Force off-main-thread ItemsSource updates to update on main thread (#11235)

* Fixed CollectionView issue adding data in different thread on Android

* Fixed build error

* Changes to fix the build

* Force off-main-thread ObservableCollection changes to marshal to main thread

* Update Xamarin.Forms.Controls.Issues.Shared.projitems

* Update interface

* Restore old method

Co-authored-by: Javier Suárez Ruiz <javiersuarezruiz@hotmail.com>
Co-authored-by: Samantha Houts <samhouts@users.noreply.github.com>
Co-authored-by: Rui Marinho <me@ruimarinho.net>

fixes #10735
fixes #9753

.Xamarin.Forms.Android.slnf

@@ -2,6 +2,8 @@
   "solution": {
     "path": "Xamarin.Forms.sln",
     "projects": [
+      "PagesGallery\\PagesGallery.Droid\\PagesGallery.Droid.csproj",
+      "PagesGallery\\PagesGallery\\PagesGallery.csproj",
       "Stubs\\Xamarin.Forms.Platform.Android\\Xamarin.Forms.Platform.Android (Forwarders).csproj",
       "XFCorePostProcessor.Tasks\\XFCorePostProcessor.Tasks.csproj",
       "Xamarin.Flex\\Xamarin.Flex.shproj",

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Issue10735.xaml

@@ -0,0 +1,46 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<controls:TestContentPage 
+    xmlns:controls="clr-namespace:Xamarin.Forms.Controls"
+    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.Issue10735"
+    Title="Issue 10735">
+   <StackLayout>
+       <Label
+           Padding="12"
+           BackgroundColor="Black"
+           TextColor="White"
+           Text="If this sample works without exceptions, the test has passed."/>
+		<CollectionView
+            x:Name="_collectionView"
+		    ItemsSource="{Binding Items}"
+		    VerticalOptions="Fill"
+		    ItemSizingStrategy="MeasureAllItems"
+		    ItemsUpdatingScrollMode="KeepLastItemInView">
+			<CollectionView.ItemTemplate>
+				<DataTemplate>
+					<Label
+                        Text="{Binding}"
+						HorizontalOptions="Center"
+						VerticalOptions="Center"
+						FontSize="30" />
+				</DataTemplate>
+			</CollectionView.ItemTemplate>
+		</CollectionView>
+
+		<StackLayout
+            VerticalOptions="End"
+			Orientation="Horizontal">
+			<Editor
+                x:Name="_editor"
+				HorizontalOptions="CenterAndExpand"
+				AutoSize="TextChanges" />
+			<Button
+                x:Name="_button" 
+				HorizontalOptions="CenterAndExpand"/>
+		</StackLayout>
+	</StackLayout>	
+</controls:TestContentPage>

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Issue10735.xaml.cs

@@ -0,0 +1,77 @@
+using Xamarin.Forms.Internals;
+using Xamarin.Forms.CustomAttributes;
+using System.Threading.Tasks;
+using System.Collections.ObjectModel;
+
+#if UITEST
+using Xamarin.Forms.Core.UITests;
+using Xamarin.UITest;
+using NUnit.Framework;
+#endif
+
+namespace Xamarin.Forms.Controls
+{
+#if UITEST
+	[Category(UITestCategories.CollectionView)]
+#endif
+	[Preserve(AllMembers = true)]
+	[Issue(IssueTracker.Github, 10735, "[Bug] [Fatal] [Android] CollectionView Causes Application Crash When Keyboard Opens", PlatformAffected.Android)]
+	public partial class Issue10735 : TestContentPage
+    {
+        readonly int _addItemDelay = 300;
+        int _item = 0;
+#if APP
+        readonly int _changeFocusDelay = 1000;
+        View _lastFocus;
+#endif
+
+        public Issue10735()
+		{
+#if APP
+			InitializeComponent();
+            BindingContext = this;
+            StartAddingMessages();
+#endif
+        }
+
+        public ObservableCollection<string> Items { get; } = new ObservableCollection<string>();
+
+        protected override void Init()
+		{
+
+        }
+
+		void StartAddingMessages()
+        {
+            Task.Run(async () =>
+            {
+                while (true)
+                {
+                    await Task.Delay(_addItemDelay);
+                    Items.Add(_item.ToString());
+                    _item++;
+                }
+            });
+#if APP
+            Task.Run(async () =>
+            {
+                while (true)
+                {
+                    await Task.Delay(_changeFocusDelay);
+                    Device.BeginInvokeOnMainThread(() =>
+                    {
+                        _lastFocus?.Unfocus();
+
+                        if (_lastFocus == _editor)
+                            _lastFocus = _button;
+                        else
+                            _lastFocus = _editor;
+                        
+                        _lastFocus.Focus();
+                    });
+                }
+            });
+#endif
+        }
+    }
+}

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

@@ -1405,10 +1405,14 @@
     <Compile Include="$(MSBuildThisFileDirectory)Issue10530.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue7780.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue8958.xaml.cs" />
+
+    <Compile Include="$(MSBuildThisFileDirectory)Issue10735.xaml.cs" />
+
     <Compile Include="$(MSBuildThisFileDirectory)Issue10497.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue10477.xaml.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue10875.xaml.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue10708.cs" />
+
     <Compile Include="$(MSBuildThisFileDirectory)Issue9711.xaml.cs">
       <DependentUpon>Issue9711.xaml</DependentUpon>
       <SubType>Code</SubType>
@@ -1654,6 +1658,9 @@
     <EmbeddedResource Include="$(MSBuildThisFileDirectory)Issue8958.xaml">
       <Generator>MSBuild:UpdateDesignTimeXaml</Generator>
     </EmbeddedResource>
+    <EmbeddedResource Include="$(MSBuildThisFileDirectory)Issue10735.xaml">
+      <Generator>MSBuild:UpdateDesignTimeXaml</Generator>
+    </EmbeddedResource>
     <EmbeddedResource Include="$(MSBuildThisFileDirectory)Issue10477.xaml">
       <SubType>Designer</SubType>
       <Generator>MSBuild:UpdateDesignTimeXaml</Generator>

Xamarin.Forms.Core.UnitTests/MarshalingObservableCollectionTests.cs

@@ -0,0 +1,338 @@
+using System;
+using System.Collections.Concurrent;
+using System.Collections.ObjectModel;
+using System.IO;
+using System.Reflection;
+using System.Threading;
+using System.Threading.Tasks;
+using NUnit.Framework;
+using Xamarin.Forms.Internals;
+
+namespace Xamarin.Forms.Core.UnitTests
+{
+	[TestFixture]
+	public class MarshalingObservableCollectionTests 
+	{
+		MarshalingTestPlatformServices _services;
+
+		[SetUp]
+		public void Setup()
+		{
+			_services = new MarshalingTestPlatformServices();
+			Device.PlatformServices = _services;
+			_services.Start();
+		}
+
+		[TearDown]
+		public void TearDown()
+		{
+			_services.Stop();
+			_services = null;
+		}
+
+		[Test]
+		[Description("Added items don't show up until they've been processed on the UI thread")]
+		public async Task AddOffUIThread()
+		{
+			int insertCount = 0;
+			var countFromThreadPool = -1;
+
+			var source = new ObservableCollection<int>();
+			var moc = new MarshalingObservableCollection(source);
+
+			moc.CollectionChanged += (sender, args) => {
+				if (args.Action == System.Collections.Specialized.NotifyCollectionChangedAction.Add)
+				{
+					insertCount += 1;
+				}
+			};
+
+			// Add an item from a threadpool thread
+			await Task.Run(() => 
+			{
+				var x = Thread.CurrentThread.ManagedThreadId;
+				source.Add(1);
+				countFromThreadPool = moc.Count;
+			});
+
+			// Check the result on the main thread
+			var onMainThreadCount = await Device.InvokeOnMainThreadAsync<int>(() => 
+			{ 
+				return moc.Count; 
+			});
+
+			Assert.That(countFromThreadPool, Is.EqualTo(0), "Count should be zero because the update on the UI thread hasn't run yet");
+			Assert.That(onMainThreadCount, Is.EqualTo(1), "Count should be 1 because the UI thread has updated");
+			Assert.That(insertCount, Is.EqualTo(1), "The CollectionChanged event should have fired with an Add exactly 1 time");
+		}
+
+		[Test]
+		[Description("Intial item count should match wrapped collection.")]
+		public async Task InitialItemCountsMatch()
+		{
+			var source = new ObservableCollection<int> { 1, 2 };
+
+			var moc = new MarshalingObservableCollection(source);
+
+			Assert.That(source.Count, Is.EqualTo(moc.Count));
+		}
+
+		[Test]
+		[Description("Clears don't show up until they've been processed on the UI thread")]
+		public async Task ClearOnUIThread()
+		{
+			var countFromThreadPool = -1;
+
+			var source = new ObservableCollection<int>
+			{
+				1,
+				2
+			};
+
+			var moc = new MarshalingObservableCollection(source);
+
+			// Call Clear from a threadpool thread
+			await Task.Run(() =>
+			{
+				source.Clear();
+				countFromThreadPool = moc.Count;
+			});
+
+			// Check the result on the main thread
+			var onMainThreadCount = await Device.InvokeOnMainThreadAsync<int>(() => moc.Count);
+
+			Assert.That(countFromThreadPool, Is.EqualTo(2), "Count should be pre-clear");
+			Assert.That(onMainThreadCount, Is.EqualTo(0), "Count should be zero because the Clear has been processed");
+		}
+
+		[Test]
+		[Description("A Reset should reflect the state at the time of the Reset")]
+		public async Task ClearAndAddOffUIThread()
+		{
+			var countFromThreadPool = -1;
+
+			var source = new ObservableCollection<int>
+			{
+				1,
+				2
+			};
+
+			var moc = new MarshalingObservableCollection(source);
+
+			// Call Clear from a threadpool thread
+			await Task.Run(() =>
+			{
+				source.Clear();
+				source.Add(4);
+				countFromThreadPool = moc.Count;
+			});
+
+			// Check the result on the main thread
+			var onMainThreadCount = await Device.InvokeOnMainThreadAsync<int>(() => moc.Count);
+
+			Assert.That(countFromThreadPool, Is.EqualTo(2), "Count should be pre-clear");
+			Assert.That(onMainThreadCount, Is.EqualTo(1), "Should have processed a Clear and an Add");
+		}
+
+		[Test]
+		[Description("Removed items are still there until they're removed on the UI thread")]
+		public async Task RemoveOffUIThread()
+		{
+			var countFromThreadPool = -1;
+
+			var source = new ObservableCollection<int> { 1, 2 };
+
+			var moc = new MarshalingObservableCollection(source);
+
+			// Call Clear from a threadpool thread
+			await Task.Run(() =>
+			{
+				source.Remove(1);
+				countFromThreadPool = moc.Count;
+			});
+
+			// Check the result on the main thread
+			var onMainThreadCount = await Device.InvokeOnMainThreadAsync<int>(() => moc.Count);
+
+			Assert.That(countFromThreadPool, Is.EqualTo(2), "Count should be pre-remove");
+			Assert.That(onMainThreadCount, Is.EqualTo(1), "Remove has now processed");
+		}
+
+		[Test]
+		[Description("Until the UI thread processes a change, the indexer should remain consistent")]
+		public async Task IndexerConsistent()
+		{
+			int itemFromThreadPool = -1;
+
+			var source = new ObservableCollection<int> { 1, 2 };
+
+			var moc = new MarshalingObservableCollection(source);
+
+			// Call Remove from a threadpool thread
+			await Task.Run(() =>
+			{
+				source.Remove(1);
+				itemFromThreadPool = (int)moc[1];
+			});
+
+			Assert.That(itemFromThreadPool, Is.EqualTo(2), "Should have indexer value from before remove");
+		}
+
+		[Test]
+		[Description("Don't show replacements until the UI thread has processed them")]
+		public async Task ReplaceOffUIThread()
+		{
+			int itemFromThreadPool = -1;
+
+			var source = new ObservableCollection<int> { 1, 2 };
+
+			var moc = new MarshalingObservableCollection(source);
+
+			// Replace a value from a threadpool thread
+			await Task.Run(() =>
+			{
+				source[0] = 42;
+				itemFromThreadPool = (int)moc[0];
+			});
+
+			// Check the result on the main thread
+			var onMainThreadValue = await Device.InvokeOnMainThreadAsync(() => moc[0]);
+
+			Assert.That(itemFromThreadPool, Is.EqualTo(1), "Should have value from before replace");
+			Assert.That(onMainThreadValue, Is.EqualTo(42), "Should have value from after replace");
+		}
+
+		[Test]
+		[Description("Don't show moves until the UI thread has processed them")]
+		public async Task MoveOffUIThread()
+		{
+			int itemFromThreadPool = -1;
+
+			var source = new ObservableCollection<int> { 1, 2 };
+
+			var moc = new MarshalingObservableCollection(source);
+
+			// Replace a value from a threadpool thread
+			await Task.Run(() =>
+			{
+				source.Move(1, 0);
+				itemFromThreadPool = (int)moc[0];
+			});
+
+			// Check the result on the main thread
+			var onMainThreadValue = await Device.InvokeOnMainThreadAsync(() => moc[0]);
+
+			Assert.That(itemFromThreadPool, Is.EqualTo(1), "Should have value from before move");
+			Assert.That(onMainThreadValue, Is.EqualTo(2), "Should have value from after move");
+		}
+
+		// This class simulates running a single UI thread with a queue and non-UI threads;
+		// this allows us to test IsInvokeRequired/BeginInvoke without having to be on an actual device
+		class MarshalingTestPlatformServices : IPlatformServices
+		{
+			int _threadId;
+			bool _running;
+			BlockingCollection<Action> _todo = new BlockingCollection<Action>();
+
+			public void Stop()
+			{
+				_running = false;
+				_todo.CompleteAdding();
+			}
+
+			public void Start()
+			{
+				_running = true;
+				Task.Run(() => {
+
+					if (_threadId == 0)
+					{
+						_threadId = Thread.CurrentThread.ManagedThreadId;
+					}
+
+					while (_running)
+					{
+						try
+						{
+							_todo.Take()?.Invoke();
+						}
+						catch (Exception ex)
+						{
+							Stop();
+						}
+					}
+				});
+			}
+
+			public bool IsInvokeRequired => Thread.CurrentThread.ManagedThreadId != _threadId;
+
+			public void BeginInvokeOnMainThread(Action action)
+			{
+				_todo.Add(action);
+			}
+
+			public OSAppTheme RequestedTheme { get; }
+			public string RuntimePlatform { get; }
+
+			public Ticker CreateTicker()
+			{
+				throw new NotImplementedException();
+			}
+
+			public Assembly[] GetAssemblies()
+			{
+				throw new NotImplementedException();
+			}
+
+			public Color GetNamedColor(string name)
+			{
+				throw new NotImplementedException();
+			}
+
+			public double GetNamedSize(NamedSize size, Type targetElementType, bool useOldSizes)
+			{
+				throw new NotImplementedException();
+			}
+
+			public SizeRequest GetNativeSize(VisualElement view, double widthConstraint, double heightConstraint)
+			{
+				throw new NotImplementedException();
+			}
+
+			public Task<Stream> GetStreamAsync(Uri uri, CancellationToken cancellationToken)
+			{
+				throw new NotImplementedException();
+			}
+
+			public IIsolatedStorageFile GetUserStoreForApplication()
+			{
+				throw new NotImplementedException();
+			}
+
+			public void OpenUriAction(Uri uri)
+			{
+				throw new NotImplementedException();
+			}
+
+			public void QuitApplication()
+			{
+				throw new NotImplementedException();
+			}
+
+			public void StartTimer(TimeSpan interval, Func<bool> callback)
+			{
+				throw new NotImplementedException();
+			}
+
+			public string GetHash(string input)
+			{
+				throw new NotImplementedException();
+			}
+
+			public string GetMD5Hash(string input)
+			{
+				throw new NotImplementedException();
+			}
+		}
+	}
+}

Xamarin.Forms.Core.UnitTests/Xamarin.Forms.Core.UnitTests.csproj

@@ -96,6 +96,7 @@
     <Compile Include="Markup\ViewInFlexLayoutExtensionsTests.cs" />
     <Compile Include="Markup\ViewInGridExtensionsTests.cs" />
     <Compile Include="Markup\VisualElementExtensionsTests.cs" />
+    <Compile Include="MarshalingObservableCollectionTests.cs" />
     <Compile Include="NumericExtensionsTests.cs" />
     <Compile Include="RefreshViewTests.cs" />
     <Compile Include="MockDispatcherProvider.cs" />
@@ -257,7 +258,7 @@
   </ItemGroup>
   <ItemGroup>
     <ProjectReference Include="..\Xamarin.Forms.Core\Xamarin.Forms.Core.csproj">
-      <Project>{57B8B73D-C3B5-4C42-869E-7B2F17D354AC}</Project>
+      <Project>{57b8b73d-c3b5-4c42-869e-7b2f17d354ac}</Project>
       <Name>Xamarin.Forms.Core</Name>
     </ProjectReference>
     <ProjectReference Include="..\Xamarin.Forms.Maps\Xamarin.Forms.Maps.csproj">
@@ -290,7 +291,7 @@
   <ItemGroup />
   <Target Name="_CopyNUnitTestAdapterFiles" AfterTargets="Build">
     <ItemGroup>
-      <_NUnitTestAdapterFiles Include="$(NuGetPackageRoot)NUnit3TestAdapter\%(Version)\build\net35\**" Condition="@(PackageReference -&gt; '%(Identity)') == 'NUnit3TestAdapter'" InProject="False" />
+      <_NUnitTestAdapterFiles Include="$(NuGetPackageRoot)NUnit3TestAdapter\%(Version)\build\net35\**" Condition="@(PackageReference -> '%(Identity)') == 'NUnit3TestAdapter'" InProject="False" />
     </ItemGroup>
     <Copy SourceFiles="@(_NUnitTestAdapterFiles)" DestinationFolder="$(SolutionDir)packages\NUnitTestAdapter.AnyVersion\tools\%(RecursiveDir)" ContinueOnError="true" Retries="0" />
     <Copy SourceFiles="@(_NUnitTestAdapterFiles)" DestinationFolder="$(SolutionDir)packages\NUnitTestAdapter.AnyVersion\build\%(RecursiveDir)" ContinueOnError="true" Retries="0" />

Xamarin.Forms.Core/Items/MarshalingObservableCollection.cs

@@ -0,0 +1,157 @@
+using System;
+using System.Collections;
+using System.Collections.Generic;
+using System.Collections.Specialized;
+
+namespace Xamarin.Forms
+{
+	// Wraps a List which implements INotifyCollectionChanged (usually an ObservableCollection)
+	// and marshals all of the list modifications to the main thread. Modifications to the underlying 
+	// collection which are made off of the main thread remain invisible to consumers on the main thread
+	// until they have been processed by the main thread.
+
+	public class MarshalingObservableCollection : List<object>, INotifyCollectionChanged
+	{
+		readonly IList _internalCollection;
+
+		public MarshalingObservableCollection(IList list)
+		{
+			if (!(list is INotifyCollectionChanged incc))
+			{
+				throw new ArgumentException($"{nameof(list)} must implement {nameof(INotifyCollectionChanged)}");
+			}
+
+			_internalCollection = list;
+			incc.CollectionChanged += InternalCollectionChanged;
+
+			foreach (var item in _internalCollection)
+			{
+				Add(item);
+			}
+		}
+
+		class ResetNotifyCollectionChangedEventArgs : NotifyCollectionChangedEventArgs 
+		{
+			public IList Items { get; }
+			public ResetNotifyCollectionChangedEventArgs(IList items) 
+				: base(NotifyCollectionChangedAction.Reset) => Items = items;
+		}
+
+		public event NotifyCollectionChangedEventHandler CollectionChanged;
+
+		void OnCollectionChanged(NotifyCollectionChangedEventArgs args) 
+		{
+			CollectionChanged?.Invoke(this, args);
+		}
+
+		void InternalCollectionChanged(object sender, NotifyCollectionChangedEventArgs args)
+		{
+			if (args.Action == NotifyCollectionChangedAction.Reset)
+			{
+				var items = new List<object>();
+				for(int n = 0; n < _internalCollection.Count; n++)
+				{
+					items.Add(_internalCollection[n]);
+				}
+
+				args = new ResetNotifyCollectionChangedEventArgs(items);
+			}
+
+			if (Device.IsInvokeRequired)
+			{
+				Device.BeginInvokeOnMainThread(() => HandleCollectionChange(args));
+			}
+			else
+			{
+				HandleCollectionChange(args);
+			}
+		}
+
+		void HandleCollectionChange(NotifyCollectionChangedEventArgs args) 
+		{
+			switch (args.Action)
+			{
+				case NotifyCollectionChangedAction.Add:
+					Add(args);
+					break;
+				case NotifyCollectionChangedAction.Move:
+					Move(args);
+					break;
+				case NotifyCollectionChangedAction.Remove:
+					Remove(args);
+					break;
+				case NotifyCollectionChangedAction.Replace:
+					Replace(args);
+					break;
+				case NotifyCollectionChangedAction.Reset:
+					Reset(args);
+					break;
+			}
+		}
+
+		void Move(NotifyCollectionChangedEventArgs args)
+		{
+			var count = args.OldItems.Count;
+
+			for (int n = 0; n < count; n++)
+			{
+				var toMove = this[args.OldStartingIndex];
+				RemoveAt(args.OldStartingIndex);
+				Insert(args.NewStartingIndex, toMove);
+			}
+
+			OnCollectionChanged(args);
+		}
+
+		void Remove(NotifyCollectionChangedEventArgs args)
+		{
+			var startIndex = args.OldStartingIndex + args.OldItems.Count - 1;
+			for (int n = startIndex; n >= args.OldStartingIndex; n--)
+			{
+				RemoveAt(n);
+			}
+
+			OnCollectionChanged(args);
+		}
+
+		void Replace(NotifyCollectionChangedEventArgs args) 
+		{
+			var startIndex = args.NewStartingIndex;
+			foreach (var item in args.NewItems)
+			{
+				this[startIndex] = item;
+				startIndex += 1;
+			}
+
+			OnCollectionChanged(args);
+		}
+
+		void Add(NotifyCollectionChangedEventArgs args) 
+		{
+			var startIndex = args.NewStartingIndex;
+			foreach (var item in args.NewItems)
+			{
+				Insert(startIndex, item);
+				startIndex += 1;
+			}
+
+			OnCollectionChanged(args);
+		}
+
+		void Reset(NotifyCollectionChangedEventArgs args)
+		{
+			if (!(args is ResetNotifyCollectionChangedEventArgs resetArgs))
+			{
+				throw new InvalidOperationException($"Cannot guarantee collection accuracy for Resets which do not use {nameof(ResetNotifyCollectionChangedEventArgs)}");
+			}
+
+			Clear();
+			foreach (var item in resetArgs.Items)
+			{
+				Add(item);
+			}
+
+			OnCollectionChanged(args);
+		}
+	}
+}

Xamarin.Forms.Platform.Android.UnitTests/ObservrableItemsSourceTests.cs

@@ -0,0 +1,194 @@
+using System.Collections.ObjectModel;
+using System.Threading.Tasks;
+using Android.OS;
+using Java.Lang;
+using NUnit.Framework;
+using NUnit.Framework.Internal;
+
+namespace Xamarin.Forms.Platform.Android.UnitTests
+{
+	[TestFixture]
+	public class ObservrableItemsSourceTests 
+	{
+		Handler _handler = new Handler(Looper.MainLooper);
+
+		[Test, Category("CollectionView")]
+		[Description("Off-main-thread modifications to the source should be reflected in the count when the main thread has processed them.")]
+		public async Task ObservableSourceItemsCountConsistent()
+		{
+			var source = new ObservableCollection<int>();
+
+			source.Add(1);
+			source.Add(2);
+
+			var ois = ItemsSourceFactory.Create(source, new MockCollectionChangedNotifier());
+
+			Assert.That(ois.Count, Is.EqualTo(2));
+
+			source.Add(3);
+
+			var count = await Device.InvokeOnMainThreadAsync(() => ois.Count);
+
+			Assert.That(ois.Count, Is.EqualTo(3));
+		}
+
+		[Test, Category("CollectionView")]
+		[Description("Off-main-thread Adds should be reflected once the main thread has processed them.")]
+		public async Task AddItemCountConsistentOnUIThread()
+		{
+			var notifier = new MockCollectionChangedNotifier();
+			var source = new ObservableCollection<int>();
+			IItemsViewSource ois = ItemsSourceFactory.Create(source, notifier);
+
+			int countBeforeNotify = -1;
+
+			// Add an item from a threadpool thread
+			await Task.Run(() => { 
+				source.Add(1);
+
+				// Post a check ahead of the queued update on the main thread
+				_handler.PostAtFrontOfQueue(() => {
+					countBeforeNotify = ois.Count;
+				});
+			});
+
+			// Check the result on the main thread
+			var onMainThreadCount = await Device.InvokeOnMainThreadAsync(() => ois.Count);
+
+			Assert.That(countBeforeNotify, Is.EqualTo(0), "Count should still be reporting no items before the notify resolves");
+			Assert.That(onMainThreadCount, Is.EqualTo(1));
+			Assert.That(notifier.InsertCount, Is.EqualTo(1), "Should have recorded exactly one Add");
+		}
+
+		[Test, Category("CollectionView")]
+		[Description("????")]
+		public async Task RemoveitemCountConsistentOnUIThread()
+		{
+			var notifier = new MockCollectionChangedNotifier();
+			var source = new ObservableCollection<int> { 1 };
+			IItemsViewSource ois = ItemsSourceFactory.Create(source, notifier);
+
+			int countBeforeNotify = -1;
+
+			// Remove an item from a threadpool thread
+			await Task.Run(() =>
+			{
+				source.Remove(1);
+				
+				// Post a check ahead of the queued update on the main thread
+				_handler.PostAtFrontOfQueue(() => countBeforeNotify = ois.Count);
+			});
+
+			// Check the result on the main thread
+			var onMainThreadCount = await Device.InvokeOnMainThreadAsync(() => ois.Count);
+
+			Assert.That(countBeforeNotify, Is.EqualTo(1));
+			Assert.That(onMainThreadCount, Is.EqualTo(0));
+			Assert.That(notifier.RemoveCount, Is.EqualTo(1));
+		}
+
+		[Test, Category("CollectionView")]
+		[Description("????")]
+		public async Task GetItemConsistentOnUIThread()
+		{
+			var notifier = new MockCollectionChangedNotifier();
+			var source = new ObservableCollection<string>
+			{
+				"zero",
+				"one",
+				"two"
+			};
+
+			IItemsViewSource ois = ItemsSourceFactory.Create(source, notifier);
+
+			string itemAtPosition2BeforeNotify = string.Empty;
+
+			// Add an item from a threadpool thread
+			await Task.Run(() => {
+				source.Insert(0, "foo");
+
+				// Post a check ahead of the queued update on the main thread
+				_handler.PostAtFrontOfQueue(() => itemAtPosition2BeforeNotify = (string)ois.GetItem(2));
+			});
+
+			// Check the result on the main thread
+			var onMainThreadGetItem = await Device.InvokeOnMainThreadAsync(() => (string)ois.GetItem(2));
+
+			Assert.That(itemAtPosition2BeforeNotify, Is.EqualTo("two"));
+			Assert.That(onMainThreadGetItem, Is.EqualTo("one"));
+			Assert.That(notifier.InsertCount, Is.EqualTo(1));
+		}
+
+		[Test, Category("CollectionView")]
+		[Description("????")]
+		public async Task GetPositionConsistentOnUIThread()
+		{
+			var notifier = new MockCollectionChangedNotifier();
+			var source = new ObservableCollection<string>
+			{
+				"zero",
+				"one",
+				"two"
+			};
+
+			IItemsViewSource ois = ItemsSourceFactory.Create(source, notifier);
+
+			int positionBeforeNotify = -1;
+
+			// Add an item from a threadpool thread
+			await Task.Run(() => { 
+				source.Insert(0, "foo");
+
+				// Post a check ahead of the queued update on the main thread
+				_handler.PostAtFrontOfQueue(() => positionBeforeNotify = ois.GetPosition("zero"));
+			});
+
+			// Check the result on the main thread
+			var onMainThreadGetItem = await Device.InvokeOnMainThreadAsync(() => ois.GetPosition("zero"));
+
+			Assert.That(positionBeforeNotify, Is.EqualTo(0));
+			Assert.That(onMainThreadGetItem, Is.EqualTo(1));
+			Assert.That(notifier.InsertCount, Is.EqualTo(1));
+		}
+
+		class MockCollectionChangedNotifier : ICollectionChangedNotifier
+		{
+			public int InsertCount;
+			public int RemoveCount;
+
+			public void NotifyDataSetChanged()
+			{
+			}
+
+			public void NotifyItemChanged(IItemsViewSource source, int startIndex)
+			{
+			}
+
+			public void NotifyItemInserted(IItemsViewSource source, int startIndex)
+			{
+				InsertCount += 1;
+			}
+
+			public void NotifyItemMoved(IItemsViewSource source, int fromPosition, int toPosition)
+			{
+			}
+
+			public void NotifyItemRangeChanged(IItemsViewSource source, int start, int end)
+			{
+			}
+
+			public void NotifyItemRangeInserted(IItemsViewSource source, int startIndex, int count)
+			{
+			}
+
+			public void NotifyItemRangeRemoved(IItemsViewSource source, int startIndex, int count)
+			{
+			}
+
+			public void NotifyItemRemoved(IItemsViewSource source, int startIndex)
+			{
+				RemoveCount += 1;
+			}
+		}
+	}
+}

Xamarin.Forms.Platform.Android.UnitTests/Xamarin.Forms.Platform.Android.UnitTests.csproj

@@ -54,6 +54,7 @@
     <Compile Include="IsEnabledTests.cs" />
     <Compile Include="Issues.cs" />
     <Compile Include="IsVisibleTests.cs" />
+    <Compile Include="ObservrableItemsSourceTests.cs" />
     <Compile Include="OpacityTests.cs" />
     <Compile Include="RendererTests.cs" />
     <Compile Include="PlatformTestFixture.cs" />

Xamarin.Forms.Platform.Android/CollectionView/ItemsSourceFactory.cs

@@ -1,6 +1,8 @@
 using System.Collections;
 using System.Collections.Generic;
+using System.Collections.ObjectModel;
 using System.Collections.Specialized;
+using System.IO;
 #if __ANDROID_29__
 using AndroidX.AppCompat.Widget;
 using AndroidX.RecyclerView.Widget;
@@ -21,8 +23,8 @@ namespace Xamarin.Forms.Platform.Android
 
 			switch (itemsSource)
 			{
-				case IList _ when itemsSource is INotifyCollectionChanged:
+				case IList list when itemsSource is INotifyCollectionChanged:
-					return new ObservableItemsSource(itemsSource as IList, notifier);
+					return new ObservableItemsSource(new MarshalingObservableCollection(list), notifier);
 				case IEnumerable _ when itemsSource is INotifyCollectionChanged:
 					return new ObservableItemsSource(itemsSource as IEnumerable, notifier);
 				case IEnumerable<object> generic:

Xamarin.Forms.Platform.Android/CollectionView/ItemsViewRenderer.cs

@@ -667,7 +667,7 @@ namespace Xamarin.Forms.Platform.Android
 		{
 			if (ItemsView.ItemsUpdatingScrollMode == ItemsUpdatingScrollMode.KeepLastItemInView)
 			{
-				ScrollTo(new ScrollToRequestEventArgs(ItemsViewAdapter.ItemCount, 0,
+				ScrollTo(new ScrollToRequestEventArgs(GetLayoutManager().ItemCount, 0,
 					Xamarin.Forms.ScrollToPosition.MakeVisible, true));
 			}
 			else if (ItemsView.ItemsUpdatingScrollMode == ItemsUpdatingScrollMode.KeepScrollOffset)