Allow binding to anchor's DataContext.

Fixes #461.

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

@@ -90,7 +90,8 @@ namespace Perspex.Markup.Xaml.Data
                 observer = CreateDataContexObserver(
                     target, 
                     pathInfo.Path,
-                    targetProperty == Control.DataContextProperty);
+                    targetProperty == Control.DataContextProperty,
+                    anchor);
             }
             else if (RelativeSource.Mode == RelativeSourceMode.TemplatedParent)
             {
@@ -161,10 +162,21 @@ namespace Perspex.Markup.Xaml.Data
         private ExpressionObserver CreateDataContexObserver(
             IPerspexObject target,
             string path,
-            bool targetIsDataContext)
+            bool targetIsDataContext,
+            object anchor)
         {
             Contract.Requires<ArgumentNullException>(target != null);
 
+            if (!(target is IControl))
+            {
+                target = anchor as IControl;
+
+                if (target == null)
+                {
+                    throw new InvalidOperationException("Cannot find a DataContext to bind to.");
+                }
+            }
+
             if (!targetIsDataContext)
             {
                 var update = target.GetObservable(Control.DataContextProperty)

tests/Perspex.Markup.Xaml.UnitTests/Data/MultiBindingTests.cs

@@ -30,7 +30,7 @@ namespace Perspex.Markup.Xaml.UnitTests.Data
                 }
             };
 
-            var target = new Mock<IPerspexObject>();
+            var target = new Mock<IPerspexObject>().As<IControl>();
             target.Setup(x => x.GetValue(Control.DataContextProperty)).Returns(source);
 
             var subject = binding.Initiate(target.Object, null).Subject;

tests/Perspex.Markup.Xaml.UnitTests/Xaml/BindingTests.cs

@@ -80,6 +80,31 @@ namespace Perspex.Markup.Xaml.UnitTests.Xaml
             }
         }
 
+        [Fact]
+        public void Can_Bind_To_DataContext_Of_Anchor_On_Non_Control()
+        {
+            using (UnitTestApplication.Start(TestServices.StyledWindow))
+            {
+                var xaml = @"
+<Window xmlns='https://github.com/perspex'
+             xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'
+             xmlns:local='clr-namespace:Perspex.Markup.Xaml.UnitTests.Xaml;assembly=Perspex.Markup.Xaml.UnitTests'>
+    <Button Name='button'>
+        <Button.Tag>
+            <local:NonControl String='{Binding Foo}'/>
+        </Button.Tag>
+    </Button>
+</Window>";
+                var loader = new PerspexXamlLoader();
+                var window = (Window)loader.Load(xaml);
+                var button = window.FindControl<Button>("button");
+
+                button.DataContext = new { Foo = "foo" };
+
+                Assert.Equal("foo", ((NonControl)button.Tag).String);
+            }
+        }
+
         [Fact]
         public void Binding_To_Window_Works()
         {

tests/Perspex.Markup.Xaml.UnitTests/Xaml/NonControl.cs

@@ -10,10 +10,19 @@ namespace Perspex.Markup.Xaml.UnitTests.Xaml
         public static readonly StyledProperty<Control> ControlProperty =
             PerspexProperty.Register<NonControl, Control>("Control");
 
+        public static readonly StyledProperty<string> StringProperty =
+            PerspexProperty.Register<NonControl, string>("String");
+
         public Control Control
         {
             get { return GetValue(ControlProperty); }
             set { SetValue(ControlProperty, value); }
         }
+
+        public string String
+        {
+            get { return GetValue(StringProperty); }
+            set { SetValue(StringProperty, value); }
+        }
     }
 }