Bind DataContext to Parent.DataContext.

Need to special-case binding to DataContext as previously it was trying
to bind to itself. When binding to DataContext, we're binding to the
*parent* DataContext.

src/Markup/Perspex.Markup.Xaml/Binding/XamlBinding.cs

@@ -29,21 +29,42 @@ namespace Perspex.Markup.Xaml.Binding
 
         public void Bind(IObservablePropertyBag instance, PerspexProperty property)
         {
-            if (property == Control.DataContextProperty && instance.InheritanceParent != null)
+            var subject = new ExpressionSubject(CreateExpressionObserver(instance, property));
+
+            if (subject != null)
             {
-                instance = instance.InheritanceParent as IObservablePropertyBag ?? instance;
+                Bind(instance, property, subject);
             }
-
-            var subject = new ExpressionSubject(CreateExpressionObserver(instance));
-            Bind(instance, property, subject);
         }
 
-        public ExpressionObserver CreateExpressionObserver(IObservablePropertyBag instance)
+        public ExpressionObserver CreateExpressionObserver(
+            IObservablePropertyBag instance, 
+            PerspexProperty property)
         {
-            var result = new ExpressionObserver(null, SourcePropertyPath);
-            var dataContext = instance.GetObservable(Control.DataContextProperty);
-            dataContext.Subscribe(x => result.Root = x);
-            return result;
+            IObservable<object> dataContext = null;
+
+            if (property != Control.DataContextProperty)
+            {
+                dataContext = instance.GetObservable(Control.DataContextProperty);
+            }
+            else
+            {
+                var parent = instance.InheritanceParent as IObservablePropertyBag;
+
+                if (parent != null)
+                {
+                    dataContext = parent.GetObservable(Control.DataContextProperty);
+                }
+            }
+
+            if (dataContext != null)
+            {
+                var result = new ExpressionObserver(null, SourcePropertyPath);
+                dataContext.Subscribe(x => result.Root = x);
+                return result;
+            }
+
+            return null;
         }
 
         internal void Bind(IObservablePropertyBag target, PerspexProperty property, ISubject<object> subject)

tests/Perspex.Markup.Xaml.UnitTests/Binding/XamlBindingTests.cs

@@ -113,6 +113,36 @@ namespace Perspex.Markup.Xaml.UnitTests.Binding
                 BindingPriority.LocalValue));
         }
 
+        [Fact]
+        public void DataContext_Binding_Should_Use_Parent_DataContext()
+        {
+            var parentDataContext = Mock.Of<IHeadered>(x => x.Header == (object)"Foo");
+
+            var parent = new Decorator
+            {
+                Child = new Control(),
+                DataContext = parentDataContext,
+            };
+
+            var binding = new XamlBinding
+            {
+                SourcePropertyPath = "Header",
+            };
+
+            binding.Bind(parent.Child, Control.DataContextProperty);
+
+            Assert.Equal("Foo", parent.Child.DataContext);
+
+            parentDataContext = Mock.Of<IHeadered>(x => x.Header == (object)"Bar");
+            parent.DataContext = parentDataContext;
+            Assert.Equal("Bar", parent.Child.DataContext);
+        }
+
+        private Mock<IObservablePropertyBag> CreateTarget(object dataContext)
+        {
+            return CreateTarget(dataContext: Observable.Never<object>().StartWith(dataContext));
+        }
+
         private Mock<IObservablePropertyBag> CreateTarget(
             IObservable<object> dataContext = null,
             IObservable<string> text = null)