[windows] fix memory leak in ListView

[jonathanpeppers]

Context: https://github.com/Tamilarasan-Paranthaman/Maui-Itemssource-Switching-Issue
Fixes #16450

In a customer sample, swapping a ListView.ItemsSource back and forth, such as:

listView.ItemsSource = employeeInfoRepository.EmployeesInfo1;
listView.ItemsSource = employeeInfoRepository.EmployeesInfo2;

Appears to create new Cell's for each call -- and each new cell lives forever. I could reproduce this issue in a device test.

Viewing a memory snapshot, each Cell via Cell.Parent subscribes to events on their parent, the ListView:

• maui/src/Controls/src/Core/Cells/Cell.cs

  Lines 197 to 201 in 9fcccd5

  	if (RealParent != null)
  	{
  	RealParent.PropertyChanged += OnParentPropertyChanged;
  	RealParent.PropertyChanging += OnParentPropertyChanging;
  	}

• maui/src/Controls/src/Core/Element/Element.cs

  Line 322 in 9fcccd5

  _changeHandlers.Add(onchanged);

This makes the ListView hold onto each Cell causing them to live forever. However, the leak would go away if you navigated away from the current Page, removed the ListView from the screen, etc.

Possible workarounds were:

1. Use two ListView instead, and show/hide them.

2. Use a CollectionView instead.

The fix I came up with appears to be reasonable. In CellControl.OnUnloaded:

void OnUnloaded(object sender, RoutedEventArgs e)
{
    // ...
    // 🚀 unsubscribe from propertychanged
    Cell.PropertyChanged -= _propertyChangedHandler;
    // Allows the Cell to unsubscribe from Parent.PropertyChanged
    Cell.Parent = null;
}

This event is fired when WinUI has removed the CellControl from the visual tree. We can safely unset the Cell's Parent, which unsubscribes from the problematic events.

If the Cell is later re-added to the visual tree, it's DataContext will be updated. In CellControl's SetCell(object newContext) the Cell.Parent is reset to the ListView.

I added two test cases for this scenario: one for the leak, and one to prove that re-setting ItemsSource still works.

[jonathanpeppers]

I haven't checked the other platforms yet... I wonder if they are going to leak as well.

[jonathanpeppers]

The CellsDoNotLeak test fails on both iOS & Android. Let me investigate more, to see if I should try to fix all 3 platforms at once.

It could be the way cells recycle on those platforms, the test might always fail.

[deviprasad987]

@jonathanpeppers @rmarinho @Eilon @samhouts @radical

In which VS version this memory leak issue will be fixed?

[Eilon]

@deviprasad987 the .NET 8 RC1 release from this week should have this fix in it. More info on the release here: https://devblogs.microsoft.com/dotnet/announcing-dotnet-8-rc1/