[WinRT/UWP] Add delay for layout in OnScrollToRequested (#988)

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Bugzilla36651.cs

@@ -0,0 +1,78 @@
+using Xamarin.Forms.CustomAttributes;
+using Xamarin.Forms.Internals;
+using System;
+
+#if UITEST
+using Xamarin.UITest;
+using NUnit.Framework;
+#endif
+
+namespace Xamarin.Forms.Controls.Issues
+{
+	[Preserve(AllMembers = true)]
+	[Issue(IssueTracker.Bugzilla, 36651, "ScrollToAsync fails to scroll on Windows Phone 8.1 RT", PlatformAffected.WinRT)]
+	public class Bugzilla36651 : TestNavigationPage
+	{
+		public class LandingPage : ContentPage
+		{
+			private static int numberOfButtons = 3;
+			StackLayout layout;
+			Button addItem, removeItem;
+			ScrollView scrollView;
+
+			public LandingPage()
+			{
+				layout = new StackLayout();
+				for (var i = 0; i < 10; i++)
+				{
+					layout.Children.Add(new Button { Text = "This is a button" });
+				}
+
+				addItem = new Button { Text = "Add Item" };
+				removeItem = new Button { Text = "Remove Item" };
+				scrollView = new ScrollView { Content = layout };
+
+				Content = new StackLayout
+				{
+					Children =
+					{
+						new Label
+						{
+							Text = "Clicking 'Add Item' should scroll to the end when added items go out of the view"
+						},
+						scrollView,
+						addItem,
+						removeItem
+					}
+				};
+
+				addItem.Clicked += (object sender, EventArgs e) =>
+				{
+					Button lastButton = null;
+					for (int i = 0; i < numberOfButtons; ++i)
+					{
+						lastButton = new Button
+						{
+							Text = String.Format("This is button #{0}", i)
+						};
+						layout.Children.Add(lastButton);
+					}
+					++numberOfButtons;
+
+					scrollView.ScrollToAsync(lastButton, ScrollToPosition.End, false);
+				};
+				removeItem.Clicked += (object sender, EventArgs e) =>
+				{
+					if (layout.Children.Count != 0)
+						layout.Children.RemoveAt(layout.Children.Count - 1);
+				};
+			}
+		}
+
+		protected override void Init()
+		{
+			var page = new LandingPage();
+			Navigation.PushAsync(page);
+		}
+	}
+}

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

@@ -84,6 +84,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)Bugzilla36559.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Bugzilla36171.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Bugzilla36780.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)Bugzilla36651.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Bugzilla36703.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Bugzilla36846.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Bugzilla36955.cs" />

Xamarin.Forms.Platform.WinRT/ScrollViewRenderer.cs

@@ -1,5 +1,6 @@
 using System;
 using System.ComponentModel;
+using System.Threading.Tasks;
 using Windows.Foundation;
 using Windows.UI.Xaml;
 using Windows.UI.Xaml.Controls;
@@ -124,8 +125,22 @@ namespace Xamarin.Forms.Platform.WinRT
 			UpdateMargins();
 		}
 
-		void OnScrollToRequested(object sender, ScrollToRequestedEventArgs e)
+		async void OnScrollToRequested(object sender, ScrollToRequestedEventArgs e)
 		{
+			// Adding items into the view while scrolling to the end can cause it to fail, as
+			// the items have not actually been laid out and return incorrect scroll position
+			// values. The ScrollViewRenderer for Android does something similar by waiting up
+			// to 10ms for layout to occur.
+			int cycle = 0;
+			while (!Element.IsInNativeLayout)
+			{
+				await Task.Delay(TimeSpan.FromMilliseconds(1));
+				cycle++;
+
+				if (cycle >= 10)
+					break;
+			}
+
 			double x = e.ScrollX, y = e.ScrollY;
 
 			ScrollToMode mode = e.Mode;