fix(Maui): parent nested region hosts in ElementParentedCallbackBehavior

Ensure region hosts inside region guests receive Parent so nested regions
register correctly after layout (GitHub #3332).

Add DryIoc region fixture test with explicit measure/arrange for nested
ContentView regions.

Author: dansiegel

src/Maui/Prism.Maui/Behaviors/ElementParentedCallbackBehavior.cs

@@ -6,7 +6,8 @@ namespace Prism.Behaviors;
 
 internal class ElementParentedCallbackBehavior : Behavior<VisualElement>
 {
-    private Action _callback { get; }
+    private readonly Action _callback;
+    private VisualElement? _target;
 
     public ElementParentedCallbackBehavior(Action callback)
     {
@@ -15,11 +16,16 @@ public ElementParentedCallbackBehavior(Action callback)
 
     protected override void OnAttachedTo(VisualElement view)
     {
+        _target = view;
+
         if (view.TryGetParentPage(out var page))
         {
             var container = page.GetContainerProvider();
             if (container is null)
+            {
+                page.PropertyChanged -= PagePropertyChanged;
                 page.PropertyChanged += PagePropertyChanged;
+            }
             else
             {
                 view.SetContainerProvider(container);
@@ -32,22 +38,46 @@ protected override void OnAttachedTo(VisualElement view)
         }
     }
 
+    protected override void OnDetachingFrom(VisualElement view)
+    {
+        view.ParentChanged -= OnParentChanged;
+        if (view.Parent is VisualElement directParent)
+            directParent.ParentChanged -= OnParentChanged;
+
+        if (view.TryGetParentPage(out var page))
+            page.PropertyChanged -= PagePropertyChanged;
+
+        _target = null;
+        base.OnDetachingFrom(view);
+    }
+
     private void OnParentChanged(object sender, EventArgs e)
     {
-        if (sender is not VisualElement view || view.Parent is null)
+        // Use _target: when listening on Parent.ParentChanged, sender is the parent, not the region host (#3332).
+        var view = _target;
+        if (view?.Parent is null)
             return;
-        else if (view.TryGetParentPage(out var page))
+
+        if (view.TryGetParentPage(out var page))
         {
-            if(page.GetContainerProvider() is not null)
+            if (page.GetContainerProvider() is not null)
             {
                 view.ParentChanged -= OnParentChanged;
+                if (view.Parent is VisualElement directParent)
+                    directParent.ParentChanged -= OnParentChanged;
+
                 _callback();
                 return;
             }
+
+            page.PropertyChanged -= PagePropertyChanged;
             page.PropertyChanged += PagePropertyChanged;
         }
-        else
-            view.Parent.ParentChanged += OnParentChanged;
+        else if (view.Parent is VisualElement parent)
+        {
+            parent.ParentChanged -= OnParentChanged;
+            parent.ParentChanged += OnParentChanged;
+        }
 
         view.ParentChanged -= OnParentChanged;
     }
@@ -59,7 +89,7 @@ private void PagePropertyChanged(object sender, PropertyChangedEventArgs e)
 
         var container = page.GetContainerProvider();
 
-        if(container is not null)
+        if (container is not null)
         {
             page.PropertyChanged -= PagePropertyChanged;
             _callback();

tests/Maui/Prism.DryIoc.Maui.Tests/Fixtures/Regions/RegionFixture.cs

@@ -1,4 +1,7 @@
-using Prism.DryIoc.Maui.Tests.Mocks.ViewModels;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Maui.Graphics;
+using Microsoft.Maui.Layouts;
+using Prism.DryIoc.Maui.Tests.Mocks.ViewModels;
 using Prism.DryIoc.Maui.Tests.Mocks.Views;
 using Prism.Navigation.Xaml;
 
@@ -286,4 +289,46 @@ public void Issue3328_WhenNavigatingToUnregisteredView_ShouldFailWithKeyNotFound
         Assert.IsType<MockRegionViewA>(page.ContentRegion.Content);
         Assert.IsType<MockRegionViewAViewModel>(page.ContentRegion.Content.BindingContext);
     }
+
+    [Fact]
+    public void Issue3332_NestedContentRegion_InnerRegionReceivesGuest_AfterLayout()
+    {
+        var mauiApp = CreateBuilder(prism => prism
+                .RegisterTypes(container =>
+                {
+                    container.RegisterForNavigation<MockNestedRegionPage, MockNestedRegionPageViewModel>();
+                    container.RegisterForRegionNavigation<MockOuterGuestView, MockOuterGuestViewModel>();
+                    container.RegisterForRegionNavigation<MockInnerGuestView, MockInnerGuestViewModel>();
+                })
+                .OnInitialized(container =>
+                {
+                    var regionManager = container.Resolve<IRegionManager>();
+                    regionManager.RegisterViewWithRegion("OuterRegion", nameof(MockOuterGuestView));
+                })
+                .CreateWindow("MockNestedRegionPage"))
+            .Build();
+
+        var window = GetWindow(mauiApp);
+        var page = Assert.IsType<MockNestedRegionPage>(window.Page);
+        Assert.NotNull(page.OuterRegion.Content);
+        var outerGuest = Assert.IsType<MockOuterGuestView>(page.OuterRegion.Content);
+
+        const double w = 400;
+        const double h = 800;
+        page.OuterRegion.Measure(w, h, MeasureFlags.IncludeMargins);
+        page.OuterRegion.Arrange(new Rect(0, 0, w, h));
+        outerGuest.InnerRegionHost.Measure(w, h, MeasureFlags.IncludeMargins);
+        outerGuest.InnerRegionHost.Arrange(new Rect(0, 0, w, h));
+
+        var regionManager = mauiApp.Services.GetRequiredService<IRegionManager>();
+        Assert.Contains(regionManager.Regions, r => r.Name == "InnerRegion");
+
+        regionManager.RegisterViewWithRegion("InnerRegion", nameof(MockInnerGuestView));
+
+        outerGuest.InnerRegionHost.Measure(w, h, MeasureFlags.IncludeMargins);
+        outerGuest.InnerRegionHost.Arrange(new Rect(0, 0, w, h));
+
+        Assert.NotNull(outerGuest.InnerRegionHost.Content);
+        Assert.IsType<MockInnerGuestView>(outerGuest.InnerRegionHost.Content);
+    }
 }

tests/Maui/Prism.DryIoc.Maui.Tests/Mocks/ViewModels/MockInnerGuestViewModel.cs

@@ -0,0 +1,5 @@
+namespace Prism.DryIoc.Maui.Tests.Mocks.ViewModels;
+
+public class MockInnerGuestViewModel
+{
+}

tests/Maui/Prism.DryIoc.Maui.Tests/Mocks/ViewModels/MockNestedRegionPageViewModel.cs

@@ -0,0 +1,5 @@
+namespace Prism.DryIoc.Maui.Tests.Mocks.ViewModels;
+
+public class MockNestedRegionPageViewModel
+{
+}

tests/Maui/Prism.DryIoc.Maui.Tests/Mocks/ViewModels/MockOuterGuestViewModel.cs

@@ -0,0 +1,5 @@
+namespace Prism.DryIoc.Maui.Tests.Mocks.ViewModels;
+
+public class MockOuterGuestViewModel
+{
+}

tests/Maui/Prism.DryIoc.Maui.Tests/Mocks/Views/MockInnerGuestView.cs

@@ -0,0 +1,10 @@
+namespace Prism.DryIoc.Maui.Tests.Mocks.Views;
+
+/// <summary>Guest view injected into the nested inner region.</summary>
+public class MockInnerGuestView : ContentView
+{
+    public MockInnerGuestView()
+    {
+        Content = new Label { Text = nameof(MockInnerGuestView) };
+    }
+}

tests/Maui/Prism.DryIoc.Maui.Tests/Mocks/Views/MockNestedRegionPage.cs

@@ -0,0 +1,14 @@
+namespace Prism.DryIoc.Maui.Tests.Mocks.Views;
+
+/// <summary>Shell page with a single outer <see cref="ContentView"/> region (GitHub #3332 repro shape).</summary>
+public class MockNestedRegionPage : ContentPage
+{
+    public MockNestedRegionPage()
+    {
+        OuterRegion = new ContentView();
+        OuterRegion.SetValue(Prism.Navigation.Regions.Xaml.RegionManager.RegionNameProperty, "OuterRegion");
+        Content = OuterRegion;
+    }
+
+    public ContentView OuterRegion { get; }
+}

tests/Maui/Prism.DryIoc.Maui.Tests/Mocks/Views/MockOuterGuestView.cs

@@ -0,0 +1,14 @@
+namespace Prism.DryIoc.Maui.Tests.Mocks.Views;
+
+/// <summary>Outer region guest that hosts an inner <see cref="ContentView"/> region.</summary>
+public class MockOuterGuestView : ContentView
+{
+    public MockOuterGuestView()
+    {
+        InnerRegionHost = new ContentView();
+        InnerRegionHost.SetValue(Prism.Navigation.Regions.Xaml.RegionManager.RegionNameProperty, "InnerRegion");
+        Content = InnerRegionHost;
+    }
+
+    public ContentView InnerRegionHost { get; }
+}