Fix DataContexts.

This makes it so that IControls are no longer DataTemplate-able,
breaking TabControl. Need to find a better way.

src/Perspex.Controls/Generators/ItemContainerGenerator.cs

@@ -142,14 +142,7 @@ namespace Perspex.Controls.Generators
         /// <returns>The created container control.</returns>
         protected virtual IControl CreateContainer(object item)
         {
-            if (item == null)
-            {
-                return null;
-            }
-            else
-            {
-                return Owner.MaterializeDataTemplate(item);
-            }
+            return Owner.MaterializeDataTemplate(item);
         }
 
         /// <summary>

src/Perspex.Controls/Generators/ItemContainerGenerator`1.cs

@@ -23,15 +23,23 @@ namespace Perspex.Controls.Generators
         /// <inheritdoc/>
         protected override IControl CreateContainer(object item)
         {
-            T result = item as T;
+            var container = item as T;
 
-            if (result == null)
+            if (item == null)
             {
-                result = new T();
+                return null;
+            }
+            else if (container != null)
+            {
+                return container;
+            }
+            else
+            {
+                var result = new T();
                 result.Content = Owner.MaterializeDataTemplate(item);
+                result.DataContext = item;
+                return result;
             }
-
-            return result;
         }
     }
 }

src/Perspex.Controls/Presenters/ContentPresenter.cs

@@ -97,7 +97,7 @@ namespace Perspex.Controls.Presenters
         {
             var old = Child;
             var content = Content;
-            var result = content != null ? this.MaterializeDataTemplate(content) : null;
+            var result = this.MaterializeDataTemplate(content);
             var logicalHost = this.FindReparentingHost();
             var logicalChildren = logicalHost?.LogicalChildren ?? LogicalChildren;
 

src/Perspex.Controls/Templates/DataTemplateExtensions.cs

@@ -19,28 +19,37 @@ namespace Perspex.Controls.Templates
         /// <returns>The data materialized as a control.</returns>
         public static IControl MaterializeDataTemplate(this IControl control, object data)
         {
-            IDataTemplate template = control.FindDataTemplate(data);
-            IControl result;
-
-            if (template != null)
+            if (data == null)
             {
-                result = template.Build(data);
+                return null;
+            }
+            else
+            {
+                var asControl = data as IControl;
 
-                if (result != null && result.DataContext == null)
+                if (asControl != null)
+                {
+                    return asControl;
+                }
+                else
                 {
+                    IDataTemplate template = control.FindDataTemplate(data);
+                    IControl result;
+
+                    if (template != null)
+                    {
+                        result = template.Build(data);
+                    }
+                    else
+                    {
+                        result = DataTemplate.Default.Build(data);
+                    }
+
                     result.DataContext = data;
+
+                    return result;
                 }
             }
-            else if (data is IControl)
-            {
-                result = (IControl)data;
-            }
-            else
-            {
-                result = DataTemplate.Default.Build(data);
-            }
-
-            return result;
         }
 
         /// <summary>

tests/Perspex.Controls.UnitTests/ItemsControlTests.cs

@@ -287,6 +287,50 @@ namespace Perspex.Controls.UnitTests
             Assert.Equal(target, ((ILogical)child).LogicalParent);
         }
 
+        [Fact]
+        public void DataContexts_Should_Be_Correctly_Set()
+        {
+            var items = new object[]
+            {
+                "Foo",
+                new Item("Bar"),
+                new TextBlock { Text = "Baz" },
+                new ListBoxItem { Content = "Qux" },
+            };
+
+            var target = new ItemsControl
+            {
+                Template = GetTemplate(),
+                DataContext = "Base",
+                DataTemplates = new DataTemplates
+                {
+                    new DataTemplate<Item>(x => new Button { Content = x })
+                },
+                Items = items,
+            };
+
+            target.ApplyTemplate();
+
+            var dataContexts = target.Presenter.Panel.Children
+                .Cast<Control>()
+                .Select(x => x.DataContext)
+                .ToList();
+
+            Assert.Equal(
+                new object[] { items[0], items[1], "Base", "Base" },
+                dataContexts);
+        }
+
+        private class Item
+        {
+            public Item(string value)
+            {
+                Value = value;
+            }
+
+            public string Value { get; }
+        }
+
         private ControlTemplate GetTemplate()
         {
             return new ControlTemplate<ItemsControl>(parent =>