[Android] Prevent white flash when switching MainPage to new MasterDetailPage (#3283) fixes #1373

* Test rapid MDP swaps

* [Android] Prevent white screen flash

* [Android] Prevent crashes on rapid page swaps

* Shorten test time

* Preserve old code just in case

* Remove flaky test from main run

Confirmed that it doesn't ever crash on UITest

* More info in 1601 test

* Update NavigateToTestPage to await test cases page

This lets the page render and then clean itself up when we switch to the test case, which allows the test case for 1601 to actually render with a compressed layout after my change to Platform.

* Remove call to FragmentManager.ExecutePendingTransactions in MasterDetailContainer

this code was only added to prevent the page flicker, and it caused more issues than it solved.

* SetPage will now delay removing views until the new views are in place

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Issue1601.cs

@@ -49,7 +49,9 @@ namespace Xamarin.Forms.Controls.Issues
 		{
 			RunningApp.Screenshot("Start G1601");
 			RunningApp.WaitForElement(q => q.Marked("CRASH!"));
+			RunningApp.Screenshot("I see the button");
 			RunningApp.Tap (q => q.Marked ("CRASH!"));
+			RunningApp.Screenshot("Didn't crash!");
 		}
 #endif
 	}

Xamarin.Forms.Controls/App.cs

@@ -32,9 +32,7 @@ namespace Xamarin.Forms.Controls
 
 			SetMainPage(CreateDefaultMainPage());
 
-			//TestBugzilla44596();
-
-			//TestBugzilla45702();
+			//TestMainPageSwitches();
 		}
 
 		protected override void OnStart()
@@ -42,23 +40,28 @@ namespace Xamarin.Forms.Controls
 			//TestIssue2393();
 		}
 
-		void TestBugzilla44596()
+		async Task TestBugzilla44596()
 		{
+			await Task.Delay(50);
 			// verify that there is no gray screen displayed between the blue splash and red MasterDetailPage.
-			SetMainPage(new Bugzilla44596SplashPage(() =>
+			SetMainPage(new Bugzilla44596SplashPage(async () =>
 			{
 				var newTabbedPage = new TabbedPage();
-				newTabbedPage.Children.Add(new ContentPage { BackgroundColor = Color.Red, Content = new Label { Text = "yay" } });
+				newTabbedPage.Children.Add(new ContentPage { BackgroundColor = Color.Red, Content = new Label { Text = "Success" } });
 				MainPage = new MasterDetailPage
 				{
 					Master = new ContentPage { Title = "Master", BackgroundColor = Color.Red },
 					Detail = newTabbedPage
 				};
+
+				await Task.Delay(50);
+				SetMainPage(CreateDefaultMainPage());
 			}));
 		}
 
-		void TestBugzilla45702()
+		async Task TestBugzilla45702()
 		{
+			await Task.Delay(50);
 			// verify that there is no crash when switching MainPage from MDP inside NavPage
 			SetMainPage(new Bugzilla45702());
 		}
@@ -91,6 +94,13 @@ namespace Xamarin.Forms.Controls
 			//SetMainPage(new Issues.Issue2004());
 		}
 
+		async Task TestMainPageSwitches()
+		{
+			await TestBugzilla45702();
+
+			await TestBugzilla44596();
+		}
+
 		public Page CreateDefaultMainPage()
 		{
 			var layout = new StackLayout { BackgroundColor = Color.Red };
@@ -123,8 +133,7 @@ namespace Xamarin.Forms.Controls
 					string page = parts[1].Trim();
 					var pageForms = Activator.CreateInstance(Type.GetType(page));
 
-					var appLinkPageGallery = pageForms as AppLinkPageGallery;
-					if (appLinkPageGallery != null)
+					if (pageForms is AppLinkPageGallery appLinkPageGallery)
 					{
 						appLinkPageGallery.ShowLabel = true;
 						(MainPage as MasterDetailPage)?.Detail.Navigation.PushAsync((pageForms as Page));
@@ -178,8 +187,7 @@ namespace Xamarin.Forms.Controls
 
 		static async Task<string> LoadResource(string filename)
 		{
-			string assemblystring;
-			Assembly assembly = GetAssembly(out assemblystring);
+			Assembly assembly = GetAssembly(out string assemblystring);
 
 			Stream stream = assembly.GetManifestResourceStream($"{assemblystring}.{filename}");
 			string text;
@@ -198,9 +206,9 @@ namespace Xamarin.Forms.Controls
 				// Set up a delegate to handle the navigation to the test page
 				EventHandler toTestPage = null;
 
-				toTestPage = delegate (object sender, EventArgs e)
+				toTestPage = async delegate (object sender, EventArgs e)
 				{
-					Current.MainPage.Navigation.PushModalAsync(TestCases.GetTestCases());
+					await Current.MainPage.Navigation.PushModalAsync(TestCases.GetTestCases());
 					TestCases.TestCaseScreen.PageToAction[test]();
 					Current.MainPage.Appearing -= toTestPage;
 				};

Xamarin.Forms.Controls/Bugzilla44596SplashPage.cs

@@ -18,7 +18,7 @@ namespace Xamarin.Forms.Controls
 		protected async override void OnAppearing()
 		{
 			base.OnAppearing();
-			await Task.Delay(2000);
+			await Task.Delay(500);
 			FinishedLoading?.Invoke();
 		}
 	}

Xamarin.Forms.Platform.Android/AppCompat/MasterDetailContainer.cs

@@ -116,18 +116,6 @@ namespace Xamarin.Forms.Platform.Android.AppCompat
 				transaction.CommitAllowingStateLossEx();
 
 				_currentFragment = fragment;
-
-				new Handler(Looper.MainLooper).PostAtFrontOfQueue(() =>
-				{
-					if (_pageContainer == null)
-					{
-						// The view we're hosting in the fragment was never created (possibly we're already 
-						// navigating to another page?) so there's nothing to commit
-						return;
-					}
-
-					FragmentManager.ExecutePendingTransactionsEx();
-				});
 			}
 		}
 

Xamarin.Forms.Platform.Android/AppCompat/Platform.cs

@@ -206,6 +206,9 @@ namespace Xamarin.Forms.Platform.Android.AppCompat
 
 		void IPlatformLayout.OnLayout(bool changed, int l, int t, int r, int b)
 		{
+			if (Page == null)
+				return;
+
 			if (changed)
 			{
 				LayoutRootPage(Page, r - l, b - t);
@@ -262,12 +265,16 @@ namespace Xamarin.Forms.Platform.Android.AppCompat
 		{
 			var layout = false;
 
+			var viewsToRemove = new List<global::Android.Views.View>();
+			var renderersToDispose = new List<IVisualElementRenderer>();
+
 			if (Page != null)
 			{
-				_renderer.RemoveAllViews();
+				for (int i = 0; i < _renderer.ChildCount; i++)
+					viewsToRemove.Add(_renderer.GetChildAt(i));
 
-				foreach (IVisualElementRenderer rootRenderer in _navModel.Roots.Select(Android.Platform.GetRenderer))
-					rootRenderer.Dispose();
+				foreach (var root in _navModel.Roots)
+					renderersToDispose.Add(Android.Platform.GetRenderer(root));
 
 				_navModel = new NavigationModel();
 
@@ -275,7 +282,10 @@ namespace Xamarin.Forms.Platform.Android.AppCompat
 			}
 
 			if (newRoot == null)
+			{
+				Cleanup(viewsToRemove, renderersToDispose);
 				return;
+			}
 
 			_navModel.Push(newRoot, null);
 
@@ -283,10 +293,25 @@ namespace Xamarin.Forms.Platform.Android.AppCompat
 			Page.Platform = this;
 			AddChild(Page, layout);
 
+			Cleanup(viewsToRemove, renderersToDispose);
+
 			Application.Current.NavigationProxy.Inner = this;
 		}
+
+		void Cleanup(List<global::Android.Views.View> viewsToRemove, List<IVisualElementRenderer> renderersToDispose)
+		{
+			foreach (var view in viewsToRemove)
+				_renderer?.RemoveView(view);
+
+			foreach (IVisualElementRenderer rootRenderer in renderersToDispose)
+				rootRenderer?.Dispose();
+		}
+
 		void AddChild(Page page, bool layout = false)
 		{
+			if (page == null)
+				return;
+
 			if (Android.Platform.GetRenderer(page) != null)
 				return;