[Bug] ios ItemsView leak when using INotifyCollectionChanged and crash

[tmijieux]

Description

When an CollectionView(or any ItemsView)'s ItemsSource is an INotifyCollectionChanged( for instance a ObservableCollection), and if that CollectionView is removed from view hierarchy(not used anymore), while the ObservableCollection continue to be used after the first CollectionView is removed, then the Event handler registered by the CollectionView on the CollectionChanged event is never unregistered.
In some cases, the application will crash because of the remaining event handlers that should have been removed(see reproduction)

associated pull request: GH-14111

Steps to Reproduce

clone, build and launch minimal reproduction https://github.com/tmijieux/XFBugItemsViewController

1. click "Fill" button (notice under the output tab of visual studio, there is 1 listener to the CollectionChanged events")
2. Rotate device
3. click "Fill" button (notice under the output tab of visual studio, there is now 2 listeners to the CollectionChanged events!")
4. Rotate device again
5. click "Fill" button (notice under the output tab of visual studio, there is now 3 listeners to the CollectionChanged events!")
6. click "Empty" button

Expected Behavior

after step 1, 3, and 5 the number of listener to the CollectionChanged is 1 each time
after step 6 the application does not crash

Actual Behavior

after step 1, 3, and 5 the number of listener to the CollectionChanged is increased by 1 each time
after step 6 the application crashes

Basic Information

• Version with issue: 5.0.0.2012

• Platform Target Frameworks:

  • iOS: Xamarin.iOS and Xamarin.Mac SDK 14.14.2.5 (3836759d4)

• NuGet Packages: Xamarin.Forms 5.0.0.2012, Xamarin.Essentials 1.6.1

• Affected Devices: tested on iPad (6th generation) with iOS 14.4.2

Environment

1. Visual Studio:

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 Microsoft Visual Studio Community 2019
Version 16.9.3
VisualStudio.16.Release/16.9.3+31129.286
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Version 4.8.04084

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Xamarin.iOS and Xamarin.Mac SDK   14.14.2.5 (3836759d4)
Xamarin.iOS and Xamarin.Mac Reference Assemblies and MSBuild support.

Build Logs

Screenshots

Reproduction Link

https://github.com/tmijieux/XFBugItemsViewController

Workaround

to avoid the crash, instead of calling the event handler directly with CollectionChanged?.Invoke()
you can iterate on invocation list with a try/catch each time

to avoid the leak the solution seems to renew the viewmodel / observablecollection, each time the view is changed, so that the old viewmodel can be garbage collected , and then the ObservableItemsSource as well.

[jtorvald]

I'm quite sure that I commented on this yesterday?
I already added a PR for this issue: #14076 together with some other events that were dangling.

[tmijieux]

@jtorvald you commented on the pull request, this is the associated issue. You probably got a little confused.

Are you sure your pull request is exactly fixing the same issue than mine ? i looked at your pull request inside the ItemsViewController you only update the Dispose method, and add that UnbindCells methods, but when Dispose is called on the ItemsViewController, it is already too late (in my issue it has already has been leaked, and is already not recoverable /not referenced anymore from the ItemsViewController) and it has nothing to do with cells ...

[jtorvald]

@tmijieux thanks Thomas, definitely confused!

Apologies, I thought it was the same issue. Looking at your code I believe your fix solves a different issue.
I had trouble debugging this in the XF Gallery because the sample code also has memory leaks. I tried to fix them for the collectionview in my pr.

[tmijieux]

fixed by GH-14111