feature: Add default WhenActivated calls to WPF base classes (#4277)

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[Contribute](https://github.com/reactiveui/reactiveui#contribute)
section of the README -->

**What kind of change does this PR introduce?**
<!-- Bug fix, feature, docs update, ... -->

update

**What is the current behavior?**
<!-- You can also link to an open issue here. -->

A IActivatableViewModel ViewModel would not trigger any events unless
WhenActivated exists in the View

**What is the new behavior?**
<!-- If this is a feature change -->

Introduced parameterless constructors in ReactivePage,
ReactiveUserControl, and ReactiveWindow that call WhenActivated with a
no-op. This ensures that IActivatableViewModel logic in the ViewModel is
triggered when the view is activated, improving consistency and
activation handling in WPF views.

**What might this PR break?**

If no WhenActivated has been created then the current code would not
execute, from now on it would be triggered correctly.

**Please check if the PR fulfills these requirements**
- [ ] Tests for the changes have been added (for bug fixes / features)
- [ ] Docs have been added / updated (for bug fixes / features)

**Other information**:

Author: ChrisPulman

src/ReactiveUI.Wpf/Common/ReactivePage.cs

@@ -160,6 +160,15 @@ protected ReactivePage(IntPtr handle)
     {
     }
 #endif
+#else
+    /// <summary>
+    /// Initializes a new instance of the <see cref="ReactivePage{TViewModel}"/> class.
+    /// </summary>
+    public ReactivePage() => this.WhenActivated(disposables =>
+                                  {
+                                      // No-op, but ensures that when the Page is activated,
+                                      // any IActivatableViewModel logic in the ViewModel is also triggered.
+                                  });
 #endif
 
     /// <summary>

src/ReactiveUI.Wpf/Common/ReactiveUserControl.cs

@@ -144,6 +144,15 @@ protected ReactiveUserControl(IntPtr handle)
     {
     }
 #endif
+#else
+    /// <summary>
+    /// Initializes a new instance of the <see cref="ReactiveUserControl{TViewModel}"/> class.
+    /// </summary>
+    public ReactiveUserControl() => this.WhenActivated(disposables =>
+                                         {
+                                             // No-op, but ensures that when the Page is activated,
+                                             // any IActivatableViewModel logic in the ViewModel is also triggered.
+                                         });
 #endif
 
     /// <summary>

src/ReactiveUI.Wpf/ReactiveWindow.cs

@@ -53,6 +53,18 @@ public class ReactiveWindow<TViewModel> :
                                     typeof(ReactiveWindow<TViewModel>),
                                     new PropertyMetadata(null));
 
+    /// <summary>
+    /// Initializes a new instance of the <see cref="ReactiveWindow{TViewModel}"/> class.
+    /// </summary>
+    /// <remarks>When the window is activated, this constructor ensures that the ViewModel's activation logic
+    /// is also triggered if the ViewModel implements IActivatableViewModel. This enables coordinated activation and
+    /// deactivation of resources tied to the window and its ViewModel.</remarks>
+    public ReactiveWindow() => this.WhenActivated(disposables =>
+                                    {
+                                        // No-op, but ensures that when the Page is activated,
+                                        // any IActivatableViewModel logic in the ViewModel is also triggered.
+                                    });
+
     /// <summary>
     /// Gets the binding root view model.
     /// </summary>

src/tests/ReactiveUI.AOTTests/TestConfiguration.cs

@@ -85,13 +85,9 @@ private sealed class ViewB : IViewFor<VmB>
         public VmB? ViewModel { get; set; }
     }
 
-    private sealed class VmA : ReactiveObject
-    {
-    }
+    private sealed class VmA : ReactiveObject;
 
-    private sealed class VmB : ReactiveObject
-    {
-    }
+    private sealed class VmB : ReactiveObject;
 
     private sealed class ViewA : IViewFor<VmA>
     {

src/tests/ReactiveUI.AOTTests/ViewLocatorAOTMappingTests.cs

@@ -143,7 +143,5 @@ protected override void OnAfterRender(bool firstRender)
     /// <summary>
     /// A concrete implementation of ReactiveComponentBase for testing activatable ViewModels.
     /// </summary>
-    public class TestActivatableComponent : ReactiveComponentBase<TestActivatableViewModel>
-    {
-    }
+    public class TestActivatableComponent : ReactiveComponentBase<TestActivatableViewModel>;
 }

src/tests/ReactiveUI.Blazor.Tests/ReactiveComponentBaseTests.cs

@@ -194,7 +194,5 @@ protected override void OnAfterRender(bool firstRender)
     /// <summary>
     /// A concrete implementation of ReactiveInjectableComponentBase for testing with activatable ViewModels.
     /// </summary>
-    public class TestActivatableInjectableComponent : ReactiveInjectableComponentBase<TestActivatableViewModel>
-    {
-    }
+    public class TestActivatableInjectableComponent : ReactiveInjectableComponentBase<TestActivatableViewModel>;
 }

src/tests/ReactiveUI.Blazor.Tests/ReactiveInjectableComponentBaseTests.cs

@@ -28,11 +28,10 @@ public async Task PageAndChildViewActivateAndDeactivate()
     {
         AppBuilder.ResetBuilderStateForTests();
         var resolver = new ModernDependencyResolver();
-        resolver.InitializeSplat();
         resolver.CreateReactiveUIBuilder()
             .WithPlatformServices()
+            .WithRegistrationOnBuild(r => r.RegisterConstant<IActivationForViewFetcher>(new ActivationForViewFetcher()))
             .BuildApp();
-        resolver.RegisterConstant<IActivationForViewFetcher>(new ActivationForViewFetcher());
 
         using (resolver.WithResolver())
         {

src/tests/ReactiveUI.Builder.Maui.Tests/Activation/ActivationForViewFetcherTests.cs

@@ -14,10 +14,8 @@ public static class AssemblyHooks
     /// Called before any tests in this assembly start.
     /// </summary>
     [Before(HookType.Assembly)]
-    public static void AssemblySetup()
-    {
+    public static void AssemblySetup() =>
         ModeDetector.OverrideModeDetector(new TestModeDetector());
-    }
 
     /// <summary>
     /// Called after all tests in this assembly complete.

src/tests/ReactiveUI.Builder.Maui.Tests/AssemblyHooks.cs

@@ -29,10 +29,9 @@ public async Task WithMaui_Should_Register_Services()
     {
         AppBuilder.ResetBuilderStateForTests();
         using var locator = new ModernDependencyResolver();
-
         locator.CreateReactiveUIBuilder()
                .WithMaui()
-               .Build();
+               .BuildApp();
 
         var observableProperty = locator.GetService<ICreatesObservableForProperty>();
         await Assert.That(observableProperty).IsNotNull();