[ios] fix memory leak with Page + Layout

Further fixes: dotnet#13520

7d0af63 did not appear to be a complete fix for dotnet#13520, as when I
retested the customer sample, I found I needed to remove a
`VerticalStackLayout` for the problem to go away.

I could reproduce the issue in a test by doing:

    var page = new ContentPage { Title = "Page 2", Content = new VerticalStackLayout { new Label() } };

After lots of debugging, I found the underlying issue was that
`MauiView` on iOS held a reference to the cross-platform `IView`.

After changing the backing field for this to be a `WeakReference<IView>`
the above problem appears to be solved.

I also removed `LayoutView.CrossplatformArrange/Measure` as we can just
use the `View` property directly instead.

src/Controls/tests/DeviceTests/Elements/NavigationPage/NavigationPageTests.cs

@@ -298,7 +298,7 @@ public async Task DoesNotLeak()
 
 			await CreateHandlerAndAddToWindow<WindowHandlerStub>(new Window(navPage), async (handler) =>
 			{
-				var page = new ContentPage { Title = "Page 2" };
+				var page = new ContentPage { Title = "Page 2", Content = new VerticalStackLayout { new Label() } };
 				pageReference = new WeakReference(page);
 				await navPage.Navigation.PushAsync(page);
 				await navPage.Navigation.PopAsync();

src/Controls/tests/DeviceTests/Elements/Shell/ShellTests.cs

@@ -932,7 +932,7 @@ await CreateHandlerAndAddToWindow<ShellHandler>(shell, async (handler) =>
 			{
 				await OnLoadedAsync(shell.CurrentPage);
 
-				var page = new ContentPage { Title = "Page 2" };
+				var page = new ContentPage { Title = "Page 2", Content = new VerticalStackLayout { new Label() } };
 				pageReference = new WeakReference(page);
 
 				await shell.Navigation.PushAsync(page);

src/Core/src/Handlers/Layout/LayoutHandler.iOS.cs

@@ -14,13 +14,7 @@ protected override LayoutView CreatePlatformView()
 				throw new InvalidOperationException($"{nameof(VirtualView)} must be set to create a LayoutViewGroup");
 			}
 
-			var view = new LayoutView
-			{
-				CrossPlatformMeasure = VirtualView.CrossPlatformMeasure,
-				CrossPlatformArrange = VirtualView.CrossPlatformArrange,
-			};
-
-			return view;
+			return new();
 		}
 
 		public override void SetVirtualView(IView view)
@@ -32,8 +26,6 @@ public override void SetVirtualView(IView view)
 			_ = MauiContext ?? throw new InvalidOperationException($"{nameof(MauiContext)} should have been set by base class.");
 
 			PlatformView.View = view;
-			PlatformView.CrossPlatformMeasure = VirtualView.CrossPlatformMeasure;
-			PlatformView.CrossPlatformArrange = VirtualView.CrossPlatformArrange;
 
 			// Remove any previous children 
 			PlatformView.ClearSubviews();

src/Core/src/Platform/iOS/LayoutView.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Drawing;
 using CoreGraphics;
 using Microsoft.Maui.Graphics;
 using UIKit;
@@ -15,15 +16,15 @@ public class LayoutView : MauiView
 		// apply to ViewHandlerExtensions.MeasureVirtualView
 		public override CGSize SizeThatFits(CGSize size)
 		{
-			if (CrossPlatformMeasure == null)
+			if (View is not ILayout layout)
 			{
 				return base.SizeThatFits(size);
 			}
 
 			var width = size.Width;
 			var height = size.Height;
 
-			var crossPlatformSize = CrossPlatformMeasure(width, height);
+			var crossPlatformSize = layout.CrossPlatformMeasure(width, height);
 			_measureValid = true;
 
 			return crossPlatformSize.ToCGSize();
@@ -36,15 +37,20 @@ public override void LayoutSubviews()
 		{
 			base.LayoutSubviews();
 
+			if (View is not ILayout layout)
+			{
+				return;
+			}
+
 			var bounds = AdjustForSafeArea(Bounds).ToRectangle();
 
 			if (!_measureValid)
 			{
-				CrossPlatformMeasure?.Invoke(bounds.Width, bounds.Height);
+				layout.CrossPlatformMeasure(bounds.Width, bounds.Height);
 				_measureValid = true;
 			}
 
-			CrossPlatformArrange?.Invoke(bounds);
+			layout.CrossPlatformArrange(bounds);
 		}
 
 		public override void SetNeedsLayout()
@@ -68,9 +74,6 @@ public override void WillRemoveSubview(UIView uiview)
 			Superview?.SetNeedsLayout();
 		}
 
-		internal Func<double, double, Size>? CrossPlatformMeasure { get; set; }
-		internal Func<Rect, Size>? CrossPlatformArrange { get; set; }
-
 		public override UIView HitTest(CGPoint point, UIEvent? uievent)
 		{
 			var result = base.HitTest(point, uievent);

src/Core/src/Platform/iOS/MauiView.cs

@@ -1,4 +1,5 @@
-using CoreGraphics;
+using System;
+using CoreGraphics;
 using ObjCRuntime;
 using UIKit;
 
@@ -8,7 +9,13 @@ public abstract class MauiView : UIView
 	{
 		static bool? _respondsToSafeArea;
 
-		public IView? View { get; set; }
+		WeakReference<IView>? _reference;
+
+		public IView? View
+		{
+			get => _reference != null && _reference.TryGetTarget(out var v) ? v : null;
+			set => _reference = value == null ? null : new(value);
+		}
 
 		bool RespondsToSafeArea()
 		{