Merge pull request #4963 from worldbeater/rx-user-controls

fix: DataContext Inheritance in ReactiveUserControl<TViewModel>

src/Avalonia.ReactiveUI/ReactiveUserControl.cs

@@ -28,11 +28,7 @@ namespace Avalonia.ReactiveUI
             // This WhenActivated block calls ViewModel's WhenActivated
             // block if the ViewModel implements IActivatableViewModel.
             this.WhenActivated(disposables => { });
-
-            this.ObservableForProperty(x => x.ViewModel, false, true)
-                .Subscribe(args => DataContext = args.Value);
-            this.ObservableForProperty(x => x.DataContext, false, true)
-                .Subscribe(args => ViewModel = args.Value as TViewModel);
+            this.GetObservable(ViewModelProperty).Subscribe(OnViewModelChanged);
         }
 
         /// <summary>
@@ -49,5 +45,22 @@ namespace Avalonia.ReactiveUI
             get => ViewModel;
             set => ViewModel = (TViewModel)value;
         }
+
+        protected override void OnDataContextChanged(EventArgs e)
+        {
+            ViewModel = DataContext as TViewModel;
+        }
+
+        private void OnViewModelChanged(object value)
+        {
+            if (value == null)
+            {
+                ClearValue(DataContextProperty);
+            }
+            else if (DataContext != value)
+            {
+                DataContext = value;
+            }
+        }
     }
 }

src/Avalonia.ReactiveUI/ReactiveWindow.cs

@@ -28,11 +28,8 @@ namespace Avalonia.ReactiveUI
             // This WhenActivated block calls ViewModel's WhenActivated
             // block if the ViewModel implements IActivatableViewModel.
             this.WhenActivated(disposables => { });
-
-            this.ObservableForProperty(x => x.ViewModel, false, true)
-                .Subscribe(args => DataContext = args.Value);
-            this.ObservableForProperty(x => x.DataContext, false, true)
-                .Subscribe(args => ViewModel = args.Value as TViewModel);
+            this.GetObservable(DataContextProperty).Subscribe(OnDataContextChanged);
+            this.GetObservable(ViewModelProperty).Subscribe(OnViewModelChanged);
         }
             
         /// <summary>
@@ -49,5 +46,29 @@ namespace Avalonia.ReactiveUI
             get => ViewModel;
             set => ViewModel = (TViewModel)value;
         }
+
+        private void OnDataContextChanged(object value)
+        {
+            if (value is TViewModel viewModel)
+            {
+                ViewModel = viewModel;
+            }
+            else
+            {
+                ViewModel = null;
+            }
+        }
+
+        private void OnViewModelChanged(object value)
+        {
+            if (value == null)
+            {
+                ClearValue(DataContextProperty);
+            }
+            else if (DataContext != value)
+            {
+                DataContext = value;
+            }
+        }
     }
 }

tests/Avalonia.ReactiveUI.UnitTests/ReactiveUserControlTest.cs

@@ -1,4 +1,5 @@
 using System.Reactive.Disposables;
+using Avalonia.Controls;
 using Avalonia.UnitTests;
 using ReactiveUI;
 using Splat;
@@ -100,5 +101,75 @@ namespace Avalonia.ReactiveUI.UnitTests
             Assert.NotNull(view.DataContext);
             Assert.False(view.ViewModel.IsActive);
         }
+        
+        [Fact]
+        public void Should_Inherit_DataContext()
+        {
+            var vm1 = new ExampleViewModel();
+            var vm2 = new ExampleViewModel();
+            var view = new ExampleView();
+            var root = new TestRoot(view);
+            
+            Assert.Null(view.DataContext);
+            Assert.Null(view.ViewModel);
+            
+            root.DataContext = vm1;
+            
+            Assert.Same(vm1, view.DataContext);
+            Assert.Same(vm1, view.ViewModel);
+            
+            root.DataContext = null;
+            
+            Assert.Null(view.DataContext);
+            Assert.Null(view.ViewModel);
+            
+            root.DataContext = vm2;
+            
+            Assert.Same(vm2, view.DataContext);
+            Assert.Same(vm2, view.ViewModel);
+        }
+
+        [Fact]
+        public void Should_Not_Overlap_Change_Notifications()
+        {
+            var vm1 = new ExampleViewModel();
+            var vm2 = new ExampleViewModel();
+
+            var view1 = new ExampleView();
+            var view2 = new ExampleView();
+            
+            Assert.Null(view1.DataContext);
+            Assert.Null(view2.DataContext);
+            Assert.Null(view1.ViewModel);
+            Assert.Null(view2.ViewModel);
+
+            view1.DataContext = vm1;
+            
+            Assert.Same(vm1, view1.DataContext);
+            Assert.Same(vm1, view1.ViewModel);
+            Assert.Null(view2.DataContext);
+            Assert.Null(view2.ViewModel);
+
+            view2.DataContext = vm2;
+
+            Assert.Same(vm1, view1.DataContext);
+            Assert.Same(vm1, view1.ViewModel);
+            Assert.Same(vm2, view2.DataContext);
+            Assert.Same(vm2, view2.ViewModel);
+
+            view1.ViewModel = null;
+
+            Assert.Null(view1.DataContext);
+            Assert.Null(view1.ViewModel);
+            Assert.Same(vm2, view2.DataContext);
+            Assert.Same(vm2, view2.ViewModel);
+
+            view2.ViewModel = null;
+
+            Assert.Null(view1.DataContext);
+            Assert.Null(view2.DataContext);
+            Assert.Null(view1.ViewModel);
+            Assert.Null(view2.ViewModel);
+        }
     }
 }