[ios] fix memory leak in Frame, VisualElementRenderer

Context: dotnet#18365

I could reproduce a leak in `Frame` by adding a parameterized test:

    [InlineData(typeof(Frame))]
    public async Task HandlerDoesNotLeak(Type type)

`FrameRenderer` has a circular reference via its base type,
`VisualElementRenderer`:

* `Frame` ->
* `FrameRenderer` / `VisualElementRenderer` ->
* `Frame` (via `VisualElementRenderer._virtualView`)

To solve this issue, I made `_virtualView` a `WeakReference`, but only
on iOS or MacCatalyst platforms. We don't necessarily need this
treatment for Windows or Android.

My initial attempt threw a `NullReferenceException`, because the
`Element` property is accessed before `SetVirtualView()` returns. I
had to create a `_tempElement` field to hold the value temporarily,
clearing it to avoid a circular reference.

The Roslyn analyzer didn't catch this cycle because it doesn't warn
about `IPlatformViewHandler`, only `NSObject`'s. Will investigate
further on this example here:

jonathanpeppers/memory-analyzers#12

src/Controls/src/Core/Compatibility/Handlers/VisualElementRenderer.cs

@@ -45,13 +45,22 @@ public abstract partial class VisualElementRenderer<TElement> : IPlatformViewHan
 
 		public static CommandMapper<TElement, IPlatformViewHandler> VisualElementRendererCommandMapper = new CommandMapper<TElement, IPlatformViewHandler>(ViewHandler.ViewCommandMapper);
 
+#if IOS || MACCATALYST
+		WeakReference<TElement>? _virtualView;
+		TElement? _tempElement;
+#else
 		TElement? _virtualView;
+#endif
 		IMauiContext? _mauiContext;
 		internal IPropertyMapper _mapper;
 		internal readonly CommandMapper? _commandMapper;
 		internal readonly IPropertyMapper _defaultMapper;
 		protected IMauiContext MauiContext => _mauiContext ?? throw new InvalidOperationException("MauiContext not set");
+#if IOS || MACCATALYST
+		public TElement? Element => _tempElement ?? (_virtualView is not null && _virtualView.TryGetTarget(out var target) ? target : null);
+#else
 		public TElement? Element => _virtualView;
+#endif
 		protected bool AutoPackage { get; set; } = true;
 
 #if ANDROID
@@ -283,14 +292,27 @@ internal static void SetVirtualView(
 
 		static partial void ProcessAutoPackage(Maui.IElement element);
 
+#if IOS || MACCATALYST
+		void IElementHandler.SetVirtualView(Maui.IElement view)
+		{
+			// _tempElement is used here, because the Element property is accessed before SetVirtualView() returns
+			_tempElement = Element;
+			SetVirtualView(view, this, OnElementChanged, ref _tempElement, ref _mapper, _defaultMapper, AutoPackage);
+
+			// We use _virtualView as a WeakReference, and clear _tempElement
+			_virtualView = _tempElement is null ? null : new(_tempElement);
+			_tempElement = null;
+		}
+#else
 		void IElementHandler.SetVirtualView(Maui.IElement view) =>
 			SetVirtualView(view, this, OnElementChanged, ref _virtualView, ref _mapper, _defaultMapper, AutoPackage);
+#endif
 
 		void IElementHandler.UpdateValue(string property)
 		{
-			if (Element != null)
+			if (Element is IElement element)
 			{
-				OnElementPropertyChanged(Element, new PropertyChangedEventArgs(property));
+				OnElementPropertyChanged(element, new PropertyChangedEventArgs(property));
 			}
 		}
 
@@ -302,8 +324,8 @@ void IElementHandler.Invoke(string command, object? args)
 		void IElementHandler.DisconnectHandler()
 		{
 			DisconnectHandlerCore();
-			if (Element != null && Element.Handler == (IPlatformViewHandler)this)
-				Element.Handler = null;
+			if (Element is IElement element && element.Handler == (IPlatformViewHandler)this)
+				element.Handler = null;
 
 			_virtualView = null;
 		}

src/Controls/src/Core/Compatibility/Handlers/iOS/FrameRenderer.cs

@@ -83,30 +83,32 @@ public virtual void SetupLayer()
 		{
 			if (_actualView == null)
 				return;
+			if (Element is not Frame element)
+				return;
 
-			float cornerRadius = Element.CornerRadius;
+			float cornerRadius = element.CornerRadius;
 
 			if (cornerRadius == -1f)
 				cornerRadius = 5f; // default corner radius
 
 			_actualView.Layer.CornerRadius = cornerRadius;
 			_actualView.Layer.MasksToBounds = cornerRadius > 0;
 
-			if (Element.BackgroundColor == null)
+			if (element.BackgroundColor == null)
 				_actualView.Layer.BackgroundColor = Microsoft.Maui.Platform.ColorExtensions.BackgroundColor.CGColor;
 			else
 			{
 				// BackgroundColor gets set on the base class too which messes with
 				// the corner radius, shadow, etc. so override that behaviour here
 				BackgroundColor = UIColor.Clear;
-				_actualView.Layer.BackgroundColor = Element.BackgroundColor.ToCGColor();
+				_actualView.Layer.BackgroundColor = element.BackgroundColor.ToCGColor();
 			}
 
 			_actualView.Layer.RemoveBackgroundLayer();
 
-			if (!Brush.IsNullOrEmpty(Element.Background))
+			if (!Brush.IsNullOrEmpty(element.Background))
 			{
-				var backgroundLayer = this.GetBackgroundLayer(Element.Background);
+				var backgroundLayer = this.GetBackgroundLayer(element.Background);
 
 				if (backgroundLayer != null)
 				{
@@ -116,17 +118,17 @@ public virtual void SetupLayer()
 				}
 			}
 
-			if (Element.BorderColor == null)
+			if (element.BorderColor == null)
 			{
 				_actualView.Layer.BorderColor = UIColor.Clear.CGColor;
 				_actualView.Layer.BorderWidth = 0;
 			}
 			else
 			{
-				var borderWidth = (int)(Element is IBorderElement be ? be.BorderWidth : 1);
+				var borderWidth = (int)(element is IBorderElement be ? be.BorderWidth : 1);
 				borderWidth = Math.Max(1, borderWidth);
 
-				_actualView.Layer.BorderColor = Element.BorderColor.ToCGColor();
+				_actualView.Layer.BorderColor = element.BorderColor.ToCGColor();
 				_actualView.Layer.BorderWidth = borderWidth;
 			}
 
@@ -136,7 +138,7 @@ public virtual void SetupLayer()
 
 			_actualView.Layer.RasterizationScale = UIScreen.MainScreen.Scale;
 			_actualView.Layer.ShouldRasterize = true;
-			_actualView.Layer.MasksToBounds = Element.IsClippedToBoundsSet(true);
+			_actualView.Layer.MasksToBounds = element.IsClippedToBoundsSet(true);
 		}
 
 		void UpdateShadow()

src/Controls/tests/DeviceTests/Memory/MemoryTests.cs

@@ -26,6 +26,7 @@ void SetupBuilder()
 				handlers.AddHandler<DatePicker, DatePickerHandler>();
 				handlers.AddHandler<Entry, EntryHandler>();
 				handlers.AddHandler<Editor, EditorHandler>();
+				handlers.AddHandler<Frame, FrameRenderer>();
 				handlers.AddHandler<GraphicsView, GraphicsViewHandler>();
 				handlers.AddHandler<Label, LabelHandler>();
 				handlers.AddHandler<ListView, ListViewRenderer>();
@@ -53,6 +54,7 @@ void SetupBuilder()
 	[InlineData(typeof(DatePicker))]
 	[InlineData(typeof(Entry))]
 	[InlineData(typeof(Editor))]
+	[InlineData(typeof(Frame))]
 	[InlineData(typeof(GraphicsView))]
 	[InlineData(typeof(Image))]
 	[InlineData(typeof(IndicatorView))]