[ios] fix memory leak in Button

Fixes: dotnet#12048
Context: https://github.com/jonathanpeppers/MemoryLeaksOniOS
Context: https://stackoverflow.com/a/13059140

Issue dotnet#12048 has four scenarios that are currently fixed in `main` for
Android and Windows. Unfortunately, one of the four scenarios fails on
iOS due to its usage of `Button`. I could reproduce the issue by
simply adding a `Button` to existing memory leak tests.

`ButtonHandler.iOS.cs` has a "cycle":

* `ButtonHandler` subscribes to events like `UIButton.TouchUpInside`
  making `UIButton` hold a strong reference to `ButtonHandler`.

* `ButtonHandler` (like all handlers) has a strong reference to the
  `Microsoft.Maui.Button`.

* `ButtonHandler` has a strong reference to `UIButton`.

* `Microsoft.Maui.Button` has a strong reference to `ButtonHandler`.

To solve this issue, we can make a nested classed named `ButtonProxy`:

* Its purpose is subscribe to `UIButton.TouchUpInside` and invoke
  `Microsoft.Maui.Button.Clicked()`.

* `ButtonProxy` has a weak reference to the `Microsoft.Maui.Button`.

Now the `UIButton`, `ButtonProxy`, `ButtonHandler`, and
`Microsoft.Maui.Button` can all be collected.

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

@@ -34,6 +34,7 @@ void SetupBuilder()
 					handlers.AddHandler<Window, WindowHandlerStub>();
 					handlers.AddHandler<Frame, FrameRenderer>();
 					handlers.AddHandler<Label, LabelHandler>();
+					handlers.AddHandler<Button, ButtonHandler>();
 				});
 			});
 		}
@@ -298,7 +299,7 @@ public async Task DoesNotLeak()
 
 			await CreateHandlerAndAddToWindow<WindowHandlerStub>(new Window(navPage), async (handler) =>
 			{
-				var page = new ContentPage { Title = "Page 2", Content = new VerticalStackLayout { new Label() } };
+				var page = new ContentPage { Title = "Page 2", Content = new VerticalStackLayout { new Label(), new Button() } };
 				pageReference = new WeakReference(page);
 				await navPage.Navigation.PushAsync(page);
 				await navPage.Navigation.PopAsync();

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

@@ -968,7 +968,7 @@ await CreateHandlerAndAddToWindow<ShellHandler>(shell, async (handler) =>
 			{
 				await OnLoadedAsync(shell.CurrentPage);
 
-				var page = new ContentPage { Title = "Page 2", Content = new VerticalStackLayout { new Label() } };
+				var page = new ContentPage { Title = "Page 2", Content = new VerticalStackLayout { new Label(), new Button() } };
 				pageReference = new WeakReference(page);
 
 				await shell.Navigation.PushAsync(page);

src/Controls/tests/DeviceTests/Extensions.cs

@@ -32,6 +32,7 @@ public static void SetupShellHandlers(this IMauiHandlersCollection handlers)
 			handlers.TryAddHandler<Layout, LayoutHandler>();
 			handlers.TryAddHandler<Image, ImageHandler>();
 			handlers.TryAddHandler<Label, LabelHandler>();
+			handlers.TryAddHandler<Button, ButtonHandler>();
 			handlers.TryAddHandler<Page, PageHandler>();
 			handlers.TryAddHandler(typeof(Toolbar), typeof(ToolbarHandler));
 			handlers.TryAddHandler(typeof(MenuBar), typeof(MenuBarHandler));

src/Core/src/Handlers/Button/ButtonHandler.iOS.cs

@@ -19,20 +19,18 @@ protected override UIButton CreatePlatformView()
 			return button;
 		}
 
+		readonly ButtonEventProxy _proxy = new ButtonEventProxy();
+
 		protected override void ConnectHandler(UIButton platformView)
 		{
-			platformView.TouchUpInside += OnButtonTouchUpInside;
-			platformView.TouchUpOutside += OnButtonTouchUpOutside;
-			platformView.TouchDown += OnButtonTouchDown;
+			_proxy.Connect(VirtualView, platformView);
 
 			base.ConnectHandler(platformView);
 		}
 
 		protected override void DisconnectHandler(UIButton platformView)
 		{
-			platformView.TouchUpInside -= OnButtonTouchUpInside;
-			platformView.TouchUpOutside -= OnButtonTouchUpOutside;
-			platformView.TouchDown -= OnButtonTouchDown;
+			_proxy.Disconnect(platformView);
 
 			base.DisconnectHandler(platformView);
 		}
@@ -166,20 +164,52 @@ static void SetControlPropertiesFromProxy(UIButton platformView)
 			}
 		}
 
-		void OnButtonTouchUpInside(object? sender, EventArgs e)
+		class ButtonEventProxy
 		{
-			VirtualView?.Released();
-			VirtualView?.Clicked();
-		}
+			WeakReference<IButton>? _virtualView;
 
-		void OnButtonTouchUpOutside(object? sender, EventArgs e)
-		{
-			VirtualView?.Released();
-		}
+			public void Connect(IButton virtualView, UIButton platformView)
+			{
+				_virtualView = new(virtualView);
 
-		void OnButtonTouchDown(object? sender, EventArgs e)
-		{
-			VirtualView?.Pressed();
+				platformView.TouchUpInside += OnButtonTouchUpInside;
+				platformView.TouchUpOutside += OnButtonTouchUpOutside;
+				platformView.TouchDown += OnButtonTouchDown;
+			}
+
+			public void Disconnect(UIButton platformView)
+			{
+				_virtualView = null;
+
+				platformView.TouchUpInside -= OnButtonTouchUpInside;
+				platformView.TouchUpOutside -= OnButtonTouchUpOutside;
+				platformView.TouchDown -= OnButtonTouchDown;
+			}
+
+			void OnButtonTouchUpInside(object? sender, EventArgs e)
+			{
+				if (_virtualView != null && _virtualView.TryGetTarget(out var virtualView))
+				{
+					virtualView.Released();
+					virtualView.Clicked();
+				}
+			}
+
+			void OnButtonTouchUpOutside(object? sender, EventArgs e)
+			{
+				if (_virtualView != null && _virtualView.TryGetTarget(out var virtualView))
+				{
+					virtualView.Released();
+				}
+			}
+
+			void OnButtonTouchDown(object? sender, EventArgs e)
+			{
+				if (_virtualView != null && _virtualView.TryGetTarget(out var virtualView))
+				{
+					virtualView?.Pressed();
+				}
+			}
 		}
 	}
 }