Fix 15052 NullReferenceException when returning null from DataTemplateSelector

Xamarin.Forms.Core.UnitTests/DataTemplateSelectorTests.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Collections.ObjectModel;
 using NUnit.Framework;
 using Xamarin.Forms;
 
@@ -37,6 +38,22 @@ public TemplateTwo() : base(typeof(EntryCell))
 			}
 		}
 
+		class TemplateThree : DataTemplate
+		{
+			public TemplateThree() : base(typeof(ContentPage))
+			{
+
+			}
+		}
+
+		class NullSelector : DataTemplateSelector
+		{
+			protected override DataTemplate OnSelectTemplate(object item, BindableObject container)
+			{
+				return null;
+			}
+		}
+
 		class TestDTS : DataTemplateSelector
 		{
 			public TestDTS()
@@ -51,36 +68,78 @@ protected override DataTemplate OnSelectTemplate(object item, BindableObject con
 					return templateOne;
 				if (item is byte)
 					return new TestDTS();
-				return templateTwo;
+				if (item is string)
+					return templateTwo;
+
+				return null;
 			}
 
 			readonly DataTemplate templateOne;
 			readonly DataTemplate templateTwo;
 		}
 
+		class TestPageDTS : DataTemplateSelector
+		{
+			public TestPageDTS()
+			{
+				templateThree = new TemplateThree();
+			}
+
+			protected override DataTemplate OnSelectTemplate(object item, BindableObject container)
+			{
+				if (item is View)
+					return templateThree;
+
+				return null;
+			}
+
+			readonly DataTemplate templateThree;
+		}
+
 		[Test]
 		public void Constructor()
 		{
 			var dts = new TestDTS();
 		}
 
+		[Test]
+		public void CanReturnNull()
+		{
+			var dts = new NullSelector();
+			Assert.IsNull(dts.SelectTemplate((short)0, null));
+		}
+
 		[Test]
 		public void ReturnsCorrectType()
 		{
 			var dts = new TestDTS();
 			Assert.IsInstanceOf<TemplateOne>(dts.SelectTemplate(1d, null));
 			Assert.IsInstanceOf<TemplateTwo>(dts.SelectTemplate("test", null));
+			Assert.IsNull(dts.SelectTemplate((short)0, null));
 		}
 
 		[Test]
 		public void ListViewSupport()
 		{
 			var listView = new ListView(ListViewCachingStrategy.RecycleElement);
-			listView.ItemsSource = new object[] { 0d, "test" };
+			listView.ItemsSource = new object[] { 0d, "test", (short)0 };
 
 			listView.ItemTemplate = new TestDTS();
 			Assert.IsInstanceOf<ViewCell>(listView.TemplatedItems[0]);
 			Assert.IsInstanceOf<EntryCell>(listView.TemplatedItems[1]);
+			Assert.IsInstanceOf<TextCell>(listView.TemplatedItems[2]);
+		}
+
+		[Test]
+		public void ListViewRecycleDTSupport()
+		{
+			var listView = new ListView(ListViewCachingStrategy.RecycleElementAndDataTemplate);
+			listView.ItemsSource = new object[] { 0d, "test", (short)0 };
+
+			listView.ItemTemplate = new TestDTS();
+			Assert.IsInstanceOf<ViewCell>(listView.TemplatedItems[0]);
+			Assert.IsInstanceOf<EntryCell>(listView.TemplatedItems[1]);
+			Assert.IsInstanceOf<TextCell>(listView.TemplatedItems[2]);
 		}
 
 		[Test]
@@ -89,6 +148,18 @@ public void NestingThrowsException()
 			var dts = new TestDTS();
 			Assert.Throws<NotSupportedException>(() => dts.SelectTemplate((byte)0, null));
 		}
+
+		[Test]
+		public void TabbedPageSupport()
+		{
+			var tabbedPage = new TabbedPage();
+			tabbedPage.ItemsSource = new object[] { new View(), "test" };
+
+			tabbedPage.ItemTemplate = new TestPageDTS();
+			Assert.IsInstanceOf<ContentPage>(tabbedPage.Children[0]);
+			Assert.IsInstanceOf<Page>(tabbedPage.Children[1]);
+			Assert.AreEqual(tabbedPage.Children[1].BindingContext, "test");
+		}
 	}
 
 	[TestFixture]

Xamarin.Forms.Core/BindableLayout.cs

@@ -275,9 +275,11 @@ View CreateEmptyView(object emptyView, DataTemplate dataTemplate)
 				return null;
 			}
 
-			if (dataTemplate != null)
+			var template = dataTemplate?.SelectDataTemplate(layout.BindingContext, layout);
+
+			if (template != null)
 			{
-				var view = (View)dataTemplate.CreateContent();
+				var view = (View)template.CreateContent();
 				view.BindingContext = layout.BindingContext;
 				return view;
 			}

Xamarin.Forms.Core/CarouselPage.cs

@@ -1,4 +1,5 @@
 using System;
+using Xamarin.Forms.Internals;
 using Xamarin.Forms.Platform;
 
 namespace Xamarin.Forms
@@ -20,11 +21,7 @@ public CarouselPage()
 
 		protected override ContentPage CreateDefault(object item)
 		{
-			var page = new ContentPage();
-			if (item != null)
-				page.Title = item.ToString();
-
-			return page;
+			return (ContentPage)DataTemplateHelpers.DefaultPageTemplate.CreateContent();
 		}
 	}
 }

Xamarin.Forms.Core/DataTemplateExtensions.cs

@@ -1,3 +1,4 @@
+using System;
 using System.ComponentModel;
 
 namespace Xamarin.Forms.Internals
@@ -16,7 +17,38 @@ public static DataTemplate SelectDataTemplate(this DataTemplate self, object ite
 
 		public static object CreateContent(this DataTemplate self, object item, BindableObject container)
 		{
-			return self.SelectDataTemplate(item, container).CreateContent();
+			return self.SelectDataTemplate(item, container)?.CreateContent();
+		}
+	}
+
+	[EditorBrowsable(EditorBrowsableState.Never)]
+	public  static class DataTemplateHelpers
+	{
+		public readonly static DataTemplate DefaultContentTemplate =
+			new Lazy<DataTemplate>(
+				() => new DataTemplate(CreateCoreContent)).Value;
+
+		public readonly static DataTemplate DefaultPageTemplate =
+			new Lazy<DataTemplate>(
+				() => new DataTemplate(() =>
+				                       {
+					                       var page = new ContentPage() { Content = CreateCoreContent() };
+					                       page.SetBinding(Page.TitleProperty, ".");
+
+					                       return page;
+				                       })).Value;
+
+		static View CreateCoreContent()
+		{
+			var label = new Label();
+			label.SetBinding(Label.TextProperty, ".");
+
+			return new StackLayout()
+			       {
+				       HorizontalOptions = LayoutOptions.Center,
+				       VerticalOptions = LayoutOptions.Center,
+				       Children = { label }
+			       };
 		}
 	}
 }

Xamarin.Forms.Core/DataTemplateSelector.cs

@@ -24,7 +24,7 @@ public DataTemplate SelectTemplate(object item, BindableObject container)
 				throw new NotSupportedException(
 					"DataTemplateSelector.OnSelectTemplate must not return another DataTemplateSelector");
 
-			if (recycle)
+			if (recycle && dataTemplate != null)
 			{
 				if (!dataTemplate.CanRecycle)
 					throw new NotSupportedException(

Xamarin.Forms.Core/Shell/BaseShellItem.cs

@@ -330,7 +330,15 @@ BindableObject NonImplicitParent
 			}
 		}
 
-		internal static DataTemplate CreateDefaultFlyoutItemCell(string textBinding, string iconBinding)
+		internal static DataTemplate MenuItemDefaultDataTemplate =
+			new Lazy<DataTemplate>(
+				() => CreateDefaultFlyoutItemCell("Text", "Icon") ).Value;
+
+		internal static DataTemplate ItemDefaultDataTemplate =
+			new Lazy<DataTemplate>(
+				() => CreateDefaultFlyoutItemCell("Title", "FlyoutIcon")).Value;
+
+		static DataTemplate CreateDefaultFlyoutItemCell(string textBinding, string iconBinding)
 		{
 			return new DataTemplate(() =>
 			{

Xamarin.Forms.Core/Shell/Shell.cs

@@ -255,35 +255,29 @@ internal static BindableObject GetBindableObjectWithFlyoutItemTemplate(BindableO
 
 		DataTemplate IShellController.GetFlyoutItemDataTemplate(BindableObject bo)
 		{
-			BindableProperty bp = null;
-			string textBinding;
-			string iconBinding;
+			BindableProperty bp;
+			DataTemplate defaultTemplate;
+
 			var bindableObjectWithTemplate = GetBindableObjectWithFlyoutItemTemplate(bo);
 
 			if (bo is IMenuItemController)
 			{
 				bp = MenuItemTemplateProperty;
-				textBinding = "Text";
-				iconBinding = "Icon";
+				defaultTemplate = BaseShellItem.MenuItemDefaultDataTemplate;
 			}
 			else
 			{
 				bp = ItemTemplateProperty;
-				textBinding = "Title";
-				iconBinding = "FlyoutIcon";
-			}
-
-			if (bindableObjectWithTemplate.IsSet(bp))
-			{
-				return (DataTemplate)bindableObjectWithTemplate.GetValue(bp);
+				defaultTemplate = BaseShellItem.ItemDefaultDataTemplate;
 			}
 
-			if (IsSet(bp))
-			{
-				return (DataTemplate)GetValue(bp);
-			}
+			DataTemplate dataTemplate = (DataTemplate)(bindableObjectWithTemplate.IsSet(bp) ?
+				bindableObjectWithTemplate.GetValue(bp) :
+				IsSet(bp) ?
+					GetValue(bp) :
+					null);
 
-			return BaseShellItem.CreateDefaultFlyoutItemCell(textBinding, iconBinding);
+			return dataTemplate?.SelectDataTemplate( bo, this ) ?? defaultTemplate;
 		}
 
 		event EventHandler IShellController.StructureChanged

Xamarin.Forms.Core/Shell/ShellContent.cs

@@ -53,8 +53,8 @@ public DataTemplate ContentTemplate
 
 		Page IShellContentController.GetOrCreateContent()
 		{
-			var template = ContentTemplate;
 			var content = Content;
+			var template = ContentTemplate?.SelectDataTemplate(content, this);
 
 			Page result = null;
 			if (template == null)
@@ -64,7 +64,7 @@ Page IShellContentController.GetOrCreateContent()
 			}
 			else
 			{
-				result = ContentCache ?? (Page)template.CreateContent(content, this);
+				result = ContentCache ?? (Page)template.CreateContent();
 				ContentCache = result;
 			}
 

Xamarin.Forms.Core/Shell/ShellTemplatedViewManager.cs

@@ -49,9 +49,11 @@ public static void OnViewTemplateChanged(
 			Shell shell)
 		{
 			View newContentView = currentViewData as View;
-			if (newViewTemplate != null)
+
+			DataTemplate template = newViewTemplate?.SelectDataTemplate(newViewTemplate, shell);
+			if (template != null)
 			{
-				newContentView = (View)newViewTemplate.CreateContent(newViewTemplate, shell);
+				newContentView = (View)template.CreateContent();
 			}
 
 			SetView(ref localViewRef,

Xamarin.Forms.Core/TabbedPage.cs

@@ -1,4 +1,5 @@
 using System;
+using Xamarin.Forms.Internals;
 using Xamarin.Forms.Platform;
 
 namespace Xamarin.Forms
@@ -49,11 +50,7 @@ public Color SelectedTabColor
 
 		protected override Page CreateDefault(object item)
 		{
-			var page = new Page();
-			if (item != null)
-				page.Title = item.ToString();
-
-			return page;
+			return (Page)DataTemplateHelpers.DefaultPageTemplate.CreateContent();
 		}
 
 		public TabbedPage()

Xamarin.Forms.Core/TemplatedItemsList.cs

@@ -532,12 +532,13 @@ public int IndexOf(TItem item)
 		[EditorBrowsable(EditorBrowsableState.Never)]
 		public DataTemplate SelectDataTemplate(object item)
 		{
-			return ItemTemplate.SelectDataTemplate(item, _itemsView);
+			return ItemTemplate?.SelectDataTemplate(item, _itemsView);
 		}
 
 		public TItem ActivateContent(int index, object item)
 		{
-			TItem content = ItemTemplate != null ? (TItem)ItemTemplate.CreateContent(item, _itemsView) : _itemsView.CreateDefault(item);
+			TItem content = (TItem)ItemTemplate?.CreateContent(item, _itemsView) ??
+			                _itemsView.CreateDefault(item);
 
 			content = UpdateContent(content, index, item);
 
@@ -740,9 +741,12 @@ TemplatedItemsList<TView, TItem> InsertGrouped(object item, int index)
 
 			groupProxy.BindingContext = item;
 
-			if (GroupHeaderTemplate != null)
+			var groupHeaderTemplate =
+				GroupHeaderTemplate?.SelectDataTemplate(groupProxy.ItemsSource, _itemsView);
+
+			if (groupHeaderTemplate != null)
 			{
-				groupProxy.HeaderContent = (TItem)GroupHeaderTemplate.CreateContent(groupProxy.ItemsSource, _itemsView);
+				groupProxy.HeaderContent = (TItem)groupHeaderTemplate.CreateContent();
 				groupProxy.HeaderContent.BindingContext = groupProxy.ItemsSource;
 				//groupProxy.HeaderContent.BindingContext = groupProxy;
 				//groupProxy.HeaderContent.SetBinding (BindingContextProperty, "ItemsSource");

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

@@ -6,8 +6,8 @@
 namespace Xamarin.Forms.Platform.Android
 {
 	public class TemplatedItemViewHolder : SelectableViewHolder
-	{
-		readonly ItemContentView _itemContentView;
+    {
+        readonly ItemContentView _itemContentView;
 		readonly DataTemplate _template;
 		DataTemplate _selectedTemplate;
 
@@ -48,11 +48,12 @@ public void Recycle(ItemsView itemsView)
 		public void Bind(object itemBindingContext, ItemsView itemsView,
 			Action<Size> reportMeasure = null, Size? size = null)
 		{
-			var template = _template.SelectDataTemplate(itemBindingContext, itemsView);
+			var template = _template?.SelectDataTemplate(itemBindingContext, itemsView) ??
+			               DataTemplateHelpers.DefaultContentTemplate;
 
 			var templateChanging = template != _selectedTemplate;
 
-			if (templateChanging)
+            if (templateChanging)
 			{
 				// Clean up any content we're still holding on to
 				_itemContentView.Recycle();