Fix Android Back Button

e2e/Maui/PrismMauiDemo.sln

@@ -29,7 +29,7 @@ Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Prism.Maui.Tests", "..\..\t
 EndProject
 Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Prism.Core.Tests", "..\..\tests\Prism.Core.Tests\Prism.Core.Tests.csproj", "{E0F13AA9-8083-47CA-B10D-93C5285D1505}"
 EndProject
-Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Prism.Events", "..\..\src\Prism.Events\Prism.Events.csproj", "{5623CB62-59C1-49BC-BB16-4C5D63D82DAC}"
+Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Prism.Events", "..\..\src\Prism.Events\Prism.Events.csproj", "{5623CB62-59C1-49BC-BB16-4C5D63D82DAC}"
 EndProject
 Global
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
@@ -95,6 +95,7 @@ Global
 		{EE6F0C99-61D1-4E2E-8185-FBA0D246D5C7} = {E91F80AA-3D61-4C28-B876-3EDFB5921E7D}
 		{F3D2DFDB-95FB-4CBB-A624-35EB6550854D} = {E91F80AA-3D61-4C28-B876-3EDFB5921E7D}
 		{E0F13AA9-8083-47CA-B10D-93C5285D1505} = {E91F80AA-3D61-4C28-B876-3EDFB5921E7D}
+		{5623CB62-59C1-49BC-BB16-4C5D63D82DAC} = {8202B92A-A573-4365-8A15-E246504A7CBD}
 	EndGlobalSection
 	GlobalSection(ExtensibilityGlobals) = postSolution
 		SolutionGuid = {50B0D1F3-D832-4C6C-858E-24F5F3B33632}

src/Maui/Prism.Maui/Common/MvvmHelpers.cs

@@ -269,13 +269,42 @@ public static void SetCurrentPageDelegate(Func<Page, Page> getCurrentPageDelegat
         if (lastModal != null)
             page = lastModal;
 
-        return GetOnNavigatedToTargetFromChild(page);
+        return EvaluateCurrentPage(page);
     };
 
+    private static Page EvaluateCurrentPage(Page target)
+    {
+        Page child = null;
+
+        if (target is FlyoutPage flyout)
+            child = flyout.Detail;
+        else if (target is TabbedPage tabbed)
+            child = tabbed.CurrentPage;
+        else if (target is NavigationPage np)
+            child = np.Navigation.NavigationStack.Last();
+
+        if (child != null)
+            target = GetOnNavigatedToTargetFromChild(child);
+
+        if (target is Page page)
+            return page.Parent switch
+            {
+                TabbedPage tab when tab.CurrentPage != target => EvaluateCurrentPage(tab.CurrentPage),
+                NavigationPage nav when nav.CurrentPage != target => EvaluateCurrentPage(nav.CurrentPage),
+                _ => target
+            };
+
+        return null;
+    }
+
     public static async Task HandleNavigationPageGoBack(NavigationPage navigationPage)
     {
         var navigationService = Navigation.Xaml.Navigation.GetNavigationService(navigationPage.CurrentPage);
-        await navigationService.GoBackAsync();
+        var result = await navigationService.GoBackAsync();
+        if(result.Exception is NavigationException navEx && navEx.Message == NavigationException.CannotPopApplicationMainPage)
+        {
+            Application.Current.Quit();
+        }
     }
 
     public static void HandleSystemGoBack(IView previousPage, IView currentPage)

src/Maui/Prism.Maui/Controls/PrismNavigationPage.cs

@@ -2,21 +2,33 @@
 
 namespace Prism.Controls;
 
+/// <summary>
+/// Provides a wrapper for the NavigationPage to better handle the OnBackButtonPressed event with Prism Navigation
+/// </summary>
 public class PrismNavigationPage : NavigationPage
 {
+    /// <summary>
+    /// Creates a new instance of the <see cref="PrismNavigationPage"/>
+    /// </summary>
     public PrismNavigationPage()
     {
         BackButtonPressed += HandleBackButtonPressed;
     }
 
+    /// <summary>
+    /// Creates a new instance of the <see cref="PrismNavigationPage"/> with a specified <see cref="Page"/> at the Root
+    /// </summary>
+    /// <param name="page"></param>
     public PrismNavigationPage(Page page)
         : base(page)
     {
         BackButtonPressed += HandleBackButtonPressed;
     }
 
+    /// <inheritdoc/>
     public event EventHandler BackButtonPressed;
 
+    /// <inheritdoc/>
     protected override bool OnBackButtonPressed()
     {
         BackButtonPressed.Invoke(this, EventArgs.Empty);
@@ -27,4 +39,4 @@ private async void HandleBackButtonPressed(object sender, EventArgs args)
     {
         await MvvmHelpers.HandleNavigationPageGoBack(this);
     }
-}
+}

src/Maui/Prism.Maui/Navigation/PrismWindow.cs

@@ -1,5 +1,11 @@
-using Prism.AppModel;
+using System.ComponentModel;
+using Prism.AppModel;
 using Prism.Common;
+using Prism.Ioc;
+using Prism.Navigation.Xaml;
+using Prism.Services;
+using Prism.Xaml;
+using TabbedPage = Microsoft.Maui.Controls.TabbedPage;
 
 namespace Prism.Navigation;
 
@@ -19,6 +25,56 @@ public PrismWindow(string name = DefaultWindowName)
 
     internal Page CurrentPage => Page is null ? null : MvvmHelpers.GetCurrentPage(Page);
 
+    internal bool IsRootPage => Page switch
+    {
+        TabbedPage tabbed => tabbed.CurrentPage,
+        NavigationPage nav => nav.RootPage,
+        _ => Page
+    } == CurrentPage;
+
+    [EditorBrowsable(EditorBrowsableState.Never)]
+    public void OnSystemBack()
+    {
+        var currentPage = CurrentPage;
+        if(currentPage?.Parent is NavigationPage navPage)
+        {
+            // The NavigationPage has already taken care of the GoBack
+            return;
+        }
+
+        var container = currentPage.GetContainerProvider();
+
+        if (IsRoot(currentPage))
+        {
+            var app = container.Resolve<IApplication>() as Application;
+            app.Quit();
+            return;
+        }
+        else if (currentPage is IDialogContainer dialogContainer)
+        {
+            if (dialogContainer.Dismiss.CanExecute(null))
+                dialogContainer.Dismiss.Execute(null);
+        }
+        else
+        {
+            var navigation = container.Resolve<INavigationService>();
+            navigation.GoBackAsync();
+        }
+    }
+
+    private bool IsRoot(Page page)
+    {
+        if (page == Page) return true;
+
+        return page.Parent switch
+        {
+            FlyoutPage flyout => IsRoot(flyout),
+            TabbedPage tabbed => IsRoot(tabbed),
+            NavigationPage navigation => IsRoot(navigation),
+            _ => false
+        };
+    }
+
     private async void PrismWindow_ModalPopping(object sender, ModalPoppingEventArgs e)
     {
         if (PageNavigationService.NavigationSource == PageNavigationSource.Device)

src/Maui/Prism.Maui/PrismAppBuilder.cs

@@ -45,28 +45,23 @@ internal PrismAppBuilder(IContainerExtension containerExtension, MauiAppBuilder
                 {
                     var root = ContainerLocator.Container;
                     if (root is null)
-                        return true;
+                        return false;
 
                     var app = root.Resolve<IApplication>();
                     var windows = app.Windows.OfType<PrismWindow>();
                     if (!windows.Any(x => x.IsActive))
-                        return true;
+                        return false;
 
                     var window = windows.First(x => x.IsActive);
-                    var currentPage = window.CurrentPage;
-                    var container = currentPage.GetContainerProvider();
-                    if(currentPage is IDialogContainer dialogContainer)
+                    if(window.IsRootPage && app is Application application)
                     {
-                        if (dialogContainer.Dismiss.CanExecute(null))
-                            dialogContainer.Dismiss.Execute(null);
-                    }
-                    else
-                    {
-                        var navigation = container.Resolve<INavigationService>();
-                        navigation.GoBackAsync();
+                        application.Quit();
+                        return false;
                     }
 
-                    return false;
+                    window.OnSystemBack();
+
+                    return true;
                 });
             });
 #endif