[iOS] Allow Forms gestures on custom renderers for controls which already have gestures (#990)

* Repro 57114 with UI test; fix for 57114 on iOS

* Repro/UI test for Windows

* Add helpful comment for posterity

* Remove stray TODO

* Only do ShouldReceiveTouch on mobile

* Explicitly require wrapped UIView to have gesture recognizers

1 files changed, 37 insertions, 5 deletions

diff --git a/Xamarin.Forms.Platform.iOS/EventTracker.cs b/Xamarin.Forms.Platform.iOS/EventTracker.cs
index 48f30648..5b2156c9 100644
--- a/Xamarin.Forms.Platform.iOS/EventTracker.cs
+++ b/Xamarin.Forms.Platform.iOS/EventTracker.cs
@@ -33,7 +33,7 @@ namespace Xamarin.Forms.Platform.MacOS
 
 		double _previousScale = 1.0;
 #if __MOBILE__
-		UITouchEventArgs _shouldReceive;
+		UITouchEventArgs _shouldReceiveTouch;
 #endif
 
 		public EventTracker(IVisualElementRenderer renderer)
@@ -82,9 +82,7 @@ namespace Xamarin.Forms.Platform.MacOS
 		{
 			if (_disposed)
 				throw new ObjectDisposedException(null);
-#if __MOBILE__
-			_shouldReceive = (r, t) => t.View is IVisualElementRenderer;
-#endif
+
 			_handler = handler;
 			OnElementChanged(this, new VisualElementChangedEventArgs(null, _renderer.Element));
 		}
@@ -281,11 +279,20 @@ namespace Xamarin.Forms.Platform.MacOS
 			return result;
 		}
 #endif
+
 		void LoadRecognizers()
 		{
 			if (ElementGestureRecognizers == null)
 				return;
 
+#if __MOBILE__
+			if (_shouldReceiveTouch == null)
+			{
+				// Cache this so we don't create a new UITouchEventArgs instance for every recognizer
+				_shouldReceiveTouch = ShouldReceiveTouch;
+			}
+#endif
+
 			foreach (var recognizer in ElementGestureRecognizers)
 			{
 				if (_gestureRecognizers.ContainsKey(recognizer))
@@ -295,7 +302,7 @@ namespace Xamarin.Forms.Platform.MacOS
 				if (nativeRecognizer != null)
 				{
 #if __MOBILE__
-					nativeRecognizer.ShouldReceiveTouch = _shouldReceive;
+					nativeRecognizer.ShouldReceiveTouch = _shouldReceiveTouch;
 #endif
 					_handler.AddGestureRecognizer(nativeRecognizer);
 
@@ -314,6 +321,31 @@ namespace Xamarin.Forms.Platform.MacOS
 			}
 		}
 
+#if __MOBILE__
+		bool ShouldReceiveTouch(UIGestureRecognizer recognizer, UITouch touch)
+		{
+			if (touch.View is IVisualElementRenderer)
+			{
+				return true;
+			}
+
+			// If the touch is coming from the UIView our renderer is wrapping (e.g., if it's  
+			// wrapping a UIView which already has a gesture recognizer), then we should let it through
+			// (This goes for children of that control as well)
+			if (_renderer?.NativeView == null)
+			{
+				return false;
+			}
+			
+			if (touch.View.IsDescendantOfView(_renderer.NativeView) && touch.View.GestureRecognizers?.Length > 0)
+			{
+				return true;
+			}
+
+			return false;
+		}
+#endif
+
 		void ModelGestureRecognizersOnCollectionChanged(object sender, NotifyCollectionChangedEventArgs notifyCollectionChangedEventArgs)
 		{
 			LoadRecognizers();