Don't force binding expression applications to queue (#6857)

* Don't force binding expression applications to queue; fixes #6609

* Make PropertyChangeBindingsOccurThroughMainThread test what it claims to test

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Issue6609.cs

@@ -0,0 +1,43 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+using Xamarin.Forms.CustomAttributes;
+using Xamarin.Forms.Internals;
+
+#if UITEST
+using Xamarin.Forms.Core.UITests;
+using Xamarin.UITest;
+using NUnit.Framework;
+#endif
+
+namespace Xamarin.Forms.Controls.Issues
+{
+#if UITEST
+	[Category(UITestCategories.CollectionView)]
+#endif
+	[Preserve(AllMembers = true)]
+	[Issue(IssueTracker.Github, 6609, "[Bug, CollectionView] SelectionChangedCommand invoked before SelectedItem is set",
+		PlatformAffected.All)]
+	public class Issue6609 : TestNavigationPage
+	{
+		protected override void Init()
+		{
+#if APP
+			FlagTestHelpers.SetCollectionViewTestFlag();
+
+			PushAsync(new GalleryPages.CollectionViewGalleries.SelectionGalleries.SelectionChangedCommandParameter());
+#endif
+		}
+
+#if UITEST
+		[Test]
+		public void SelectionChangedCommandParameterBoundToSelectedItemShouldMatchSelectedItem()
+		{
+			RunningApp.WaitForElement("Item 2");
+			RunningApp.Tap("Item 2");
+
+			RunningApp.WaitForElement("Success");
+		}
+#endif
+	}
+}

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Xamarin.Forms.Controls.Issues.Shared.projitems

@@ -13,6 +13,7 @@
       <DependentUpon>Issue5046.xaml</DependentUpon>
       <SubType>Code</SubType>
     </Compile>
+    <Compile Include="$(MSBuildThisFileDirectory)Issue6609.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)ShellInsets.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)CollectionViewGrouping.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue5412.cs" />
@@ -528,7 +529,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)Bugzilla39853.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)MultipleClipToBounds.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)_TemplateMarkup.xaml.cs">
-	  <DependentUpon>_TemplateMarkup.xaml</DependentUpon>
+      <DependentUpon>_TemplateMarkup.xaml</DependentUpon>
       <SubType>Code</SubType>
     </Compile>
     <Compile Include="$(MSBuildThisFileDirectory)PerformanceGallery\PerformanceDataManager.cs" />

Xamarin.Forms.Controls/GalleryPages/CollectionViewGalleries/SelectionGalleries/SelectionChangedCommandParameter.xaml

@@ -0,0 +1,40 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
+             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+             xmlns:d="http://xamarin.com/schemas/2014/forms/design"
+             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+             mc:Ignorable="d"
+             x:Class="Xamarin.Forms.Controls.GalleryPages.CollectionViewGalleries.SelectionGalleries.SelectionChangedCommandParameter">
+    <ContentPage.Content>
+        <StackLayout>
+
+            <Label x:Name="Result" Text="Pending..."></Label>
+            
+            <CollectionView ItemsSource="{Binding Items}"
+                    SelectionMode="Single"
+                    SelectionChangedCommandParameter="{Binding SelectedItem,Source={x:Reference MyCollectionView}}"
+                    SelectedItem="{Binding SelectedItem, Mode=TwoWay}"
+                    SelectionChangedCommand="{Binding SelectionChangedCommand}"
+                    x:Name="MyCollectionView">
+                <CollectionView.ItemTemplate>
+                    <DataTemplate>
+                        <StackLayout Padding="10">
+                            <Label Text="{Binding Text}" 
+                                    d:Text="{Binding .}"
+                                    LineBreakMode="NoWrap" 
+                                    Style="{DynamicResource ListItemTextStyle}" 
+                                    FontSize="16" />
+                            <Label Text="{Binding Description}" 
+                                    d:Text="Item description"
+                                    LineBreakMode="NoWrap"
+                                    Style="{DynamicResource ListItemDetailTextStyle}"
+                                    FontSize="13" />
+                        </StackLayout>
+                    </DataTemplate>
+                </CollectionView.ItemTemplate>
+            </CollectionView>
+        </StackLayout>
+        
+        
+    </ContentPage.Content>
+</ContentPage>

Xamarin.Forms.Controls/GalleryPages/CollectionViewGalleries/SelectionGalleries/SelectionChangedCommandParameter.xaml.cs

@@ -0,0 +1,88 @@
+using System;
+using System.Collections.Generic;
+using System.Collections.ObjectModel;
+using System.ComponentModel;
+using System.Linq;
+using System.Runtime.CompilerServices;
+using System.Text;
+using System.Threading.Tasks;
+
+using Xamarin.Forms;
+using Xamarin.Forms.Internals;
+using Xamarin.Forms.Xaml;
+
+namespace Xamarin.Forms.Controls.GalleryPages.CollectionViewGalleries.SelectionGalleries
+{
+	[XamlCompilation(XamlCompilationOptions.Compile)]
+	public partial class SelectionChangedCommandParameter : ContentPage
+	{
+		public SelectionChangedCommandParameter()
+		{
+			InitializeComponent();
+			BindingContext = new ItemsViewModel(Result);
+		}
+	}
+
+	[Preserve(AllMembers = true)]
+	class Item
+	{
+		public string Id { get; set; }
+		public string Text { get; set; }
+		public string Description { get; set; }
+	}
+
+	[Preserve(AllMembers = true)]
+	class ItemsViewModel : INotifyPropertyChanged
+	{
+		public ObservableCollection<Item> Items { get; set; }
+		public Command LoadItemsCommand { get; set; }
+
+		Item _selectedItem;
+		readonly Label _result;
+
+		public event PropertyChangedEventHandler PropertyChanged;
+
+		public Item SelectedItem
+		{
+			get => _selectedItem;
+			set { _selectedItem = value; OnPropertyChanged(); }
+		}
+
+		public Command<Item> SelectionChangedCommand { get; }
+
+		protected void OnPropertyChanged([CallerMemberName] string propertyName = "")
+		{
+			var changed = PropertyChanged;
+			if (changed == null)
+				return;
+
+			changed.Invoke(this, new PropertyChangedEventArgs(propertyName));
+		}
+
+		public ItemsViewModel(Label result)
+		{
+			Items = new ObservableCollection<Item>();
+
+			for (int n = 0; n < 10; n++)
+			{
+				Items.Add(new Item { Id = n.ToString(), Text = $"Item {n}", Description = $"This is item {n}" });
+			}
+
+			SelectionChangedCommand = new Command<Item>(item =>
+			{
+				var fromParameter = item;
+				var fromSelectedItem = SelectedItem;
+
+				if (fromParameter != fromSelectedItem)
+				{
+					_result.Text = "Fail";
+				}
+				else
+				{
+					_result.Text = "Success";
+				}
+			});
+			_result = result;
+		}
+	}
+}

Xamarin.Forms.Controls/GalleryPages/CollectionViewGalleries/SelectionGalleries/SelectionGallery.cs

@@ -26,6 +26,8 @@
 							new SingleBoundSelection(), Navigation),
 						GalleryBuilder.NavButton("Multiple Selection, Bound", () =>
 							new MultipleBoundSelection(), Navigation),
+						GalleryBuilder.NavButton("SelectionChangedCommandParameter", () =>
+							new SelectionChangedCommandParameter(), Navigation),
 					}
 				}
 			};

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

@@ -65,6 +65,9 @@
     <EmbeddedResource Update="GalleryPages\CollectionViewGalleries\DataTemplateSelectorGallery.xaml">
       <Generator>MSBuild:UpdateDesignTimeXaml</Generator>
     </EmbeddedResource>
+    <EmbeddedResource Update="GalleryPages\CollectionViewGalleries\SelectionGalleries\SelectionChangedCommandParameter.xaml">
+      <Generator>MSBuild:UpdateDesignTimeXaml</Generator>
+    </EmbeddedResource>
     <EmbeddedResource Update="GalleryPages\MapGallery.xaml">
       <Generator>MSBuild:UpdateDesignTimeXaml</Generator>
     </EmbeddedResource>

Xamarin.Forms.Core.UnitTests/BindingBaseUnitTests.cs

@@ -614,7 +614,7 @@ namespace Xamarin.Forms.Core.UnitTests
 			bindable.SetBinding(MockBindable.TextProperty, binding);
 
 			bool invokeOnMainThreadWasCalled = false;
-			Device.PlatformServices = new MockPlatformServices(a => invokeOnMainThreadWasCalled = true);
+			Device.PlatformServices = new MockPlatformServices(a => invokeOnMainThreadWasCalled = true, isInvokeRequired: true);
 
 			vm.Text = "updated";
 

Xamarin.Forms.Core.UnitTests/MockPlatformServices.cs

@@ -25,17 +25,19 @@ namespace Xamarin.Forms.Core.UnitTests
 		Func<Uri, CancellationToken, Task<Stream>> getStreamAsync;
 		Func<VisualElement, double, double, SizeRequest> getNativeSizeFunc;
 		readonly bool useRealisticLabelMeasure;
+		readonly bool _isInvokeRequired;
 
 		public MockPlatformServices (Action<Action> invokeOnMainThread = null, Action<Uri> openUriAction = null, 
 			Func<Uri, CancellationToken, Task<Stream>> getStreamAsync = null, 
 			Func<VisualElement, double, double, SizeRequest> getNativeSizeFunc = null, 
-			bool useRealisticLabelMeasure = false)
+			bool useRealisticLabelMeasure = false, bool isInvokeRequired = false)
 		{
 			this.invokeOnMainThread = invokeOnMainThread;
 			this.openUriAction = openUriAction;
 			this.getStreamAsync = getStreamAsync;
 			this.getNativeSizeFunc = getNativeSizeFunc;
 			this.useRealisticLabelMeasure = useRealisticLabelMeasure;
+			_isInvokeRequired = isInvokeRequired;
 		}
 
 		static MD5CryptoServiceProvider checksum = new MD5CryptoServiceProvider ();
@@ -85,7 +87,7 @@ namespace Xamarin.Forms.Core.UnitTests
 
 		public bool IsInvokeRequired
 		{
-			get { return false; }
+			get { return _isInvokeRequired; }
 		}
 
 		public string RuntimePlatform { get; set; }

Xamarin.Forms.Core/BindingExpression.cs

@@ -615,7 +615,14 @@ namespace Xamarin.Forms
 					}
 				}
 
-				Device.BeginInvokeOnMainThread(() => _expression.Apply());
+				if (Device.IsInvokeRequired)
+				{
+					Device.BeginInvokeOnMainThread(() => _expression.Apply());
+				}
+				else
+				{
+					_expression.Apply();
+				}
 			}
 
 			public bool TryGetValue(object source, out object value)