[Android] Make ScrollViewRenderer handle double disposal; fixes #1931 , fixes #2011 (#3025)

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Issue1931.cs

@@ -0,0 +1,109 @@
+using System.Collections.Generic;
+using Xamarin.Forms.CustomAttributes;
+using Xamarin.Forms.Internals;
+
+#if UITEST
+using Xamarin.UITest;
+using NUnit.Framework;
+using Xamarin.Forms.Core.UITests;
+#endif
+
+namespace Xamarin.Forms.Controls.Issues
+{
+#if UITEST
+	[Category(UITestCategories.ScrollView)]
+	[Category(UITestCategories.ListView)]
+#endif
+
+	[Preserve(AllMembers = true)]
+	[Issue(IssueTracker.Github, 1931, 
+		"Xamarin Forms on Android: ScrollView on ListView header crashes app when closing page",
+		PlatformAffected.Android)]
+	public class Issue1931 : TestNavigationPage
+	{
+		const string Go = "Go";
+		const string Back = "Back";
+		const string Success = "Success";
+		Label _result;
+		Label _instructions2;
+
+		ContentPage RootPage()
+		{
+			var page = new ContentPage();
+			page.Title = "GH1931 Root";
+
+			var button = new Button { Text = Go };
+			button.Clicked += (sender, args) => PushAsync(ListViewPage());
+
+			var instructions = new Label { Text = $"Tap the {Go} button" };
+
+			_result = new Label { Text = Success, IsVisible = false };
+			_instructions2 = new Label { Text = "If you can see this, the test has passed", IsVisible = false };
+
+			var layout = new StackLayout();
+			layout.Children.Add(instructions);
+			layout.Children.Add(button);
+			layout.Children.Add(_result);
+			layout.Children.Add(_instructions2);
+			page.Content = layout;
+
+			return page;
+		}
+
+		ContentPage ListViewPage()
+		{
+			var page = new ContentPage();
+
+			var layout = new StackLayout();
+
+			var listView = new ListView();
+
+			var scrollView = new ScrollView { Content = new BoxView { Color = Color.Green } };
+
+			listView.Header = scrollView;
+
+			listView.ItemsSource = new List<string> { "One", "Two", "Three" };
+
+			page.Title = "GH1931 Test";
+
+			var instructions = new Label { Text = $"Tap the {Back} button" };
+
+			var button = new Button { Text = Back };
+			button.Clicked += (sender, args) => PopAsync();
+
+			layout.Children.Add(instructions);
+			layout.Children.Add(button);
+			layout.Children.Add(listView);
+
+			page.Content = layout;
+
+			page.Appearing += (sender, args) =>
+			{
+				_instructions2.IsVisible = true;
+				_result.IsVisible = true;
+			};
+
+			return page;
+		}
+
+		protected override void Init()
+		{
+			PushAsync(RootPage());
+		}
+
+		
+#if UITEST
+		[Test]
+		public void ScrollViewInHeaderDisposesProperly()
+		{
+			RunningApp.WaitForElement(Go);
+			RunningApp.Tap(Go);
+
+			RunningApp.WaitForElement(Back);
+			RunningApp.Tap(Back);
+
+			RunningApp.WaitForElement(Success);
+		}
+#endif
+	}
+}

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

@@ -240,6 +240,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)Effects\AttachedStateEffect.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Effects\AttachedStateEffectLabel.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Effects\AttachedStateEffectList.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)Issue1931.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue2767.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue2499.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)GitHub1878.cs" />
@@ -907,4 +908,4 @@
       <Generator>MSBuild:UpdateDesignTimeXaml</Generator>
     </EmbeddedResource>
   </ItemGroup>
-</Project>
+</Project>

Xamarin.Forms.Platform.Android/Renderers/ScrollViewRenderer.cs

@@ -24,6 +24,7 @@ namespace Xamarin.Forms.Platform.Android
 		ScrollView _view;
 		int _previousBottom;
 		bool _isEnabled;
+		bool _disposed;
 
 		public ScrollViewRenderer(Context context) : base(context)
 		{
@@ -176,18 +177,24 @@ namespace Xamarin.Forms.Platform.Android
 
 		protected override void Dispose(bool disposing)
 		{
-			base.Dispose(disposing);
+			if (_disposed)
+			{
+				return;
+			}
 
-			SetElement(null);
+			_disposed = true;
 
 			if (disposing)
 			{
-				Tracker.Dispose();
+				SetElement(null);
+				Tracker?.Dispose();
 				Tracker = null;
 				RemoveAllViews();
-				_container.Dispose();
+				_container?.Dispose();
 				_container = null;
 			}
+
+			base.Dispose(disposing);
 		}
 
 		protected override void OnAttachedToWindow()