Avalonia/src/Avalonia.Base/Data/Core/IndexerNodeBase.cs

using System;
using System.Collections;
using System.Collections.Generic;
using System.Collections.Specialized;
using System.ComponentModel;
using System.Linq;
using System.Reactive.Linq;
using Avalonia.Utilities;

namespace Avalonia.Data.Core
{
    public abstract class IndexerNodeBase : SettableNode
    {
        private IDisposable? _subscription;
        
        protected override void StartListeningCore(WeakReference<object?> reference)
        {
            reference.TryGetTarget(out var target);

            var incc = target as INotifyCollectionChanged;
            var inpc = target as INotifyPropertyChanged;
            var inputs = new List<IObservable<object?>>();

            if (incc != null)
            {
                inputs.Add(WeakObservable.FromEventPattern(
                    incc, WeakEvents.CollectionChanged)
                    .Where(x => ShouldUpdate(x.Sender, x.EventArgs))
                    .Select(_ => GetValue(target)));
            }

            if (inpc != null)
            {
                inputs.Add(WeakObservable.FromEventPattern(
                    inpc, WeakEvents.PropertyChanged)
                    .Where(x => ShouldUpdate(x.Sender, x.EventArgs))
                    .Select(_ => GetValue(target)));
            }

            _subscription = Observable.Merge(inputs).StartWith(GetValue(target)).Subscribe(ValueChanged);
        }

        protected override void StopListeningCore()
        {
            _subscription?.Dispose();
        }

        protected abstract object? GetValue(object? target);

        protected abstract int? TryGetFirstArgumentAsInt();

        private bool ShouldUpdate(object? sender, NotifyCollectionChangedEventArgs e)
        {
            if (sender is IList)
            {
                var index = TryGetFirstArgumentAsInt();

                if (index == null)
                {
                    return false;
                }

                switch (e.Action)
                {
                    case NotifyCollectionChangedAction.Add:
                        return index >= e.NewStartingIndex;
                    case NotifyCollectionChangedAction.Remove:
                        return index >= e.OldStartingIndex;
                    case NotifyCollectionChangedAction.Replace:
                        return index >= e.NewStartingIndex &&
                               index < e.NewStartingIndex + e.NewItems!.Count;
                    case NotifyCollectionChangedAction.Move:
                        return (index >= e.NewStartingIndex &&
                                index < e.NewStartingIndex + e.NewItems!.Count) ||
                               (index >= e.OldStartingIndex &&
                                index < e.OldStartingIndex + e.OldItems!.Count);
                    case NotifyCollectionChangedAction.Reset:
                        return true;
                }
            }

            return true; // Implementation defined meaning for the index, so just try to update anyway
        }

        protected abstract bool ShouldUpdate(object? sender, PropertyChangedEventArgs e);
    }
}