[ios] fix memory leaks in MauiDoneAccessoryView (#16380)

Context: #16346

This addresses the memory leak discovered by:

    src/Core/src/Platform/iOS/MauiDoneAccessoryView.cs(11,19): error MA0002: Member '_doneClicked' could cause memory leaks in an NSObject subclass. Remove the member, store the value as a WeakReference, or add the [UnconditionalSuppressMessage("Memory", "MA0002")] attribute with a justification as to why the member will not leak.

After writing a test for it, I found a pattern the analyze doesn't
currently catch:

    public MauiDoneAccessoryView(Action doneClicked)
    {
        //...
        var doneButton = new UIBarButtonItem(UIBarButtonSystemItem.Done, OnClicked);
        SetItems(new[] { spacer, doneButton }, false);
    }

This creates a cycle:

* `MauiDoneAccessoryView` -> `UIBarButtonItem` via list of items

* `UIBarButtonItem` -> `Action` -> `MauiDoneAccessoryView` via `OnClicked`

`MauiDoneAccessoryView` lives forever, as well as the owners of any
`Action` delegate values passed in.

I could resolve these problems by creating a new non-NSObject
`BarButtonItemProxy` type to handle events.

This solves one leak with the following controls that use
`MauiDoneAccessoryView`:

* `Editor`
* `Picker`
* `DatePicker`
* `TimePicker`

However, I'll need to send future PRs to verify all these controls are
100% leak free.

src/Core/src/Platform/iOS/MauiDoneAccessoryView.cs

@@ -7,44 +7,60 @@ namespace Microsoft.Maui.Platform
 {
 	internal class MauiDoneAccessoryView : UIToolbar
 	{
-		WeakReference<object>? _data;
-		Action<object>? _doneClicked;
+		readonly BarButtonItemProxy _proxy;
 
 		public MauiDoneAccessoryView() : base(new CGRect(0, 0, UIScreen.MainScreen.Bounds.Width, 44))
 		{
+			_proxy = new BarButtonItemProxy();
 			BarStyle = UIBarStyle.Default;
 			Translucent = true;
 			var spacer = new UIBarButtonItem(UIBarButtonSystemItem.FlexibleSpace);
-			var doneButton = new UIBarButtonItem(UIBarButtonSystemItem.Done, OnClicked);
+			var doneButton = new UIBarButtonItem(UIBarButtonSystemItem.Done, _proxy.OnDataClicked);
 
 			SetItems(new[] { spacer, doneButton }, false);
 		}
 
-		void OnClicked(object? sender, EventArgs e)
-		{
-			if (_data?.TryGetTarget(out object? target) == true)
-				_doneClicked?.Invoke(target);
-		}
+		internal void SetDoneClicked(Action<object>? value) => _proxy.SetDoneClicked(value);
 
-		internal void SetDoneClicked(Action<object>? value) => _doneClicked = value;
-		internal void SetDataContext(object? dataContext)
-		{
-			_data = null;
-			if (dataContext == null)
-				return;
 
-			_data = new WeakReference<object>(dataContext);
-		}
+		internal void SetDataContext(object? dataContext) => _proxy.SetDataContext(dataContext);
 
 		public MauiDoneAccessoryView(Action doneClicked) : base(new CGRect(0, 0, UIScreen.MainScreen.Bounds.Width, 44))
 		{
+			_proxy = new BarButtonItemProxy(doneClicked);
 			BarStyle = UIBarStyle.Default;
 			Translucent = true;
 
 			var spacer = new UIBarButtonItem(UIBarButtonSystemItem.FlexibleSpace);
-			var doneButton = new UIBarButtonItem(UIBarButtonSystemItem.Done, (o, a) => doneClicked?.Invoke());
+			var doneButton = new UIBarButtonItem(UIBarButtonSystemItem.Done, _proxy.OnClicked);
 			SetItems(new[] { spacer, doneButton }, false);
 		}
+
+		class BarButtonItemProxy
+		{
+			readonly Action? _doneClicked;
+			Action<object>? _doneWithDataClicked;
+			WeakReference<object>? _data;
+
+			public BarButtonItemProxy() { }
+
+			public BarButtonItemProxy(Action doneClicked)
+			{
+				_doneClicked = doneClicked;
+			}
+
+			public void SetDoneClicked(Action<object>? value) => _doneWithDataClicked = value;
+
+			public void SetDataContext(object? dataContext) => _data = dataContext is null ? null : new(dataContext);
+
+			public void OnDataClicked(object? sender, EventArgs e)
+			{
+				if (_data is not null && _data.TryGetTarget(out var data))
+					_doneWithDataClicked?.Invoke(data);
+			}
+
+			public void OnClicked(object? sender, EventArgs e) => _doneClicked?.Invoke();
+		}
 	}
 }
 #endif

src/Core/tests/DeviceTests/Memory/UIViewSubclassTests.cs

@@ -0,0 +1,53 @@
+using System;
+using System.Threading.Tasks;
+using Xunit;
+
+#if IOS || MACCATALYST
+
+namespace Microsoft.Maui.DeviceTests.Memory
+{
+	// Set of tests to verify UIView subclasses do not leak
+	[Category(TestCategory.Memory)]
+	public class UIViewSubclassTests : TestBase
+	{
+		#if IOS // MauiDoneAccessoryView is iOS only
+		void DoAction() { }
+		void DoAction(object data) { }
+
+		[Fact]
+		public async Task MauiDoneAccessoryView_Ctor()
+		{
+			WeakReference reference = null;
+			Action action = DoAction;
+
+			await InvokeOnMainThreadAsync(() =>
+			{
+				var accessory = new MauiDoneAccessoryView(action);
+				reference = new(accessory);
+			});
+
+			await AssertionExtensions.WaitForGC(reference);
+			Assert.False(reference.IsAlive, "MauiDoneAccessoryView should not be alive!");
+		}
+
+		[Fact]
+		public async Task MauiDoneAccessoryView_SetDoneClicked()
+		{
+			WeakReference reference = null;
+			Action<object> action = DoAction;
+
+			await InvokeOnMainThreadAsync(() =>
+			{
+				var accessory = new MauiDoneAccessoryView();
+				reference = new(accessory);
+				accessory.SetDoneClicked(action);
+			});
+
+			await AssertionExtensions.WaitForGC(reference);
+			Assert.False(reference.IsAlive, "MauiDoneAccessoryView should not be alive!");
+		}
+		#endif
+	}
+}
+
+#endif

src/Core/tests/DeviceTests/TestCategory.cs

@@ -26,6 +26,7 @@ public static class TestCategory
 		public const string IndicatorView = "IndicatorView";
 		public const string Label = "Label";
 		public const string Layout = "Layout";
+		public const string Memory = "Memory";
 		public const string NavigationView = "NavigationView";
 		public const string Page = "Page";
 		public const string Picker = "Picker";