Fix "ScrollView resets position" issue during changing TranslationX/TranslationY

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Issue3840.cs

@@ -0,0 +1,91 @@
+using System;
+using System.Collections.Generic;
+using System.Collections.ObjectModel;
+using System.Text;
+using Xamarin.Forms.CustomAttributes;
+using Xamarin.Forms.Internals;
+using System.Threading.Tasks;
+
+
+#if UITEST
+using Xamarin.UITest;
+using NUnit.Framework;
+using Xamarin.Forms.Core.UITests;
+#endif
+
+namespace Xamarin.Forms.Controls.Issues
+{
+	[Preserve(AllMembers = true)]
+	[Issue(IssueTracker.Github, 3840, "[iOS] Translation change causes ScrollView to reset to initial position (0, 0)",
+		PlatformAffected.iOS)]
+#if UITEST
+	[NUnit.Framework.Category(UITestCategories.ScrollView)]
+#endif
+	public class Issue3840 : TestContentPage
+	{
+		const string _failedText = "Test Failed if Visible";
+		const string _button1 = "FirstClick";
+		const string _button2 = "SecondClick";
+
+		protected override void Init()
+		{
+			ScrollView scroll = null;
+			scroll = new ScrollView
+			{
+				Content = new StackLayout
+				{
+					Children =
+					{
+						new Label()
+						{
+							Text = _failedText
+						},
+						new Button()
+						{
+							Text = "Click Me First",
+							AutomationId = _button1,
+							Command = new Command(async () =>
+							{
+								await scroll.ScrollToAsync(0, 100, true);
+							}),
+							HorizontalOptions = LayoutOptions.Start
+						},
+						new BoxView { Color = Color.Red, HeightRequest = 500 },
+						new Button()
+						{
+							Text = "Click Me Second",
+							AutomationId = _button2,
+							Command = new Command(async () =>
+							{
+								scroll.TranslationX = 100;
+								await Task.Delay(100);
+								// using one because of a bug on UWP that doesn't react to being set back to zero
+								scroll.TranslationX = 1;
+
+							}),
+							HorizontalOptions = LayoutOptions.Start
+						},
+						new BoxView { Color = Color.Gray, HeightRequest = 500 },
+						new BoxView { Color = Color.Yellow, HeightRequest = 500 }
+					}
+				}
+			};
+
+			var mainLayout = new AbsoluteLayout();
+			mainLayout.Children.Add(scroll, new Rectangle(0, 0, 1, 1), AbsoluteLayoutFlags.All);
+			Content = mainLayout;
+		}
+
+
+#if UITEST
+		[Test]
+		public void TranslatingViewKeepsScrollViewPosition()
+		{
+			RunningApp.WaitForElement(_failedText);
+			RunningApp.Tap(_button1);
+			RunningApp.Tap(_button2);
+			RunningApp.WaitForNoElement(_failedText);
+		}
+#endif
+	}
+}

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

@@ -393,6 +393,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)Issue3398.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue3558.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue3541.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)Issue3840.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue3913.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)LegacyComponents\NonAppCompatSwitch.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)MapsModalCrash.cs" />
@@ -997,7 +998,7 @@
   <ItemGroup>
     <EmbeddedResource Include="$(MSBuildThisFileDirectory)Issue1588.xaml">
       <SubType>Designer</SubType>
-      <Generator>MSBuild:Compile</Generator>
+      <Generator>MSBuild:UpdateDesignTimeXaml</Generator>
     </EmbeddedResource>
   </ItemGroup>
   <ItemGroup>

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

@@ -236,8 +236,8 @@ void HandleScrolled(object sender, EventArgs e)
 
 		void OnNativeControlUpdated(object sender, EventArgs eventArgs)
 		{
-			ContentSize = Bounds.Size;
-			UpdateContentSize();
+			var elementContentSize = RetrieveElementContentSize();
+			ContentSize = elementContentSize.IsEmpty ? Bounds.Size : elementContentSize;
 		}
 
 		void OnScrollToRequested(object sender, ScrollToRequestedEventArgs e)
@@ -287,11 +287,16 @@ void UpdateBackgroundColor()
 
 		void UpdateContentSize()
 		{
-			var contentSize = ((ScrollView)Element).ContentSize.ToSizeF();
+			var contentSize = RetrieveElementContentSize();
 			if (!contentSize.IsEmpty)
 				ContentSize = contentSize;
 		}
 
+		CoreGraphics.CGSize RetrieveElementContentSize()
+		{
+			return ((ScrollView)Element).ContentSize.ToSizeF();
+		}
+
 		void UpdateScrollPosition()
 		{
 			if (ScrollView != null)
@@ -303,4 +308,4 @@ void IEffectControlProvider.RegisterEffect(Effect effect)
 			VisualElementRenderer<VisualElement>.RegisterEffect(effect, this, NativeView);
 		}
 	}
-}
+}