -35,7 +35,16 @@ namespace Xamarin.Forms
 			BindableProperty.CreateAttached("NavBarIsVisible", typeof(bool), typeof(Shell), true);
 
 		public static readonly BindableProperty SearchHandlerProperty =
-			BindableProperty.CreateAttached("SearchHandler", typeof(SearchHandler), typeof(Shell), null, BindingMode.OneTime);
+			BindableProperty.CreateAttached("SearchHandler", typeof(SearchHandler), typeof(Shell), null, BindingMode.OneTime,
+				propertyChanged: OnSearchHandlerPropertyChanged);
+
+		static void OnSearchHandlerPropertyChanged(BindableObject bindable, object oldValue, object newValue)
+		{
+			if (oldValue is SearchHandler oldHandler)
+				SetInheritedBindingContext(oldHandler, null);
+			if (newValue is SearchHandler newHandler)
+				SetInheritedBindingContext(newHandler, bindable.BindingContext);
+		}
 
 		public static readonly BindableProperty SetPaddingInsetsProperty =
 			BindableProperty.CreateAttached("SetPaddingInsets", typeof(bool), typeof(Shell), false);
@@ -1069,14 +1078,11 @@ namespace Xamarin.Forms
 			return element;
 		}
 
-
-		#region IPropertyPropagationController
 		void IPropertyPropagationController.PropagatePropertyChanged(string propertyName)
 		{
 			PropertyPropagationExtensions.PropagatePropertyChanged(propertyName, this, LogicalChildren);
 			if (FlyoutHeaderView != null)
 				PropertyPropagationExtensions.PropagatePropertyChanged(propertyName, this, new[] { FlyoutHeaderView });
 		}
-		#endregion
 	}
 }

Xamarin.Forms.Xaml.UnitTests/Issues/Gh5705.xaml

@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<Shell xmlns="http://xamarin.com/schemas/2014/forms" xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" x:Class="Xamarin.Forms.Xaml.UnitTests.Gh5705">
+    <Shell.SearchHandler>
+        <SearchHandler x:Name="searchHandler"/>
+    </Shell.SearchHandler>
+</Shell>

Xamarin.Forms.Xaml.UnitTests/Issues/Gh5705.xaml.cs

@@ -0,0 +1,45 @@
+using System;
+using System.Collections.Generic;
+using NUnit.Framework;
+using Xamarin.Forms;
+using Xamarin.Forms.Core.UnitTests;
+
+namespace Xamarin.Forms.Xaml.UnitTests
+{
+	public partial class Gh5705 : Shell
+	{
+		public Gh5705() => InitializeComponent();
+		public Gh5705(bool useCompiledXaml)
+		{
+			//this stub will be replaced at compile time
+		}
+
+		[TestFixture] class Tests
+		{
+			IReadOnlyList<string> _flags;
+
+			[SetUp]
+			public void Setup()
+			{
+				Device.PlatformServices = new MockPlatformServices();
+				_flags = Device.Flags;
+				Device.SetFlags(new List<string>() { ExperimentalFlags.ShellExperimental});
+			}
+
+			[TearDown]
+			public void TearDown()
+			{
+				Device.PlatformServices = null;
+				Device.SetFlags(_flags);
+			}
+
+			[Test]
+			public void SearchHandlerIneritBC([Values(false, true)]bool useCompiledXaml)
+			{
+				var vm = new object();
+				var shell = new Gh5705(useCompiledXaml) { BindingContext = vm };
+				Assert.That(shell.searchHandler.BindingContext, Is.EqualTo(vm));
+			}
+		}
+	}
+}