[ios/catalyst] fix leak in NavigationPage

Fixes: #20119

PR #13833 fixed a leak in child pages of a `NavigationPage`, but it
appears there are several cycles that would prevent the `NavigationPage`
itself from going away.

So, if you did something like this:

    // The original NavigationPage & children leak
    Application.Current.MainPage = new NavigationPage(new Page1());
    Application.Current.MainPage = new Page2();

I could reproduce the same problem in a new device test.

The cycles (and solutions) are:

1. `NavigationPage` -> `NavigationRenderer` -> `ParentingViewController` -> `NavigationPage`

    * Solution: make `_child` a `WeakReference`

2. `NavigationPage` -> `MenuItemTracker` -> `NavigationPage`

    * Solution: make `_target` a `WeakReference`, and `_additionalTargets` a `List<WeakReference>`

3. `NavigationPage` -> `MenuItemTracker.CollectionChanged` -> `NavigationPage`

    * Solution: subscribe/unsubscribe in `WillMoveToParentViewController`

After these changes the sample app works, and the new device test passes.

src/Controls/src/Core/Compatibility/Handlers/NavigationPage/iOS/NavigationRenderer.cs

@@ -674,11 +674,11 @@ void RemovePage(Page page)
 		void RemoveViewControllers(bool animated)
 		{
 			var controller = TopViewController as ParentingViewController;
-			if (controller == null || controller.Child == null || controller.Child.Handler == null)
+			if (controller?.Child is not Page child || child.Handler == null)
 				return;
 
 			// Gesture in progress, lets not be proactive and just wait for it to finish
-			var task = GetAppearedOrDisappearedTask(controller.Child);
+			var task = GetAppearedOrDisappearedTask(child);
 
 			task.ContinueWith(t =>
 			{
@@ -1113,7 +1113,7 @@ class ParentingViewController : UIViewController
 		{
 			readonly WeakReference<NavigationRenderer> _navigation;
 
-			Page _child;
+			WeakReference<Page> _child;
 			bool _disposed;
 			ToolbarTracker _tracker = new ToolbarTracker();
 
@@ -1128,19 +1128,25 @@ public ParentingViewController(NavigationRenderer navigation)
 
 			public Page Child
 			{
-				get { return _child; }
+				get => _child?.GetTargetOrDefault();
 				set
 				{
-					if (_child == value)
+					var child = Child;
+					if (child == value)
 						return;
 
-					if (_child != null)
-						_child.PropertyChanged -= HandleChildPropertyChanged;
-
-					_child = value;
+					if (child is not null)
+						child.PropertyChanged -= HandleChildPropertyChanged;
 
-					if (_child != null)
-						_child.PropertyChanged += HandleChildPropertyChanged;
+					if (value is not null)
+					{
+						_child = new(value);
+						value.PropertyChanged += HandleChildPropertyChanged;
+					}
+					else
+					{
+						_child = null;
+					}
 
 					UpdateHasBackButton();
 					UpdateLargeTitles();
@@ -1228,7 +1234,6 @@ public override void ViewDidLoad()
 
 				_tracker.Target = Child;
 				_tracker.AdditionalTargets = Child.GetParentPages();
-				_tracker.CollectionChanged += TrackerOnCollectionChanged;
 
 				UpdateToolbarItems();
 			}
@@ -1247,6 +1252,20 @@ public override void ViewWillAppear(bool animated)
 				base.ViewWillAppear(animated);
 			}
 
+			public override void WillMoveToParentViewController(UIViewController parent)
+			{
+				base.WillMoveToParentViewController(parent);
+
+				if (parent is null)
+				{
+					_tracker.CollectionChanged -= TrackerOnCollectionChanged;
+				}
+				else
+				{
+					_tracker.CollectionChanged += TrackerOnCollectionChanged;
+				}
+			}
+
 			protected override void Dispose(bool disposing)
 			{
 				if (_disposed)
@@ -1258,10 +1277,10 @@ protected override void Dispose(bool disposing)
 
 				if (disposing)
 				{
-					if (Child != null)
+					if (Child is Page child)
 					{
-						Child.SendDisappearing();
-						Child.PropertyChanged -= HandleChildPropertyChanged;
+						child.SendDisappearing();
+						child.PropertyChanged -= HandleChildPropertyChanged;
 						Child = null;
 					}
 
@@ -1517,17 +1536,17 @@ void TrackerOnCollectionChanged(object sender, EventArgs eventArgs)
 
 			void UpdateHasBackButton()
 			{
-				if (Child == null || NavigationItem.HidesBackButton == !NavigationPage.GetHasBackButton(Child))
+				if (Child is not Page child || NavigationItem.HidesBackButton == !NavigationPage.GetHasBackButton(child))
 					return;
 
-				NavigationItem.HidesBackButton = !NavigationPage.GetHasBackButton(Child);
+				NavigationItem.HidesBackButton = !NavigationPage.GetHasBackButton(child);
 
 				NavigationRenderer n;
 				if (!_navigation.TryGetTarget(out n))
 					return;
 
 				if (!(OperatingSystem.IsIOSVersionAtLeast(11) || OperatingSystem.IsMacCatalystVersionAtLeast(11)) || n._parentFlyoutPage != null)
-					UpdateTitleArea(Child);
+					UpdateTitleArea(child);
 			}
 
 			void UpdateNavigationBarVisibility(bool animated)

src/Controls/src/Core/Menu/MenuItemTracker.cs

@@ -12,7 +12,8 @@ internal abstract class MenuItemTracker<TMenuItem>
 		where TMenuItem : BaseMenuItem
 	{
 		int _flyoutDetails;
-		Page _target;
+		WeakReference<Page> _target;
+		List<WeakReference<Page>> _additionalTargets = new();
 		public MenuItemTracker()
 		{
 		}
@@ -21,7 +22,28 @@ public MenuItemTracker()
 
 		protected abstract IComparer<TMenuItem> CreateComparer();
 
-		public IEnumerable<Page> AdditionalTargets { get; set; }
+		public IEnumerable<Page> AdditionalTargets
+		{
+			get
+			{
+				foreach (var target in _additionalTargets)
+				{
+					if (target.TryGetTarget(out var page))
+						yield return page;
+				}
+			}
+			set
+			{
+				_additionalTargets.Clear();
+				if (value is not null)
+				{
+					foreach	(var page in value)
+					{
+						_additionalTargets.Add(new(page));
+					}
+				}
+			}
+		}
 
 		public bool HaveFlyoutPage
 		{
@@ -32,17 +54,18 @@ public bool HaveFlyoutPage
 
 		public Page Target
 		{
-			get { return _target; }
+			get =>  _target is not null && _target.TryGetTarget(out var target) ? target : null;
 			set
 			{
-				if (_target == value)
+				var target = Target;
+				if (target == value)
 					return;
 
-				UntrackTarget(_target);
-				_target = value;
+				UntrackTarget(target);
+				_target = value is null ? null : new(value);
 
-				if (_target != null)
-					TrackTarget(_target);
+				if (value != null)
+					TrackTarget(value);
 				EmitCollectionChanged();
 			}
 		}

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

@@ -303,9 +303,33 @@ await CreateHandlerAndAddToWindow<WindowHandlerStub>(new Window(navPage), async
 			});
 		}
 
-		[Fact(DisplayName = "NavigationPage Does Not Leak")]
+		[Fact(DisplayName = "Does Not Leak")]
 		public async Task DoesNotLeak()
 		{
+			SetupBuilder();
+			WeakReference pageReference = null;
+			WeakReference handlerReference = null;
+
+			{
+				var navPage = new NavigationPage(new ContentPage());
+				var window = new Window(navPage);
+
+				await CreateHandlerAndAddToWindow<WindowHandlerStub>(window, (handler) =>
+				{
+					pageReference = new WeakReference(navPage);
+					handlerReference = new WeakReference(handler);
+
+					// Just replace the page with a new one
+					window.Page = new ContentPage();
+				});
+			}
+
+			await AssertionExtensions.WaitForGC(pageReference, handlerReference);
+		}
+
+		[Fact(DisplayName = "Child Pages Do Not Leak")]
+		public async Task ChildPagesDoNotLeak()
+		{
 
 #if ANDROID
 			if (!OperatingSystem.IsAndroidVersionAtLeast(30))