Prevent FormsAppCompatActivity from loading fragments we don't use on restart (#246)

* Prevent FormsAppCompatActivity from loading fragments we don't use on restart

* Adding issues lost in merge

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Bugzilla42075.cs

@@ -0,0 +1,39 @@
+using Xamarin.Forms.CustomAttributes;
+using Xamarin.Forms.Internals;
+
+namespace Xamarin.Forms.Controls
+{
+	[Preserve (AllMembers = true)]
+	[Issue (IssueTracker.Bugzilla, 42075, "IllegalStateException - Fragment does not have a view", PlatformAffected.Android)]
+	public class Bugzilla42075 : TestTabbedPage
+	{
+		protected override void Init()
+		{
+			Title = "Outer";
+
+			const string text = @"To run this test, you'll need to have an emulator or device in Developer mode, with the ""Don't Keep Activities"" setting turned on.
+Hit the Home button to dismiss the application. Then bring up the Overview (recent apps) screen and select the Control Gallery.
+If the application crashes with ""Java.Lang.IllegalStateException: Fragment does not have a view"", this test has failed. If the application does not crash or crashes with a different exception, this test has passed.";
+
+			var directions = new ContentPage
+			{
+				Content = new StackLayout()
+				{
+					Children =
+					{
+						new Label()
+						{
+							Text = text
+						}
+					}
+				}
+			};
+
+			var tabbedPage = new TabbedPage() {Title = "Inner"};
+			tabbedPage.Children.Add(new ContentPage());
+
+			Children.Add(directions);
+			Children.Add(tabbedPage);
+		}
+	}
+}

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Xamarin.Forms.Controls.Issues.Shared.projitems

@@ -110,6 +110,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)Bugzilla41205.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Bugzilla41424.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Bugzilla42074.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)Bugzilla42075.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)CarouselAsync.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Bugzilla34561.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Bugzilla34727.cs" />

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

@@ -44,6 +44,9 @@ namespace Xamarin.Forms.Platform.Android
 		int _statusBarHeight = -1;
 		global::Android.Views.View _statusBarUnderlay;
 
+		// Override this if you want to handle the default Android behavior of restoring fragments on an application restart
+		protected virtual bool AllowFragmentRestore => false;
+
 		protected FormsAppCompatActivity()
 		{
 			_previousState = AndroidApplicationLifecycleState.Uninitialized;
@@ -139,8 +142,17 @@ namespace Xamarin.Forms.Platform.Android
 				callback(resultCode, data);
 		}
 
+
 		protected override void OnCreate(Bundle savedInstanceState)
 		{
+			if (!AllowFragmentRestore)
+			{
+				// Remove the automatically persisted fragment structure; we don't need them
+				// because we're rebuilding everything from scratch. This saves a bit of memory
+				// and prevents loading errors from child fragment managers
+				savedInstanceState?.Remove("android:support:fragments");
+			}
+
 			base.OnCreate(savedInstanceState);
 
 			AToolbar bar;