[iOS] Prevent multiple ListView cells from being swiped simultaneously (

xamarin#578)

* disable multiple cell swipe

* add sample code

* refactored

* convert to weakreference

* remove null setting

* change weakreference setting place

* remove if

* revert isopen changes

* add instructions

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Bugzilla43735.cs

@@ -0,0 +1,61 @@
+using System.Collections.Generic;
+using Xamarin.Forms.CustomAttributes;
+using Xamarin.Forms.Internals;
+
+#if UITEST
+using Xamarin.UITest;
+using NUnit.Framework;
+#endif
+
+namespace Xamarin.Forms.Controls.Issues
+{
+	[Preserve(AllMembers = true)]
+	[Issue(IssueTracker.Bugzilla, 43735, "Multiple Swipe on ContextActions", PlatformAffected.iOS)]
+	public class Bugzilla43735 : TestContentPage // or TestMasterDetailPage, etc ...
+	{
+		protected override void Init()
+		{
+			var stackLayout = new StackLayout();
+
+			var l = new Label
+			{
+				Text = "Swipe multiple cells at the same time. Only one cell should show its context actions."
+			};
+			stackLayout.Children.Add(l);
+
+			var list = new List<int>();
+			for (var i = 0; i < 20; i++)
+				list.Add(i);
+
+			var listView = new ListView
+			{
+				ItemsSource = list,
+				ItemTemplate = new DataTemplate(() =>
+				{
+					var label = new Label();
+					label.SetBinding(Label.TextProperty, new Binding("."));
+
+					return new ViewCell
+					{
+						View = new ContentView
+						{
+							Content = label,
+						},
+						ContextActions = { new MenuItem
+						{
+							Text = "Action"
+						},
+						new MenuItem
+						{
+							Text = "Delete",
+							IsDestructive = true
+						} }
+					};
+				})
+			};
+			stackLayout.Children.Add(listView);
+
+			Content = stackLayout;
+		}
+	}
+}

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

@@ -139,6 +139,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)Bugzilla43469.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Bugzilla43516.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Bugzilla43663.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)Bugzilla43735.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Bugzilla44453.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Bugzilla44944.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Bugzilla44166.cs" />

Xamarin.Forms.Platform.iOS/ContextScrollViewDelegate.cs

@@ -45,7 +45,7 @@ internal class ContextScrollViewDelegate : UIScrollViewDelegate
 		GlobalCloseContextGestureRecognizer _globalCloser;
 
 		bool _isDisposed;
-
+		static WeakReference<UIScrollView> s_scrollViewBeingScrolled;
 		UITableView _table;
 
 		public ContextScrollViewDelegate(UIView container, List<UIButton> buttons, bool isOpen)
@@ -72,6 +72,11 @@ public ContextScrollViewDelegate(UIView container, List<UIButton> buttons, bool
 
 		public override void DraggingStarted(UIScrollView scrollView)
 		{
+			if (ShouldIgnoreScrolling(scrollView))
+				return;
+
+			s_scrollViewBeingScrolled = new WeakReference<UIScrollView>(scrollView);
+
 			if (!IsOpen)
 				SetButtonsShowing(true);
 
@@ -90,6 +95,9 @@ public void PrepareForDeselect(UIScrollView scrollView)
 
 		public override void Scrolled(UIScrollView scrollView)
 		{
+			if (ShouldIgnoreScrolling(scrollView))
+				return;
+
 			var width = _finalButtonSize;
 			var count = _buttons.Count;
 
@@ -116,10 +124,9 @@ public override void Scrolled(UIScrollView scrollView)
 				SetButtonsShowing(false);
 				RestoreHighlight(scrollView);
 
+				s_scrollViewBeingScrolled = null;
 				ClearCloserRecognizer(scrollView);
-
-				if (ClosedCallback != null)
-					ClosedCallback();
+				ClosedCallback?.Invoke();
 			}
 		}
 
@@ -131,6 +138,9 @@ public void Unhook(UIScrollView scrollView)
 
 		public override void WillEndDragging(UIScrollView scrollView, PointF velocity, ref PointF targetContentOffset)
 		{
+			if (ShouldIgnoreScrolling(scrollView))
+				return;
+
 			var width = ButtonsWidth;
 			var x = targetContentOffset.X;
 			var parentThreshold = scrollView.Frame.Width * .4f;
@@ -187,6 +197,19 @@ public override void WillEndDragging(UIScrollView scrollView, PointF velocity, r
 			}
 		}
 
+		static bool ShouldIgnoreScrolling(UIScrollView scrollView)
+		{
+			if (s_scrollViewBeingScrolled == null)
+				return false;
+
+			UIScrollView scrollViewBeingScrolled;
+			if (!s_scrollViewBeingScrolled.TryGetTarget(out scrollViewBeingScrolled) || ReferenceEquals(scrollViewBeingScrolled, scrollView))
+				return false;
+
+			scrollView.SetContentOffset(new PointF(0, 0), false);
+			return true;
+		}
+
 		protected override void Dispose(bool disposing)
 		{
 			if (_isDisposed)