TwoWay DataContext and ViewModel sync

src/Avalonia.ReactiveUI/ReactiveUserControl.cs

@@ -1,3 +1,4 @@
+using System;
 using Avalonia;
 using Avalonia.VisualTree;
 using Avalonia.Controls;
@@ -20,7 +21,10 @@ namespace Avalonia.ReactiveUI
         /// </summary>
         public ReactiveUserControl()
         {
-            DataContextChanged += (sender, args) => ViewModel = DataContext as TViewModel;
+            this.WhenAnyValue(x => x.DataContext)
+                .Subscribe(context => ViewModel = context as TViewModel);
+            this.WhenAnyValue(x => x.ViewModel)
+                .Subscribe(model => DataContext = model);
         }
 
         /// <summary>

src/Avalonia.ReactiveUI/ReactiveWindow.cs

@@ -1,3 +1,4 @@
+using System;
 using Avalonia;
 using Avalonia.VisualTree;
 using Avalonia.Controls;
@@ -20,7 +21,10 @@ namespace Avalonia.ReactiveUI
         /// </summary>
         public ReactiveWindow()
         {
-            DataContextChanged += (sender, args) => ViewModel = DataContext as TViewModel;
+            this.WhenAnyValue(x => x.DataContext)
+                .Subscribe(context => ViewModel = context as TViewModel);
+            this.WhenAnyValue(x => x.ViewModel)
+                .Subscribe(model => DataContext = model);
         }
             
         /// <summary>

tests/Avalonia.ReactiveUI.UnitTests/AvaloniaActivationForViewFetcherTest.cs

@@ -88,12 +88,12 @@ namespace Avalonia.ReactiveUI.UnitTests
             }
         }
 
-        public AvaloniaActivationForViewFetcherTest()
-        {
-            Locator.CurrentMutable.RegisterConstant(
-                new AvaloniaActivationForViewFetcher(), 
-                typeof(IActivationForViewFetcher));
-        }
+        public AvaloniaActivationForViewFetcherTest() =>
+            Locator
+                .CurrentMutable
+                .RegisterConstant(
+                    new AvaloniaActivationForViewFetcher(),
+                    typeof(IActivationForViewFetcher));
 
         [Fact]
         public void Visual_Element_Is_Activated_And_Deactivated()

tests/Avalonia.ReactiveUI.UnitTests/ReactiveUserControlTest.cs

@@ -1,4 +1,3 @@
-using Avalonia.Controls;
 using Avalonia.UnitTests;
 using ReactiveUI;
 using Splat;
@@ -12,10 +11,20 @@ namespace Avalonia.ReactiveUI.UnitTests
 
         public class ExampleView : ReactiveUserControl<ExampleViewModel> { }
 
+        public ReactiveUserControlTest() =>
+            Locator
+                .CurrentMutable
+                .RegisterConstant(
+                    new AvaloniaActivationForViewFetcher(),
+                    typeof(IActivationForViewFetcher));
+
         [Fact]
         public void Data_Context_Should_Stay_In_Sync_With_Reactive_User_Control_View_Model() 
         {
+            var root = new TestRoot();
             var view = new ExampleView();
+            root.Child = view;
+
             var viewModel = new ExampleViewModel();
             Assert.Null(view.ViewModel);
 
@@ -26,6 +35,14 @@ namespace Avalonia.ReactiveUI.UnitTests
             view.DataContext = null;
             Assert.Null(view.ViewModel);
             Assert.Null(view.DataContext);
+
+            view.ViewModel = viewModel;
+            Assert.Equal(viewModel, view.ViewModel);
+            Assert.Equal(viewModel, view.DataContext);
+
+            view.ViewModel = null;
+            Assert.Null(view.ViewModel);
+            Assert.Null(view.DataContext);
         }
     }
-}
+}

tests/Avalonia.ReactiveUI.UnitTests/ReactiveWindowTest.cs

@@ -1,4 +1,3 @@
-using Avalonia.Controls;
 using Avalonia.UnitTests;
 using ReactiveUI;
 using Splat;
@@ -12,6 +11,13 @@ namespace Avalonia.ReactiveUI.UnitTests
 
         public class ExampleWindow : ReactiveWindow<ExampleViewModel> { }
 
+        public ReactiveWindowTest() =>
+            Locator
+                .CurrentMutable
+                .RegisterConstant(
+                    new AvaloniaActivationForViewFetcher(),
+                    typeof(IActivationForViewFetcher));
+
         [Fact]
         public void Data_Context_Should_Stay_In_Sync_With_Reactive_Window_View_Model() 
         {
@@ -19,16 +25,27 @@ namespace Avalonia.ReactiveUI.UnitTests
             {
                 var view = new ExampleWindow();
                 var viewModel = new ExampleViewModel();
+                view.Show();
+
                 Assert.Null(view.ViewModel);
+                Assert.Null(view.DataContext);
 
                 view.DataContext = viewModel;
-                Assert.Equal(view.ViewModel, viewModel);
-                Assert.Equal(view.DataContext, viewModel);
+                Assert.Equal(viewModel, view.ViewModel);
+                Assert.Equal(viewModel, view.DataContext);
 
                 view.DataContext = null;
                 Assert.Null(view.ViewModel);
                 Assert.Null(view.DataContext);
+
+                view.ViewModel = viewModel;
+                Assert.Equal(viewModel, view.ViewModel);
+                Assert.Equal(viewModel, view.DataContext);
+
+                view.ViewModel = null;
+                Assert.Null(view.ViewModel);
+                Assert.Null(view.DataContext);
             }
         }
     }
-}
+}