feature: Support ReactiveUI WhenActivated

src/Avalonia.ReactiveUI/AppBuilderExtensions.cs

@@ -4,6 +4,7 @@
 using Avalonia.Controls;
 using Avalonia.Threading;
 using ReactiveUI;
+using Splat;
 
 namespace Avalonia
 {
@@ -15,6 +16,9 @@ namespace Avalonia
             return builder.AfterSetup(_ =>
             {
                 RxApp.MainThreadScheduler = AvaloniaScheduler.Instance;
+                Locator.CurrentMutable.Register(
+                    () => new AvaloniaActivationForViewFetcher(), 
+                    typeof(IActivationForViewFetcher));
             });
         }
     }

src/Avalonia.ReactiveUI/AvaloniaActivationForViewFetcher.cs

@@ -0,0 +1,38 @@
+// Copyright (c) The Avalonia Project. All rights reserved.
+// Licensed under the MIT license. See licence.md file in the project root for full license information.
+
+using System;
+using System.Reflection;
+using System.Reactive.Linq;
+using Avalonia;
+using Avalonia.VisualTree;
+using ReactiveUI;
+
+namespace Avalonia
+{
+    public class AvaloniaActivationForViewFetcher : IActivationForViewFetcher
+    {
+        public int GetAffinityForView(Type view)
+        {
+            return typeof(IVisual).GetTypeInfo().IsAssignableFrom(view.GetTypeInfo()) ? 10 : 0;
+        }
+
+        public IObservable<bool> GetActivationForView(IActivatable view)
+        {
+            if (!(view is IVisual visual)) return Observable.Return(false);
+             var viewLoaded = Observable
+                .FromEventPattern<VisualTreeAttachmentEventArgs>(
+                    x => visual.AttachedToVisualTree += x,
+                    x => visual.DetachedFromVisualTree -= x)
+                .Select(args => true);
+             var viewUnloaded = Observable
+                .FromEventPattern<VisualTreeAttachmentEventArgs>(
+                    x => visual.DetachedFromVisualTree += x,
+                    x => visual.DetachedFromVisualTree -= x)
+                .Select(args => false);
+             return viewLoaded
+                .Merge(viewUnloaded)
+                .DistinctUntilChanged();
+        }
+    }
+}