ObservableCollections.cs

// similar as ObservableCollection <-> CollectionView

public delegate void NotifyCollectionChangedEventHandler<T>(in NotifyCollectionChangedEventArgs<T> e);

public interface INotifyCollectionChanged<T>
{
    event NotifyCollectionChangedEventHandler<T>? CollectionChanged;
    ISynchronizedView<T, TView> CreateView<TView>(Func<T, TView> transform, bool reverse = false);
    ISynchronizedView<T, TView> CreateSortedView<TView>(Func<T, TView> transform, IComparer<T> comparer);
    IGroupedSynchronizedView<T, TKey, TView> CreateGroupedView<TKey, TView>(Func<T, TKey> keySelector, Func<T, TView> transform);
}

public readonly ref struct NotifyCollectionChangedEventArgs<T>
{
    public readonly NotifyCollectionChangedAction Action;
    public readonly ReadOnlySpan<T> NewItems;
    public readonly ReadOnlySpan<T> OldItems;
    public readonly int NewStartingIndex;
    public readonly int OldStartingIndex;

    // ctor()
}

public interface ISynchronizedView<T, TView> : IReadOnlyList<TView>, IDisposable
{
    void AttachFilter(Func<T, TView> filter);
    void ResetFilter();
}

public interface IGroupedSynchronizedView<T, TKey, TView> : IGrouping<TKey, TView>, IDisposable
{
    void AttachFilter(Func<T, TView> filter);
    void ResetFilter();
}