From 34165550f5757d2fe3a17b9a3108109d0fbdfcce Mon Sep 17 00:00:00 2001
From: "E.Z. Hart" <hartez@users.noreply.github.com>
Date: Tue, 28 May 2019 11:54:09 -0600
Subject: [PATCH] Make CollectionView SelectedItem and SelectedItems binding
 function correctly (#6085)

* Add automated test for CollectionView single selection bound item

* Make SelectedItem Two-Way

* Multiple selection test page

* Bindable SelectedItems implementation

* Add automated test

* Simplify null checks

* Add Preserve attribute so linker doesn't break test

* Make multi-item select test smaller so it passes UITests on smaller screens

* Clearer list-to-string method

* Clear native selection on iOS when SelectedItem set to null

fixes #6158
fixes #5832
---
 .../CollectionViewBoundMultiSelection.cs           | 63 +++++++++++++++++
 .../CollectionViewBoundSingleSelection.cs          | 44 ++++++++++++
 .../Xamarin.Forms.Controls.Issues.Shared.projitems |  2 +
 .../SelectionGalleries/BoundSelectionModel.cs      | 82 ++++++++++++++++++++++
 .../SelectionGalleries/MultipleBoundSelection.xaml | 34 +++++++++
 .../MultipleBoundSelection.xaml.cs                 | 42 +++++++++++
 .../SelectionGalleries/SelectionGallery.cs         |  4 ++
 .../SelectionGalleries/SelectionHelpers.cs         | 18 +++++
 .../SelectionModeGallery.xaml.cs                   | 17 +----
 .../SelectionGalleries/SingleBoundSelection.xaml   | 33 +++++++++
 .../SingleBoundSelection.xaml.cs                   | 28 ++++++++
 .../Xamarin.Forms.Controls.csproj                  |  3 +
 Xamarin.Forms.Core/Items/SelectableItemsView.cs    | 46 ++++++++++--
 Xamarin.Forms.Core/Items/SelectionList.cs          | 69 +++++++++++-------
 .../SelectableItemsViewController.cs               | 16 +++++
 15 files changed, 457 insertions(+), 44 deletions(-)
 create mode 100644 Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/CollectionViewBoundMultiSelection.cs
 create mode 100644 Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/CollectionViewBoundSingleSelection.cs
 create mode 100644 Xamarin.Forms.Controls/GalleryPages/CollectionViewGalleries/SelectionGalleries/BoundSelectionModel.cs
 create mode 100644 Xamarin.Forms.Controls/GalleryPages/CollectionViewGalleries/SelectionGalleries/MultipleBoundSelection.xaml
 create mode 100644 Xamarin.Forms.Controls/GalleryPages/CollectionViewGalleries/SelectionGalleries/MultipleBoundSelection.xaml.cs
 create mode 100644 Xamarin.Forms.Controls/GalleryPages/CollectionViewGalleries/SelectionGalleries/SelectionHelpers.cs
 create mode 100644 Xamarin.Forms.Controls/GalleryPages/CollectionViewGalleries/SelectionGalleries/SingleBoundSelection.xaml
 create mode 100644 Xamarin.Forms.Controls/GalleryPages/CollectionViewGalleries/SelectionGalleries/SingleBoundSelection.xaml.cs

diff --git a/Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/CollectionViewBoundMultiSelection.cs b/Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/CollectionViewBoundMultiSelection.cs
new file mode 100644
index 0000000..815befa
--- /dev/null
+++ b/Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/CollectionViewBoundMultiSelection.cs
@@ -0,0 +1,63 @@
+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.None, 47803, "CollectionView: Multi Selection Binding", PlatformAffected.All)]
+	public class CollectionViewBoundMultiSelection : TestNavigationPage
+	{
+		protected override void Init()
+		{
+#if APP
+			Device.SetFlags(new List<string>(Device.Flags ?? new List<string>()) { "CollectionView_Experimental" });
+
+			PushAsync(new GalleryPages.CollectionViewGalleries.SelectionGalleries.MultipleBoundSelection());
+#endif
+		}
+
+#if UITEST
+		[Test]
+		public void ItemsFromViewModelShouldBeSelected()
+		{
+			// Initially Items 1 and 2 should be selected (from the view model)
+			RunningApp.WaitForElement("Selected: Item 1, Item 2");
+
+			// Tapping Item 3 should select it and updating the binding
+			RunningApp.Tap("Item 3");	
+			RunningApp.WaitForElement("Selected: Item 1, Item 2, Item 3");
+
+			// Test clearing the selection from the view model and updating it
+			RunningApp.Tap("ClearAndAdd");	
+			RunningApp.WaitForElement("Selected: Item 1, Item 2");
+
+			// Test removing an item from the selection
+			RunningApp.Tap("Item 2");
+			RunningApp.WaitForElement("Selected: Item 1");
+
+			// Test setting a new selection list in the view mdoel 
+			RunningApp.Tap("Reset");	
+			RunningApp.WaitForElement("Selected: Item 1, Item 2");
+
+			RunningApp.Tap("Item 0");
+			
+			// Test setting the selection directly with CollectionView.SelectedItems 
+			RunningApp.Tap("DirectUpdate");	
+			RunningApp.WaitForElement("Selected: Item 0, Item 3");
+		}
+#endif
+	}
+}
\ No newline at end of file
diff --git a/Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/CollectionViewBoundSingleSelection.cs b/Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/CollectionViewBoundSingleSelection.cs
new file mode 100644
index 0000000..cb782e0
--- /dev/null
+++ b/Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/CollectionViewBoundSingleSelection.cs
@@ -0,0 +1,44 @@
+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.None, 4539134, "CollectionView: Single Selection Binding", PlatformAffected.All)]
+	public class CollectionViewBoundSingleSelection : TestNavigationPage
+	{
+		protected override void Init()
+		{
+#if APP
+			Device.SetFlags(new List<string>(Device.Flags ?? new List<string>()) { "CollectionView_Experimental" });
+
+			PushAsync(new GalleryPages.CollectionViewGalleries.SelectionGalleries.SingleBoundSelection());
+#endif
+		}
+
+#if UITEST
+		[Test]
+		public void SelectionShouldUpdateBinding()
+		{
+			// Initially Item 2 should be selected (from the view model)
+			RunningApp.WaitForElement("Selected: Item: 2");
+
+			// Tapping Item 3 should select it and updating the binding
+			RunningApp.Tap("Item 3");	
+			RunningApp.WaitForElement("Selected: Item: 3");
+		}
+#endif
+	}
+}
diff --git a/Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Xamarin.Forms.Controls.Issues.Shared.projitems b/Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Xamarin.Forms.Controls.Issues.Shared.projitems
index c5a8752..bdd2727 100644
--- a/Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Xamarin.Forms.Controls.Issues.Shared.projitems
+++ b/Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Xamarin.Forms.Controls.Issues.Shared.projitems
@@ -14,6 +14,8 @@
     <Compile Include="$(MSBuildThisFileDirectory)Bugzilla59172.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)FlagTestHelpers.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue5766.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)CollectionViewBoundMultiSelection.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)CollectionViewBoundSingleSelection.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue4684.xaml.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue4992.xaml.cs">
       <DependentUpon>Issue4992.xaml</DependentUpon>
diff --git a/Xamarin.Forms.Controls/GalleryPages/CollectionViewGalleries/SelectionGalleries/BoundSelectionModel.cs b/Xamarin.Forms.Controls/GalleryPages/CollectionViewGalleries/SelectionGalleries/BoundSelectionModel.cs
new file mode 100644
index 0000000..838103c
--- /dev/null
+++ b/Xamarin.Forms.Controls/GalleryPages/CollectionViewGalleries/SelectionGalleries/BoundSelectionModel.cs
@@ -0,0 +1,82 @@
+using System;
+using System.Collections.ObjectModel;
+using System.ComponentModel;
+using System.Runtime.CompilerServices;
+using Xamarin.Forms.Internals;
+
+namespace Xamarin.Forms.Controls.GalleryPages.CollectionViewGalleries.SelectionGalleries
+{
+	[Preserve(AllMembers = true)]
+	internal class BoundSelectionModel : INotifyPropertyChanged
+	{
+		private CollectionViewGalleryTestItem _selectedItem;
+		private ObservableCollection<CollectionViewGalleryTestItem> _items;
+		private ObservableCollection<object> _selectedItems;
+
+		public event PropertyChangedEventHandler PropertyChanged;
+
+		public BoundSelectionModel()
+		{
+			Items = new ObservableCollection<CollectionViewGalleryTestItem>();
+
+			for (int n = 0; n < 4; n++)
+			{
+				Items.Add(new CollectionViewGalleryTestItem(DateTime.Now.AddDays(n), $"Item {n}", "coffee.png", n));
+			}
+
+			SelectedItem = Items[2];
+
+			SelectedItems = new ObservableCollection<object>()
+			{
+				Items[1], Items[2]
+			};
+		}
+
+		private void SelectedItemsCollectionChanged(object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e)
+		{
+			OnPropertyChanged(nameof(SelectedItemsText));
+		}
+
+		void OnPropertyChanged([CallerMemberName] string propertyName = null)
+		{
+			PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
+		}
+
+		public CollectionViewGalleryTestItem SelectedItem
+		{
+			get => _selectedItem;
+			set
+			{
+				_selectedItem = value;
+				OnPropertyChanged();
+			}
+		}
+
+		public ObservableCollection<object> SelectedItems
+		{
+			get => _selectedItems;
+			set
+			{
+				if (_selectedItems != null)
+				{
+					_selectedItems.CollectionChanged -= SelectedItemsCollectionChanged;
+				}
+
+				_selectedItems = value;
+
+				_selectedItems.CollectionChanged += SelectedItemsCollectionChanged;
+
+				OnPropertyChanged();
+				OnPropertyChanged(nameof(SelectedItemsText));
+			}
+		}
+
+		public ObservableCollection<CollectionViewGalleryTestItem> Items
+		{
+			get => _items;
+			set { _items = value; OnPropertyChanged(); }
+		}
+
+		public string SelectedItemsText => SelectedItems.ToCommaSeparatedList();
+	}
+}
\ No newline at end of file
diff --git a/Xamarin.Forms.Controls/GalleryPages/CollectionViewGalleries/SelectionGalleries/MultipleBoundSelection.xaml b/Xamarin.Forms.Controls/GalleryPages/CollectionViewGalleries/SelectionGalleries/MultipleBoundSelection.xaml
new file mode 100644
index 0000000..d78cf95
--- /dev/null
+++ b/Xamarin.Forms.Controls/GalleryPages/CollectionViewGalleries/SelectionGalleries/MultipleBoundSelection.xaml
@@ -0,0 +1,34 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
+             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+             x:Class="Xamarin.Forms.Controls.GalleryPages.CollectionViewGalleries.SelectionGalleries.MultipleBoundSelection">
+    <ContentPage.Content>
+        <StackLayout Spacing="2">
+
+            <Label Text="The selected items in the CollectionView should always match the 'Selected' Label below. If it does not, this test has failed."
+                FontSize="10" VerticalOptions="CenterAndExpand" HorizontalOptions="CenterAndExpand" />
+
+            <Label Text="{Binding SelectedItemsText, StringFormat='{}Selected: {0}'}" FontAttributes="Bold"
+                FontSize="10" VerticalOptions="CenterAndExpand" HorizontalOptions="CenterAndExpand" />
+
+            <Button AutomationId="ClearAndAdd" HeightRequest="35" FontSize="10" Text="Clear VM selection and add Items 1 and 2" Clicked="ClearAndAdd" />
+
+            <Button AutomationId="Reset" HeightRequest="35" FontSize="10" Text="Set VM selection to new list" Clicked="ResetClicked" />
+
+            <Button AutomationId="DirectUpdate" HeightRequest="35" FontSize="10" Text="Clear CV selection and add Items 0 and 3" Clicked="DirectUpdateClicked" />
+
+            <CollectionView x:Name="CollectionView" ItemsSource="{Binding Items}" 
+                            SelectionMode="Multiple" SelectedItems="{Binding SelectedItems}">
+                <CollectionView.ItemTemplate>
+                    <DataTemplate>
+                        <StackLayout>
+                            <Image Source="{Binding Image}" HeightRequest="30" />
+                            <Label FontSize="10" Text="{Binding Caption}"></Label>
+                        </StackLayout>
+                    </DataTemplate>
+                </CollectionView.ItemTemplate>
+            </CollectionView>
+
+        </StackLayout>
+    </ContentPage.Content>
+</ContentPage>
\ No newline at end of file
diff --git a/Xamarin.Forms.Controls/GalleryPages/CollectionViewGalleries/SelectionGalleries/MultipleBoundSelection.xaml.cs b/Xamarin.Forms.Controls/GalleryPages/CollectionViewGalleries/SelectionGalleries/MultipleBoundSelection.xaml.cs
new file mode 100644
index 0000000..955f68e
--- /dev/null
+++ b/Xamarin.Forms.Controls/GalleryPages/CollectionViewGalleries/SelectionGalleries/MultipleBoundSelection.xaml.cs
@@ -0,0 +1,42 @@
+using System;
+using System.Collections.ObjectModel;
+using Xamarin.Forms.Xaml;
+
+namespace Xamarin.Forms.Controls.GalleryPages.CollectionViewGalleries.SelectionGalleries
+{
+	[XamlCompilation(XamlCompilationOptions.Compile)]
+	public partial class MultipleBoundSelection : ContentPage
+	{
+		BoundSelectionModel _vm;
+
+		public MultipleBoundSelection()
+		{
+			_vm = new BoundSelectionModel();
+			BindingContext = _vm;
+			InitializeComponent();
+		}
+
+		private void ClearAndAdd(object sender, EventArgs e)
+		{
+			_vm.SelectedItems.Clear();
+			_vm.SelectedItems.Add(_vm.Items[1]);
+			_vm.SelectedItems.Add(_vm.Items[2]);
+		}
+
+		private void ResetClicked(object sender, EventArgs e)
+		{
+			_vm.SelectedItems = new ObservableCollection<object>
+			{
+				_vm.Items[1],
+				_vm.Items[2]
+			};
+		}
+
+		private void DirectUpdateClicked(object sender, EventArgs e)
+		{
+			CollectionView.SelectedItems.Clear();
+			CollectionView.SelectedItems.Add(_vm.Items[0]);
+			CollectionView.SelectedItems.Add(_vm.Items[3]);
+		}
+	}
+}
\ No newline at end of file
diff --git a/Xamarin.Forms.Controls/GalleryPages/CollectionViewGalleries/SelectionGalleries/SelectionGallery.cs b/Xamarin.Forms.Controls/GalleryPages/CollectionViewGalleries/SelectionGalleries/SelectionGallery.cs
index a4c1517..6a10d5f 100644
--- a/Xamarin.Forms.Controls/GalleryPages/CollectionViewGalleries/SelectionGalleries/SelectionGallery.cs
+++ b/Xamarin.Forms.Controls/GalleryPages/CollectionViewGalleries/SelectionGalleries/SelectionGallery.cs
@@ -22,6 +22,10 @@
 							new PreselectedItemGallery(), Navigation),
 						GalleryBuilder.NavButton("Preselected Items", () =>
 							new PreselectedItemsGallery(), Navigation),
+						GalleryBuilder.NavButton("Single Selection, Bound", () =>
+							new SingleBoundSelection(), Navigation),
+						GalleryBuilder.NavButton("Multiple Selection, Bound", () =>
+							new MultipleBoundSelection(), Navigation),
 					}
 				}
 			};
diff --git a/Xamarin.Forms.Controls/GalleryPages/CollectionViewGalleries/SelectionGalleries/SelectionHelpers.cs b/Xamarin.Forms.Controls/GalleryPages/CollectionViewGalleries/SelectionGalleries/SelectionHelpers.cs
new file mode 100644
index 0000000..9042c18
--- /dev/null
+++ b/Xamarin.Forms.Controls/GalleryPages/CollectionViewGalleries/SelectionGalleries/SelectionHelpers.cs
@@ -0,0 +1,18 @@
+using System.Collections.Generic;
+using System.Linq;
+
+namespace Xamarin.Forms.Controls.GalleryPages.CollectionViewGalleries.SelectionGalleries
+{
+	internal static class SelectionHelpers
+	{
+		public static string ToCommaSeparatedList(this IEnumerable<object> items)
+		{
+			if (items == null)
+			{
+				return string.Empty;
+			}
+
+			return string.Join(", ", items.Cast<CollectionViewGalleryTestItem>().Select(i => i.Caption));
+		}
+	}
+}
\ No newline at end of file
diff --git a/Xamarin.Forms.Controls/GalleryPages/CollectionViewGalleries/SelectionGalleries/SelectionModeGallery.xaml.cs b/Xamarin.Forms.Controls/GalleryPages/CollectionViewGalleries/SelectionGalleries/SelectionModeGallery.xaml.cs
index 8363e67..12c5f6e 100644
--- a/Xamarin.Forms.Controls/GalleryPages/CollectionViewGalleries/SelectionGalleries/SelectionModeGallery.xaml.cs
+++ b/Xamarin.Forms.Controls/GalleryPages/CollectionViewGalleries/SelectionGalleries/SelectionModeGallery.xaml.cs
@@ -35,8 +35,8 @@ namespace Xamarin.Forms.Controls.GalleryPages.CollectionViewGalleries.SelectionG
 
 		void UpdateSelectionInfo(IEnumerable<object> currentSelectedItems, IEnumerable<object> previousSelectedItems)
 		{
-			var previous = ToList(previousSelectedItems);
-			var current = ToList(currentSelectedItems);
+			var previous = previousSelectedItems.ToCommaSeparatedList();
+			var current = currentSelectedItems.ToCommaSeparatedList();
 
 			if (string.IsNullOrEmpty(previous))
 			{
@@ -58,7 +58,7 @@ namespace Xamarin.Forms.Controls.GalleryPages.CollectionViewGalleries.SelectionG
 
 			if(CollectionView.SelectionMode == SelectionMode.Multiple)
 			{
-				current = ToList(CollectionView?.SelectedItems);
+				current = CollectionView?.SelectedItems.ToCommaSeparatedList();
 			}
 			else if (CollectionView.SelectionMode == SelectionMode.Single)
 			{
@@ -67,16 +67,5 @@ namespace Xamarin.Forms.Controls.GalleryPages.CollectionViewGalleries.SelectionG
 
 			SelectedItemsCommand.Text = $"Selection (command): {current}";
 		}
-
-		static string ToList(IEnumerable<object> items)
-		{
-			if (items == null)
-			{
-				return string.Empty;
-			}
-
-			return items.Aggregate(string.Empty, 
-				(s, o) => s + (s.Length == 0 ? "" : ", ") + ((CollectionViewGalleryTestItem)o).Caption);
-		}
 	}
 }
\ No newline at end of file
diff --git a/Xamarin.Forms.Controls/GalleryPages/CollectionViewGalleries/SelectionGalleries/SingleBoundSelection.xaml b/Xamarin.Forms.Controls/GalleryPages/CollectionViewGalleries/SelectionGalleries/SingleBoundSelection.xaml
new file mode 100644
index 0000000..70e561f
--- /dev/null
+++ b/Xamarin.Forms.Controls/GalleryPages/CollectionViewGalleries/SelectionGalleries/SingleBoundSelection.xaml
@@ -0,0 +1,33 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
+             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+             x:Class="Xamarin.Forms.Controls.GalleryPages.CollectionViewGalleries.SelectionGalleries.SingleBoundSelection">
+    <ContentPage.Content>
+        <StackLayout Spacing="5">
+
+            <Label Text="The selected item in the CollectionView should match the 'Selected' Label below. If it does not, this test has failed."
+                VerticalOptions="CenterAndExpand" 
+                HorizontalOptions="CenterAndExpand" />
+
+            <Label Text="{Binding SelectedItem, StringFormat='{}Selected: {0}'}"
+                VerticalOptions="CenterAndExpand" 
+                HorizontalOptions="CenterAndExpand" />
+
+            <Button AutomationId="Reset" Text="Reset Selection to Item 0" Clicked="ResetClicked" />
+            
+            <Button AutomationId="Clear" Text="Clear Selection" Clicked="ClearClicked" />
+
+            <CollectionView ItemsSource="{Binding Items}" SelectionMode="Single" SelectedItem="{Binding SelectedItem}">
+                <CollectionView.ItemTemplate>
+                    <DataTemplate>
+                        <StackLayout>
+                            <Image Source="{Binding Image}" HeightRequest="50" />
+                            <Label Text="{Binding Caption}"></Label>
+                        </StackLayout>
+                    </DataTemplate>
+                </CollectionView.ItemTemplate>
+            </CollectionView>
+
+        </StackLayout>
+    </ContentPage.Content>
+</ContentPage>
\ No newline at end of file
diff --git a/Xamarin.Forms.Controls/GalleryPages/CollectionViewGalleries/SelectionGalleries/SingleBoundSelection.xaml.cs b/Xamarin.Forms.Controls/GalleryPages/CollectionViewGalleries/SelectionGalleries/SingleBoundSelection.xaml.cs
new file mode 100644
index 0000000..c106e04
--- /dev/null
+++ b/Xamarin.Forms.Controls/GalleryPages/CollectionViewGalleries/SelectionGalleries/SingleBoundSelection.xaml.cs
@@ -0,0 +1,28 @@
+using System;
+using Xamarin.Forms.Xaml;
+
+namespace Xamarin.Forms.Controls.GalleryPages.CollectionViewGalleries.SelectionGalleries
+{
+	[XamlCompilation(XamlCompilationOptions.Compile)]
+	public partial class SingleBoundSelection : ContentPage
+	{
+		BoundSelectionModel _vm;
+
+		public SingleBoundSelection()
+		{
+			InitializeComponent();
+			_vm = new BoundSelectionModel();
+			BindingContext = _vm;
+		}
+
+		private void ResetClicked(object sender, EventArgs e)
+		{
+			_vm.SelectedItem = _vm.Items[0];
+		}
+
+		private void ClearClicked(object sender, EventArgs e)
+		{
+			_vm.SelectedItem = null;
+		}
+	}
+}
\ No newline at end of file
diff --git a/Xamarin.Forms.Controls/Xamarin.Forms.Controls.csproj b/Xamarin.Forms.Controls/Xamarin.Forms.Controls.csproj
index 02bb02c..5ef591f 100644
--- a/Xamarin.Forms.Controls/Xamarin.Forms.Controls.csproj
+++ b/Xamarin.Forms.Controls/Xamarin.Forms.Controls.csproj
@@ -56,6 +56,9 @@
     <EmbeddedResource Update="GalleryPages\CollectionViewGalleries\EmptyViewGalleries\EmptyViewSwapGallery.xaml">
 	<Generator>MSBuild:UpdateDesignTimeXaml</Generator>
     </EmbeddedResource>
+    <EmbeddedResource Update="GalleryPages\CollectionViewGalleries\SelectionGalleries\MultipleBoundSelection.xaml">
+      <Generator>MSBuild:UpdateDesignTimeXaml</Generator>
+    </EmbeddedResource>
     <EmbeddedResource Update="GalleryPages\CollectionViewGalleries\SelectionGalleries\PreselectedItemsGallery.xaml">
        <Generator>MSBuild:UpdateDesignTimeXaml</Generator>
     </EmbeddedResource>
diff --git a/Xamarin.Forms.Core/Items/SelectableItemsView.cs b/Xamarin.Forms.Core/Items/SelectableItemsView.cs
index 0faeb05..72b76e6 100644
--- a/Xamarin.Forms.Core/Items/SelectableItemsView.cs
+++ b/Xamarin.Forms.Core/Items/SelectableItemsView.cs
@@ -12,12 +12,15 @@ namespace Xamarin.Forms
 
 		public static readonly BindableProperty SelectedItemProperty =
 			BindableProperty.Create(nameof(SelectedItem), typeof(object), typeof(SelectableItemsView), default(object),
+				defaultBindingMode: BindingMode.TwoWay,
 				propertyChanged: SelectedItemPropertyChanged);
 
-		static readonly BindablePropertyKey SelectedItemsPropertyKey =
-			BindableProperty.CreateReadOnly(nameof(SelectedItems), typeof(IList<object>), typeof(SelectableItemsView), null);
-
-		public static readonly BindableProperty SelectedItemsProperty = SelectedItemsPropertyKey.BindableProperty;
+		public static readonly BindableProperty SelectedItemsProperty =
+			BindableProperty.Create(nameof(SelectedItems), typeof(IList<object>), typeof(SelectableItemsView), null,
+				defaultBindingMode: BindingMode.OneWay,
+				propertyChanged: SelectedItemsPropertyChanged,
+				coerceValue: CoerceSelectedItems,
+				defaultValueCreator: DefaultValueCreator);
 
 		public static readonly BindableProperty SelectionChangedCommandProperty =
 			BindableProperty.Create(nameof(SelectionChangedCommand), typeof(ICommand), typeof(SelectableItemsView));
@@ -26,10 +29,10 @@ namespace Xamarin.Forms
 			BindableProperty.Create(nameof(SelectionChangedCommandParameter), typeof(object),
 				typeof(SelectableItemsView));
 
+		static readonly IList<object> s_empty = new List<object>(0);
+
 		public SelectableItemsView()
 		{
-			var selectionList = new SelectionList(this);
-			SetValue(SelectedItemsPropertyKey, selectionList);
 		}
 
 		public object SelectedItem
@@ -41,6 +44,7 @@ namespace Xamarin.Forms
 		public IList<object> SelectedItems
 		{
 			get => (IList<object>)GetValue(SelectedItemsProperty);
+			set => SetValue(SelectedItemsProperty, new SelectionList(this, value));
 		}
 
 		public ICommand SelectionChangedCommand
@@ -67,9 +71,39 @@ namespace Xamarin.Forms
 		{
 		}
 
+		static object CoerceSelectedItems(BindableObject bindable, object value)
+		{
+			if (value == null)
+			{
+				return new SelectionList((SelectableItemsView)bindable);
+			}
+
+			if(value is SelectionList)
+			{
+				return value;
+			}
+
+			return new SelectionList((SelectableItemsView)bindable, value as IList<object>);
+		}
+
+		static object DefaultValueCreator(BindableObject bindable)
+		{
+			return new SelectionList((SelectableItemsView)bindable);
+		}
+
+		static void SelectedItemsPropertyChanged(BindableObject bindable, object oldValue, object newValue)
+		{
+			var selectableItemsView = (SelectableItemsView)bindable;
+			var oldSelection = (IList<object>)oldValue ?? s_empty;
+			var newSelection = (IList<object>)newValue ?? s_empty;
+
+			selectableItemsView.SelectedItemsPropertyChanged(oldSelection, newSelection);
+		}
+
 		internal void SelectedItemsPropertyChanged(IList<object> oldSelection, IList<object> newSelection)
 		{
 			SelectionPropertyChanged(this, new SelectionChangedEventArgs(oldSelection, newSelection));
+			
 			OnPropertyChanged(SelectedItemsProperty.PropertyName);
 		}
 
diff --git a/Xamarin.Forms.Core/Items/SelectionList.cs b/Xamarin.Forms.Core/Items/SelectionList.cs
index 467594d..5bfe0ea 100644
--- a/Xamarin.Forms.Core/Items/SelectionList.cs
+++ b/Xamarin.Forms.Core/Items/SelectionList.cs
@@ -1,42 +1,55 @@
 using System;
 using System.Collections;
 using System.Collections.Generic;
+using System.Collections.Specialized;
 
 namespace Xamarin.Forms
 {
 	// Used by the SelectableItemsView to keep track of (and respond to changes in) the SelectedItems property
 	internal class SelectionList : IList<object>
 	{
-		readonly SelectableItemsView _selectableItemsView;
-		List<object> _internal;
 		static readonly IList<object> s_empty = new List<object>(0);
+		readonly SelectableItemsView _selectableItemsView;
+		readonly IList<object> _internal;
+		IList<object> _shadow;
+		bool _externalChange;
 
-		public SelectionList(SelectableItemsView selectableItemsView)
+		public SelectionList(SelectableItemsView selectableItemsView, IList<object> items = null)
 		{
 			_selectableItemsView = selectableItemsView ?? throw new ArgumentNullException(nameof(selectableItemsView));
-			_internal = new List<object>();
+			_internal = items ?? new List<object>();
+			_shadow = Copy();
+
+			if (items is INotifyCollectionChanged incc)
+			{
+				incc.CollectionChanged += OnCollectionChanged;
+			}
 		}
 
 		public object this[int index] { get => _internal[index]; set => _internal[index] = value; }
 
 		public int Count => _internal.Count;
+
 		public bool IsReadOnly => false;
 
 		public void Add(object item)
 		{
-			var oldItems = Copy();
-
+			_externalChange = true;
 			_internal.Add(item);
+			_externalChange = false;
 
-			_selectableItemsView.SelectedItemsPropertyChanged(oldItems, Copy());
+			_selectableItemsView.SelectedItemsPropertyChanged(_shadow, _internal);
+			_shadow.Add(item);
 		}
 
 		public void Clear()
 		{
-			var oldItems = Copy();
+			_externalChange = true;
 			_internal.Clear();
+			_externalChange = false;
 
-			_selectableItemsView.SelectedItemsPropertyChanged(oldItems, s_empty);
+			_selectableItemsView.SelectedItemsPropertyChanged(_shadow, s_empty);
+			_shadow.Clear();
 		}
 
 		public bool Contains(object item)
@@ -61,22 +74,24 @@ namespace Xamarin.Forms
 
 		public void Insert(int index, object item)
 		{
-			var oldItems = Copy();
-
+			_externalChange = true;
 			_internal.Insert(index, item);
+			_externalChange = false;
 
-			_selectableItemsView.SelectedItemsPropertyChanged(oldItems, Copy());
+			_selectableItemsView.SelectedItemsPropertyChanged(_shadow, _internal);
+			_shadow.Insert(index, item);
 		}
 
 		public bool Remove(object item)
 		{
-			var oldItems = Copy();
-
+			_externalChange = true;
 			var removed = _internal.Remove(item);
+			_externalChange = false;
 
 			if (removed)
 			{
-				_selectableItemsView.SelectedItemsPropertyChanged(oldItems, Copy());
+				_selectableItemsView.SelectedItemsPropertyChanged(_shadow, _internal);
+				_shadow.Remove(item);
 			}
 
 			return removed;
@@ -84,11 +99,12 @@ namespace Xamarin.Forms
 
 		public void RemoveAt(int index)
 		{
-			var oldItems = Copy();
-
+			_externalChange = true;
 			_internal.RemoveAt(index);
+			_externalChange = false;
 
-			_selectableItemsView.SelectedItemsPropertyChanged(oldItems, Copy());
+			_selectableItemsView.SelectedItemsPropertyChanged(_shadow, _internal);
+			_shadow.RemoveAt(index);
 		}
 
 		IEnumerator IEnumerable.GetEnumerator()
@@ -107,14 +123,19 @@ namespace Xamarin.Forms
 			return items;
 		}
 
-		public void ClearQuietly()
+		void OnCollectionChanged(object sender, NotifyCollectionChangedEventArgs args)
 		{
-			_internal.Clear();
-		}
+			if (_externalChange)
+			{
+				// If this change was initiated by a renderer or direct manipulation of ColllectionView.SelectedItems,
+				// we don't need to send a selection change notification
+				return;
+			}
 
-		public void AddQuietly(object item)
-		{
-			_internal.Add(item);
+			// This change is coming from a bound viewmodel property
+			// Emit a selection change notification, then bring the shadow copy up-to-date
+			_selectableItemsView.SelectedItemsPropertyChanged(_shadow, _internal);
+			_shadow = Copy();
 		}
 	}
 }
diff --git a/Xamarin.Forms.Platform.iOS/CollectionView/SelectableItemsViewController.cs b/Xamarin.Forms.Platform.iOS/CollectionView/SelectableItemsViewController.cs
index 73fb7f0..ae4e6ee 100644
--- a/Xamarin.Forms.Platform.iOS/CollectionView/SelectableItemsViewController.cs
+++ b/Xamarin.Forms.Platform.iOS/CollectionView/SelectableItemsViewController.cs
@@ -34,6 +34,17 @@ namespace Xamarin.Forms.Platform.iOS
 			CollectionView.SelectItem(index, true, UICollectionViewScrollPosition.None);
 		}
 
+		// Called by Forms to clear the native selection
+		internal void ClearSelection()
+		{
+			var selectedItemIndexes = CollectionView.GetIndexPathsForSelectedItems();
+
+			foreach (var index in selectedItemIndexes)
+			{
+				CollectionView.DeselectItem(index, true);
+			}
+		}
+
 		void FormsSelectItem(NSIndexPath indexPath)
 		{
 			var mode = SelectableItemsView.SelectionMode;
@@ -87,6 +98,11 @@ namespace Xamarin.Forms.Platform.iOS
 					{
 						SelectItem(selectedItem);
 					}
+					else
+					{
+						// SelectedItem has been set to null; if an item is selected, we need to de-select it
+						ClearSelection();
+					}
 				
 					return;
 				case SelectionMode.Multiple:
-- 
2.7.4