Add back support for using MainPage instead of CreateWindow

src/Compatibility/ControlGallery/src/Core/App.cs

@@ -35,13 +35,11 @@ public App()
 
 			//TestMainPageSwitches();
 
-			if (Device.RuntimePlatform != Device.iOS)
-				SetMainPage(new ImageSourcesGallery());
-		}
+			SetMainPage(CreateDefaultMainPage());
 
-		protected override IWindow CreateWindow(IActivationState activationState)
-		{
-			return new Window(CreateDefaultMainPage());
+			//TestMainPageSwitches();
+
+			//SetMainPage(new ImageSourcesGallery());
 		}
 
 		protected override void OnStart()

src/Controls/samples/Controls.Sample/XamlApp.xaml.cs

@@ -17,13 +17,10 @@ public XamlApp(IServiceProvider services, ITextService textService)
 
 			Debug.WriteLine($"The .NET Purple color is {Resources["DotNetPurple"]}");
 			Debug.WriteLine($"The injected text service had a message: '{textService.GetText()}'");
+
+			MainPage = Services.GetRequiredService<Page>();
 		}
 
 		public IServiceProvider Services { get; }
-
-		protected override IWindow CreateWindow(IActivationState activationState)
-		{
-			return Services.GetRequiredService<IWindow>();
-		}
 	}
 }

src/Controls/src/Core/Application.cs

@@ -63,82 +63,60 @@ public IAppLinks AppLinks
 
 		public static Application Current { get; set; }
 
-		// TODO MAUI. What should this be?
 		public Page MainPage
 		{
 			get
 			{
 				if (Windows.Count == 0)
 					return null;
 
-				return Windows[0].View as Page;
+				return Windows[0].Page;
 			}
 			set
 			{
 				if (value == null)
-					throw new ArgumentNullException("value");
+					throw new ArgumentNullException(nameof(value));
 
 				if (MainPage == value)
 					return;
 
+				OnPropertyChanging();
+
+				var previousPage = MainPage;
+
+				if (previousPage != null)
+					previousPage.Parent = null;
+
 				if (Windows.Count == 0)
 				{
-					OnPropertyChanging();
+					// there are no windows, so add a new window
+
 					AddWindow(new Window(value));
-					OnPropertyChanged();
 				}
 				else
 				{
-					var mainPage = MainPage;
-
-					if (mainPage == value)
-						return;
-
-					OnPropertyChanging();
-					if (mainPage != null)
-						mainPage.Parent = null;
-
-					Windows[0].View = (IView)value;
+					// find the best window and replace the page
+
+					var theWindow = Windows[0];
+					foreach (var window in Windows)
+					{
+						if (window.Page == previousPage)
+						{
+							theWindow = window;
+							break;
+						}
+					}
+
+					theWindow.Page = value;
+				}
 
-					if (mainPage != null)
-						mainPage.NavigationProxy.Inner = NavigationProxy;
+				if (previousPage != null)
+					previousPage.NavigationProxy.Inner = NavigationProxy;
 
-					OnPropertyChanged();
-				}
+				OnPropertyChanged();
 			}
 		}
 
-		//public Page MainPage
-		//{
-		//	get { return _mainPage; }
-		//	set
-		//	{
-		//		if (value == null)
-		//			throw new ArgumentNullException("value");
-
-		//		if (_mainPage == value)
-		//			return;
-
-		//		OnPropertyChanging();
-		//		if (_mainPage != null)
-		//		{
-		//			InternalChildren.Remove(_mainPage);
-		//			_mainPage.Parent = null;
-		//		}
-
-		//		_mainPage = value;
-
-		//		if (_mainPage != null)
-		//		{
-		//			_mainPage.Parent = this;
-		//			_mainPage.NavigationProxy.Inner = NavigationProxy;
-		//			InternalChildren.Add(_mainPage);
-		//		}
-
-		//		OnPropertyChanged();
-		//	}
-		//}
-
 		public IDictionary<string, object> Properties
 		{
 			get

src/Controls/src/Core/HandlerImpl/Application.Impl.cs

@@ -5,19 +5,23 @@ namespace Microsoft.Maui.Controls
 {
 	public partial class Application : IApplication
 	{
-		List<IWindow> _windows = new List<IWindow>();
-		public IReadOnlyList<IWindow> Windows => _windows.AsReadOnly();
+		readonly List<Window> _windows = new();
+
+		IReadOnlyList<IWindow> IApplication.Windows => Windows;
+
+		public IReadOnlyList<Window> Windows => _windows.AsReadOnly();
 
 		IWindow IApplication.CreateWindow(IActivationState activationState)
 		{
-			IWindow window = CreateWindow(activationState);
+			var window = CreateWindow(activationState);
 
-			AddWindow(window);
+			if (!_windows.Contains(window))
+				AddWindow(window);
 
 			return window;
 		}
 
-		void AddWindow(IWindow window)
+		void AddWindow(Window window)
 		{
 			_windows.Add(window);
 
@@ -32,9 +36,12 @@ void AddWindow(IWindow window)
 				ne.NavigationProxy.Inner = NavigationProxy;
 		}
 
-		protected virtual IWindow CreateWindow(IActivationState activationState)
+		protected virtual Window CreateWindow(IActivationState activationState)
 		{
-			throw new NotImplementedException();
+			if (Windows.Count > 0)
+				return Windows[0];
+
+			throw new NotImplementedException($"Either set {nameof(MainPage)} or override {nameof(Application.CreateWindow)}.");
 		}
 	}
 }

src/Controls/tests/Core.UnitTests/ApplicationTests.cs

@@ -0,0 +1,152 @@
+using System;
+using NUnit.Framework;
+
+namespace Microsoft.Maui.Controls.Core.UnitTests
+{
+	[TestFixture]
+	public class ApplicationTests : BaseTestFixture
+	{
+		[Test]
+		public void NewApplicationHasNoWindowsNorPage()
+		{
+			var app = new Application();
+
+			Assert.Null(app.MainPage);
+			Assert.IsEmpty(app.Windows);
+		}
+
+		[Test]
+		public void SettingMainPageSetsMainPageAndWindow()
+		{
+			var app = new Application();
+			var page = new ContentPage();
+
+			app.MainPage = page;
+
+			Assert.AreEqual(page, app.MainPage);
+			Assert.AreEqual(1, app.Windows.Count);
+			Assert.AreEqual(page, app.Windows[0].Page);
+		}
+
+		[Test]
+		public void CreateWindowUsesMainPage()
+		{
+			var app = new Application();
+			var iapp = app as IApplication;
+			var page = new ContentPage();
+
+			app.MainPage = page;
+
+			var window = iapp.CreateWindow(null);
+
+			Assert.AreEqual(page, app.MainPage);
+			Assert.IsNotEmpty(app.Windows);
+			Assert.AreEqual(1, app.Windows.Count);
+			Assert.AreEqual(window, app.Windows[0]);
+			Assert.AreEqual(page, app.Windows[0].Page);
+		}
+
+		[Test]
+		public void SettingMainPageUpdatesWindow()
+		{
+			var app = new Application();
+			var iapp = app as IApplication;
+			var page = new ContentPage();
+
+			app.MainPage = page;
+			var window = iapp.CreateWindow(null);
+
+			var page2 = new ContentPage();
+			app.MainPage = page2;
+
+			Assert.AreEqual(page2, app.MainPage);
+			Assert.IsNotEmpty(app.Windows);
+			Assert.AreEqual(1, app.Windows.Count);
+			Assert.AreEqual(window, app.Windows[0]);
+			Assert.AreEqual(page2, app.Windows[0].Page);
+		}
+
+		[Test]
+		public void NotSettingMainPageThrows()
+		{
+			var app = new Application();
+			var iapp = app as IApplication;
+
+			Assert.Throws<NotImplementedException>(() => iapp.CreateWindow(null));
+		}
+
+		[Test]
+		public void CanUseExistingWindow()
+		{
+			var window = new Window();
+
+			var app = new StubApp { MainWindow = window };
+			var iapp = app as IApplication;
+
+			var win = iapp.CreateWindow(null);
+
+			Assert.AreEqual(window, win);
+			Assert.Null(app.MainPage);
+		}
+
+		[Test]
+		public void CanUseExistingWindowWithPage()
+		{
+			var window = new Window { Page = new ContentPage() };
+
+			var app = new StubApp { MainWindow = window };
+			var iapp = app as IApplication;
+
+			var win = iapp.CreateWindow(null);
+
+			Assert.AreEqual(window, win);
+			Assert.AreEqual(window.Page, app.MainPage);
+		}
+
+		[Test]
+		public void SettingMainPageOverwritesExistingPage()
+		{
+			var window = new Window { Page = new ContentPage() };
+
+			var app = new StubApp { MainWindow = window };
+			var iapp = app as IApplication;
+
+			var win = iapp.CreateWindow(null);
+
+			var page2 = new ContentPage();
+			app.MainPage = page2;
+
+			Assert.AreEqual(window, win);
+			Assert.AreEqual(page2, app.MainPage);
+			Assert.AreEqual(window.Page, app.MainPage);
+		}
+
+		[Test]
+		public void SettingWindowPageOverwritesMainPage()
+		{
+			var window = new Window { Page = new ContentPage() };
+
+			var app = new StubApp { MainWindow = window };
+			var iapp = app as IApplication;
+
+			var win = iapp.CreateWindow(null);
+
+			var page2 = new ContentPage();
+			window.Page = page2;
+
+			Assert.AreEqual(window, win);
+			Assert.AreEqual(page2, app.MainPage);
+			Assert.AreEqual(window.Page, app.MainPage);
+		}
+
+		class StubApp : Application
+		{
+			public Window MainWindow { get; set; }
+
+			protected override Window CreateWindow(IActivationState activationState)
+			{
+				return MainWindow;
+			}
+		}
+	}
+}

src/Controls/tests/Core.UnitTests/WindowsTests.cs

@@ -72,7 +72,7 @@ public class TestApp : Application
 			public TestWindow CreateWindow() =>
 				(TestWindow)(this as IApplication).CreateWindow(null);
 
-			protected override IWindow CreateWindow(IActivationState activationState)
+			protected override Window CreateWindow(IActivationState activationState)
 			{
 				return new TestWindow(new ContentPage());
 			}

src/Core/src/LifecycleEvents/Android/AndroidLifecycle.cs

@@ -8,6 +8,13 @@ namespace Microsoft.Maui.LifecycleEvents
 {
 	public static class AndroidLifecycle
 	{
+		// Events called by the Application
+		public delegate void OnApplicationCreating(Application application);
+		public delegate void OnApplicationCreate(Application application);
+		public delegate void OnApplicationLowMemory(Application application);
+		public delegate void OnApplicationTrimMemory(Application application, TrimMemory level);
+		public delegate void OnApplicationConfigurationChanged(Application application, Configuration newConfig);
+
 		// Events called by the ActivityLifecycleCallbacks
 		public delegate void OnCreate(Activity activity, Bundle? savedInstanceState);
 		public delegate void OnStart(Activity activity);

src/Core/src/LifecycleEvents/Android/AndroidLifecycleBuilderExtensions.cs

@@ -2,6 +2,12 @@
 {
 	public static class AndroidLifecycleBuilderExtensions
 	{
+		public static IAndroidLifecycleBuilder OnApplicationCreating(this IAndroidLifecycleBuilder lifecycle, AndroidLifecycle.OnApplicationCreating del) => lifecycle.OnEvent(del);
+		public static IAndroidLifecycleBuilder OnApplicationCreate(this IAndroidLifecycleBuilder lifecycle, AndroidLifecycle.OnApplicationCreate del) => lifecycle.OnEvent(del);
+		public static IAndroidLifecycleBuilder OnApplicationLowMemory(this IAndroidLifecycleBuilder lifecycle, AndroidLifecycle.OnApplicationLowMemory del) => lifecycle.OnEvent(del);
+		public static IAndroidLifecycleBuilder OnApplicationTrimMemory(this IAndroidLifecycleBuilder lifecycle, AndroidLifecycle.OnApplicationTrimMemory del) => lifecycle.OnEvent(del);
+		public static IAndroidLifecycleBuilder OnApplicationConfigurationChanged(this IAndroidLifecycleBuilder lifecycle, AndroidLifecycle.OnApplicationConfigurationChanged del) => lifecycle.OnEvent(del);
+
 		public static IAndroidLifecycleBuilder OnActivityResult(this IAndroidLifecycleBuilder lifecycle, AndroidLifecycle.OnActivityResult del) => lifecycle.OnEvent(del);
 		public static IAndroidLifecycleBuilder OnBackPressed(this IAndroidLifecycleBuilder lifecycle, AndroidLifecycle.OnBackPressed del) => lifecycle.OnEvent(del);
 		public static IAndroidLifecycleBuilder OnConfigurationChanged(this IAndroidLifecycleBuilder lifecycle, AndroidLifecycle.OnConfigurationChanged del) => lifecycle.OnEvent(del);