When binding a Object that implements INotifyCollectionChanged to a ItemsControl, update the object doesn't trigger view updating

[cdytoby]

Describe the bug

• In ItemsControl, ItemsSource is binded to a custom object that implments INotifyCollectionChanged, IEnumerable and IReadOnlyCollection
• that custom object gets updated, and CollectionChanged event is fired
• In debugger, the values of the custom object are all correct
• Bug: The actual ItemsControl view is empty

To Reproduce

[DumbRenamer.zip](https://github.com/AvaloniaUI/Avalonia/files/15210435/DumbRenamer.zip)

This is my project (just started not long ago, to experiment things)

Just build and run the project: DumbRenamer.AvaloniaApp.Desktop

In file MainViewModel.cs, line 64, in the method FileInputViewOnCollectionChanged(), you can put a break point there, and this break point gets triggered.

Update:
I created a public repo for bug reporting (I think this probably won't be the last one).
https://github.com/cdytoby/AvaloniaBugReport/tree/bug/observable-list

For this bug specifically, it is the branch "bug/observable-list".

Just build and run the project "AvaloniaBugReport"

In file MainWindowViewModel.cs, line 37, method TestNotifiedEnumerableOnCollectionChanged(), you can set breakpoint there, it's the proof that INotifyCollectionChanged is called.

Expected behavior

The ItemsControl displays the items correctly

Avalonia version

11.0.10

OS

Windows

Additional context

I need a better ObservableCollection, so I found a third party one, they have their own solutions and I'd like to use it.
https://github.com/Cysharp/ObservableCollections

To my understanding of MVVM, as long as INotifyCollectionChanged is correctly fired, view should be updated. (experience from WPF)

(I could use Avalonia ones but I don't want avalonia package involved when design view model. And the Avalonia.Collection namespace is in the Avalonia main package, it's not a independent package.)

[stevemonaco]

INCC needs to be on the UI thread. Using their ObservableList<T> works fine. It's only an issue with the INotifyCollectionChangedSynchronizedView<T>. Their WPF sample shows using BindingOperations.EnableCollectionSynchronization, so not being on the UI thread seems to be a possible scenario. I would have expected an exception or log message on Verbose, but I don't seem to get either.

[cdytoby]

I saw it, but I don't need to synchronize across thread (at least not right now), so I thought this is optional. ObservableList<T> don't have INCC, so INotifyCollectionChangedSynchronizedView<T> is the only option here if I want to keep using this.

[timunie]

Can you please file a minimum sample? I'll not explore a random file renamer as that could be harmful to my machine.

[cdytoby]

Hi, this name "DumbRenamer" is a project I just started, despite the solution name, there is not much code, and not much going on, that's why I upload it, and it's without any dll or built binary.

But I understand your concern. Here are the some sample code files in a zip (.cs file and .axaml files are not directly allowed somehow)

Code.zip

PS: CommunityToolkit.Mvvm package and ObservableCollections package from "https://github.com/Cysharp/ObservableCollections" are required.

[timunie]

If you want us to investigate an issue, we need a very minimal repo to understand that issue. Not a full app, not some fragments (fragments are okay if the problem is reproducible and easy to understand). I still believe it is an async issue as stated below.

[timunie]

You need the Dispather if you update things from a Task.

private async Task ChangeValue()
{
    await Task.Delay(1000);
    Dispatcher.UiThread.Post(() => { TestInt = 10 });
    ExecutePreview();
}

[cdytoby]

I just tried this, but it still doesn't work.

In the project I mentioned, the new items in the list are still not appear in the UI.

[cdytoby]

Hi @timunie and everyone,

I created a public repo for bug reporting (I think this probably won't be the last one).

https://github.com/cdytoby/AvaloniaBugReport/tree/bug/observable-list

For this bug specifically, it is the branch "bug/observable-list".

I put the information to the first post too.

[timunie]

the docs state this for WPF: https://github.com/Cysharp/ObservableCollections?tab=readme-ov-file#wpf

BindingOperations.EnableCollectionSynchronization(ItemsView, new object()); // for ui synchronization safety of viewmodel

So you should use another approach, for example DynamicData. It's by design with this lib.

[cdytoby]

OK, I see, thanks for the answer. I'll look into other approaches.