[Android] Fix crash adding items to the CollectionView (#7670)

* Fixed 7510 - Crash adding items to the CollectionView on Android

* Removed code used for testing

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Issue7510.cs

@@ -0,0 +1,144 @@
+using System;
+using System.Collections.Generic;
+using System.Collections.ObjectModel;
+using System.Diagnostics;
+using System.Linq;
+using System.Threading.Tasks;
+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, 7510, "CollectionView crashes on adding item", PlatformAffected.All)]
+	public class Issue7510 : TestContentPage
+	{
+		public Issue7510()
+		{
+			Title = "Issue 7510";
+			BindingContext = new Issue7510ModelViewModel();
+		}
+
+		protected override void OnAppearing()
+		{
+			if (this.BindingContext is Issue7510ModelViewModel vm)
+			{
+				vm.Init();
+			}
+
+			base.OnAppearing();
+		}
+
+		protected override void Init()
+		{
+			var infoLabel = new Label
+			{
+				Text = "If you press the button and there is no crash, everything goes correctly."
+			};
+
+			var addButton = new Button
+			{
+				Text = "Add items",
+				AutomationId = "AddBtn"
+			};
+
+			addButton.SetBinding(Button.CommandProperty, "MoreCommand");
+
+			var collectionView = new CollectionView
+			{
+				AutomationId = "Collection",
+				SelectionMode = SelectionMode.None,
+				ItemTemplate = CreateDataTemplate()
+			};
+
+			collectionView.SetBinding(ItemsView.ItemsSourceProperty, "Items");
+
+			var stack = new StackLayout();
+
+			stack.Children.Add(infoLabel);
+			stack.Children.Add(addButton);
+			stack.Children.Add(collectionView);
+
+			Content = stack;
+		}
+
+		private DataTemplate CreateDataTemplate()
+		{
+			DataTemplate template = new DataTemplate(() =>
+			{
+				var grid = new Grid() { Padding = new Thickness(0), Margin = 0, RowSpacing = 0, ColumnSpacing = 0 };
+
+				grid.RowDefinitions.Clear();
+				grid.ColumnDefinitions.Clear();
+				grid.Children.Clear();
+
+				var cell = new Label();
+				cell.SetBinding(Label.TextProperty, "Title");
+				grid.Children.Add(cell, 0, 0);
+
+				return grid;
+			});
+
+			return template;
+		}
+	}
+
+	[Preserve(AllMembers = true)]
+	public class Issue7510Model
+	{
+		public Guid Id { get; set; }
+		public string Title { get; set; }
+	}
+
+	[Preserve(AllMembers = true)]
+	public class Issue7510ModelViewModel : BindableObject
+	{
+		private ObservableCollection<Issue7510Model> _items;
+
+		public ObservableCollection<Issue7510Model> Items
+		{
+			get { return _items; }
+			set
+			{
+				_items = value;
+				OnPropertyChanged();
+			}
+		}
+
+		public Command MoreCommand => new Command(async () => await LoadMore());
+
+		private async Task LoadMore()
+		{
+			await Task.Delay(100);
+
+			var postCount = Items.Count();
+
+			for (var i = 0; i < 20; i++)
+			{
+				Items.Add(new Issue7510Model { Id = Guid.NewGuid(), Title = $"Added Item {postCount + i}" });
+				Debug.WriteLine(postCount + i);
+			}
+		}
+
+		internal void Init()
+		{
+			var posts = new List<Issue7510Model>();
+
+			for (var i = 0; i < 20; i++)
+			{
+				posts.Add(new Issue7510Model { Id = Guid.NewGuid(), Title = $"Item {i}" });
+			}
+
+			Items = new ObservableCollection<Issue7510Model>(posts);
+		}
+	}
+}

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

@@ -1143,6 +1143,7 @@
       <DependentUpon>Issue7886.xaml</DependentUpon>
     </Compile>
     <Compile Include="$(MSBuildThisFileDirectory)Issue7898.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)Issue7510.cs" />
   </ItemGroup>
   <ItemGroup>
     <EmbeddedResource Include="$(MSBuildThisFileDirectory)Bugzilla22229.xaml">

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

@@ -14,42 +14,58 @@ namespace Xamarin.Forms.Platform.Android
 
 		public void NotifyDataSetChanged()
 		{
-			_adapter.NotifyDataSetChanged();
+			if (IsValidAdapter())
+				_adapter.NotifyDataSetChanged();
 		}
 
 		public void NotifyItemChanged(IItemsViewSource source, int startIndex)
 		{
-			_adapter.NotifyItemChanged(startIndex);
+			if (IsValidAdapter())
+				_adapter.NotifyItemChanged(startIndex);
 		}
 
 		public void NotifyItemInserted(IItemsViewSource source, int startIndex)
 		{
-			_adapter.NotifyItemInserted(startIndex);
+			if (IsValidAdapter())
+				_adapter.NotifyItemInserted(startIndex);
 		}
 
 		public void NotifyItemMoved(IItemsViewSource source, int fromPosition, int toPosition)
 		{
-			_adapter.NotifyItemMoved(fromPosition, toPosition);
+			if (IsValidAdapter())
+				_adapter.NotifyItemMoved(fromPosition, toPosition);
 		}
 
 		public void NotifyItemRangeChanged(IItemsViewSource source, int start, int end)
 		{
-			_adapter.NotifyItemRangeChanged(start, end);
+			if (IsValidAdapter())
+				_adapter.NotifyItemRangeChanged(start, end);
 		}
 
 		public void NotifyItemRangeInserted(IItemsViewSource source, int startIndex, int count)
 		{
-			_adapter.NotifyItemRangeInserted(startIndex, count);
+			if (IsValidAdapter())
+				_adapter.NotifyItemRangeInserted(startIndex, count);
 		}
 
 		public void NotifyItemRangeRemoved(IItemsViewSource source, int startIndex, int count)
 		{
-			_adapter.NotifyItemRangeRemoved(startIndex, count);
+			if (IsValidAdapter())
+				_adapter.NotifyItemRangeRemoved(startIndex, count);
 		}
 
 		public void NotifyItemRemoved(IItemsViewSource source, int startIndex)
 		{
-			_adapter.NotifyItemRemoved(startIndex);
+			if (IsValidAdapter())
+				_adapter.NotifyItemRemoved(startIndex);
+		}
+
+		internal bool IsValidAdapter()
+		{
+			if (_adapter == null || _adapter.IsDisposed())
+				return false;
+
+			return true;
 		}
 	}
-}
+}