Fix for #3700 Maui Activation fails (#3703)

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[Contribute](https://github.com/reactiveui/reactiveui#contribute)
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**What kind of change does this PR introduce?**
<!-- Bug fix, feature, docs update, ... -->

Fix for #3700 

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

Maui Activation fails.

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

Maui Activation updated to include all relevant types

**What might this PR break?**

none expected

**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**:

src/ReactiveUI.Maui/ActivationForViewFetcher.cs

@@ -6,15 +6,13 @@
 using System.Reflection;
 #if WINUI_TARGET
 using Microsoft.UI.Xaml;
-
 using Windows.Foundation;
 #endif
 
 #if IS_WINUI
 namespace ReactiveUI.WinUI;
 #endif
 #if IS_MAUI
-using System.ComponentModel;
 using Microsoft.Maui.Controls;
 
 namespace ReactiveUI.Maui;
@@ -28,12 +26,10 @@ public class ActivationForViewFetcher : IActivationForViewFetcher
 {
     /// <inheritdoc/>
     public int GetAffinityForView(Type view) =>
-#if WINUI_TARGET
-#if IS_MAUI
-       typeof(Page).GetTypeInfo().IsAssignableFrom(view.GetTypeInfo()) ||
-#endif
+#if IS_WINUI
        typeof(FrameworkElement).GetTypeInfo().IsAssignableFrom(view.GetTypeInfo())
-#else
+#endif
+#if IS_MAUI
        typeof(Page).GetTypeInfo().IsAssignableFrom(view.GetTypeInfo()) ||
        typeof(View).GetTypeInfo().IsAssignableFrom(view.GetTypeInfo()) ||
        typeof(Cell).GetTypeInfo().IsAssignableFrom(view.GetTypeInfo())
@@ -45,12 +41,10 @@ public IObservable<bool> GetActivationForView(IActivatableView view)
     {
         var activation =
             GetActivationFor(view as ICanActivate) ??
-#if WINUI_TARGET
+#if IS_WINUI
             GetActivationFor(view as FrameworkElement) ??
-#if IS_MAUI
-            GetActivationFor(view as Page) ??
 #endif
-#else
+#if IS_MAUI
             GetActivationFor(view as Page) ??
             GetActivationFor(view as View) ??
             GetActivationFor(view as Cell) ??
@@ -63,7 +57,7 @@ public IObservable<bool> GetActivationForView(IActivatableView view)
     private static IObservable<bool>? GetActivationFor(ICanActivate? canActivate) =>
         canActivate?.Activated.Select(_ => true).Merge(canActivate.Deactivated.Select(_ => false));
 
-#if !WINUI_TARGET || (WINUI_TARGET && IS_MAUI)
+#if IS_MAUI
     private static IObservable<bool>? GetActivationFor(Page? page)
     {
         if (page is null)
@@ -93,7 +87,7 @@ public IObservable<bool> GetActivationForView(IActivatableView view)
     }
 #endif
 
-#if !WINUI_TARGET
+#if IS_MAUI
     private static IObservable<bool>? GetActivationFor(View? view)
     {
         if (view is null)

src/ReactiveUI.Maui/ReactiveUI.Maui.csproj

@@ -31,6 +31,8 @@
 
   <ItemGroup>
     <Compile Remove="Common\RoutedViewHost.cs" />
+    <Compile Remove="Common\ViewModelViewHost.cs" />
+    <Compile Remove="WinUI\TransitioningContentControl.Empty.cs" />
   </ItemGroup>
 
 </Project>

src/ReactiveUI.Maui/ViewModelViewHost.cs

@@ -60,9 +60,7 @@ public ViewModelViewHost()
                                                                               this.WhenAnyObservable(x => x.ViewContractObservable),
                                                                               (vm, contract) => new { ViewModel = vm, Contract = contract, });
 
-        this.WhenActivated(() =>
-        {
-            return new[]
+        this.WhenActivated(() => new[]
             {
                 vmAndContract.Subscribe(x =>
                 {
@@ -75,13 +73,7 @@ public ViewModelViewHost()
                     }
 
                     var viewLocator = ViewLocator ?? ReactiveUI.ViewLocator.Current;
-                    var view = viewLocator.ResolveView(x.ViewModel, x.Contract) ?? viewLocator.ResolveView(x.ViewModel);
-
-                    if (view is null)
-                    {
-                        throw new Exception($"Couldn't find view for '{x.ViewModel}'.");
-                    }
-
+                    var view = (viewLocator.ResolveView(x.ViewModel, x.Contract) ?? viewLocator.ResolveView(x.ViewModel)) ?? throw new Exception($"Couldn't find view for '{x.ViewModel}'.");
                     if (view is not View castView)
                     {
                         throw new Exception($"View '{view.GetType().FullName}' is not a subclass of '{typeof(View).FullName}'.");
@@ -90,8 +82,7 @@ public ViewModelViewHost()
                     view.ViewModel = x.ViewModel;
                     Content = castView;
                 })
-            };
-        });
+            });
     }
 
     /// <summary>
@@ -137,4 +128,4 @@ public View DefaultContent
     /// Gets or sets the override for the view locator to use when resolving the view. If unspecified, <see cref="ViewLocator.Current"/> will be used.
     /// </summary>
     public IViewLocator? ViewLocator { get; set; }
-}
+}