ObservableCollections/src/ObservableCollections.R3/ObservableCollectionR3Extensions.cs

using System;
using System.Collections.Specialized;
using System.Threading;
using R3;

namespace ObservableCollections;

public readonly record struct CollectionAddEvent<T>(int Index, T Value);
public readonly record struct CollectionRemoveEvent<T>(int Index, T Value);
public readonly record struct CollectionReplaceEvent<T>(int Index, T OldValue, T NewValue);
public readonly record struct CollectionMoveEvent<T>(int OldIndex, int NewIndex, T Value);

public static class ObservableCollectionR3Extensions
{
    public static Observable<CollectionAddEvent<T>> ObserveAdd<T>(this IObservableCollection<T> source, CancellationToken cancellationToken = default)
    {
        return new ObservableCollectionAdd<T>(source, cancellationToken);
    }

    public static Observable<CollectionRemoveEvent<T>> ObserveRemove<T>(this IObservableCollection<T> source, CancellationToken cancellationToken = default)
    {
        return new ObservableCollectionRemove<T>(source, cancellationToken);
    }

    public static Observable<CollectionReplaceEvent<T>> ObserveReplace<T>(this IObservableCollection<T> source, CancellationToken cancellationToken = default)
    {
        return new ObservableCollectionReplace<T>(source, cancellationToken);
    }
    
    public static Observable<CollectionMoveEvent<T>> ObserveMove<T>(this IObservableCollection<T> source, CancellationToken cancellationToken = default)
    {
        return new ObservableCollectionMove<T>(source, cancellationToken);
    }
    
    public static Observable<Unit> ObserveReset<T>(this IObservableCollection<T> source, CancellationToken cancellationToken = default)
    {
        return new ObservableCollectionReset<T>(source, cancellationToken);
    }

    public static Observable<int> ObserveCountChanged<T>(this IObservableCollection<T> source, bool notifyCurrentCount = false, CancellationToken cancellationToken = default)
    {
        return new ObservableCollectionCountChanged<T>(source, notifyCurrentCount, cancellationToken);
    }
}

sealed class ObservableCollectionAdd<T>(IObservableCollection<T> collection, CancellationToken cancellationToken)
    : Observable<CollectionAddEvent<T>>
{
    protected override IDisposable SubscribeCore(Observer<CollectionAddEvent<T>> observer)
    {
        return new _ObservableCollectionAdd(collection, observer, cancellationToken);
    }

    sealed class _ObservableCollectionAdd(
        IObservableCollection<T> collection,
        Observer<CollectionAddEvent<T>> observer,
        CancellationToken cancellationToken)
        : ObservableCollectionObserverBase<T, CollectionAddEvent<T>>(collection, observer, cancellationToken)
    {
        protected override void Handler(in NotifyCollectionChangedEventArgs<T> eventArgs)
        {
            if (eventArgs.Action == NotifyCollectionChangedAction.Add)
            {
                if (eventArgs.IsSingleItem)
                {
                    observer.OnNext(new CollectionAddEvent<T>(eventArgs.NewStartingIndex, eventArgs.NewItem));
                }
                else
                {
                    var i = eventArgs.NewStartingIndex;
                    foreach (var item in eventArgs.NewItems)
                    {
                        observer.OnNext(new CollectionAddEvent<T>(i++, item));
                    }
                }
            }
        }
    }
}

sealed class ObservableCollectionRemove<T>(IObservableCollection<T> collection, CancellationToken cancellationToken)
    : Observable<CollectionRemoveEvent<T>>
{
    protected override IDisposable SubscribeCore(Observer<CollectionRemoveEvent<T>> observer)
    {
        return new _ObservableCollectionRemove(collection, observer, cancellationToken);
    }

    sealed class _ObservableCollectionRemove(
        IObservableCollection<T> collection,
        Observer<CollectionRemoveEvent<T>> observer,
        CancellationToken cancellationToken)
        : ObservableCollectionObserverBase<T, CollectionRemoveEvent<T>>(collection, observer, cancellationToken)
    {
        protected override void Handler(in NotifyCollectionChangedEventArgs<T> eventArgs)
        {
            if (eventArgs.Action == NotifyCollectionChangedAction.Remove)
            {
                if (eventArgs.IsSingleItem)
                {
                    observer.OnNext(new CollectionRemoveEvent<T>(eventArgs.OldStartingIndex, eventArgs.OldItem));
                }
                else
                {
                    var i = eventArgs.OldStartingIndex;
                    foreach (var item in eventArgs.OldItems)
                    {
                        observer.OnNext(new CollectionRemoveEvent<T>(i++, item));
                    }
                }
            }
        }
    }
}

sealed class ObservableCollectionReplace<T>(IObservableCollection<T> collection, CancellationToken cancellationToken)
    : Observable<CollectionReplaceEvent<T>>
{
    protected override IDisposable SubscribeCore(Observer<CollectionReplaceEvent<T>> observer)
    {
        return new _ObservableCollectionReplace(collection, observer, cancellationToken);
    }
    
    sealed class _ObservableCollectionReplace(
        IObservableCollection<T> collection,
        Observer<CollectionReplaceEvent<T>> observer,
        CancellationToken cancellationToken)
        : ObservableCollectionObserverBase<T, CollectionReplaceEvent<T>>(collection, observer, cancellationToken)
    {
        protected override void Handler(in NotifyCollectionChangedEventArgs<T> eventArgs)
        {
            if (eventArgs.Action == NotifyCollectionChangedAction.Replace)
            {
                observer.OnNext(new CollectionReplaceEvent<T>(eventArgs.NewStartingIndex, eventArgs.OldItem, eventArgs.NewItem));
            }
        }
    }
}

sealed class ObservableCollectionMove<T>(IObservableCollection<T> collection, CancellationToken cancellationToken)
    : Observable<CollectionMoveEvent<T>>
{
    protected override IDisposable SubscribeCore(Observer<CollectionMoveEvent<T>> observer)
    {
        return new _ObservableCollectionMove(collection, observer, cancellationToken);
    }
    
    sealed class _ObservableCollectionMove(
        IObservableCollection<T> collection,
        Observer<CollectionMoveEvent<T>> observer,
        CancellationToken cancellationToken)
        : ObservableCollectionObserverBase<T, CollectionMoveEvent<T>>(collection, observer, cancellationToken)
    {
        protected override void Handler(in NotifyCollectionChangedEventArgs<T> eventArgs)
        {
            if (eventArgs.Action == NotifyCollectionChangedAction.Move)
            {
                observer.OnNext(new CollectionMoveEvent<T>(eventArgs.OldStartingIndex, eventArgs.NewStartingIndex, eventArgs.NewItem));
            }
        }
    }
}

sealed class ObservableCollectionReset<T>(IObservableCollection<T> collection, CancellationToken cancellationToken)
    : Observable<Unit>
{
    protected override IDisposable SubscribeCore(Observer<Unit> observer)
    {
        return new _ObservableCollectionReset(collection, observer, cancellationToken);
    }
    
    sealed class _ObservableCollectionReset(
        IObservableCollection<T> collection,
        Observer<Unit> observer,
        CancellationToken cancellationToken)
        : ObservableCollectionObserverBase<T, Unit>(collection, observer, cancellationToken)
    {
        protected override void Handler(in NotifyCollectionChangedEventArgs<T> eventArgs)
        {
            if (eventArgs.Action == NotifyCollectionChangedAction.Reset)
            {
                observer.OnNext(Unit.Default);
            }
        }
    }
}

sealed class ObservableCollectionCountChanged<T>(IObservableCollection<T> collection, bool notifyCurrentCount, CancellationToken cancellationToken)
    : Observable<int>
{
    protected override IDisposable SubscribeCore(Observer<int> observer)
    {
        return new _ObservableCollectionCountChanged(collection, notifyCurrentCount, observer, cancellationToken);
    }
    
    sealed class _ObservableCollectionCountChanged : ObservableCollectionObserverBase<T, int>
    {
        int countPrev;

        public _ObservableCollectionCountChanged(
            IObservableCollection<T> collection,
            bool notifyCurrentCount,
            Observer<int> observer,
            CancellationToken cancellationToken) : base(collection, observer, cancellationToken)
        {
            this.countPrev = collection.Count;
            if (notifyCurrentCount)
            {
                observer.OnNext(collection.Count);
            }
        }

        protected override void Handler(in NotifyCollectionChangedEventArgs<T> eventArgs)
        {
            switch (eventArgs.Action)
            {
                case NotifyCollectionChangedAction.Add:
                case NotifyCollectionChangedAction.Remove:
                case NotifyCollectionChangedAction.Reset when countPrev != collection.Count:
                    observer.OnNext(collection.Count);
                    break;
            }
            countPrev = collection.Count;
        }
    }
}


abstract class ObservableCollectionObserverBase<T, TEvent> : IDisposable
{
    protected readonly IObservableCollection<T> collection;
    protected readonly Observer<TEvent> observer;
    readonly CancellationTokenRegistration cancellationTokenRegistration;
    readonly NotifyCollectionChangedEventHandler<T> handlerDelegate;

    public ObservableCollectionObserverBase(IObservableCollection<T> collection, Observer<TEvent> observer, CancellationToken cancellationToken)
    {
        this.collection = collection;
        this.observer = observer;
        this.handlerDelegate = Handler;

        collection.CollectionChanged += handlerDelegate;
            
        if (cancellationToken.CanBeCanceled)
        {
            cancellationTokenRegistration = cancellationToken.UnsafeRegister(static state =>
            {
                var s = (ObservableCollectionObserverBase<T, TEvent>)state!;
                s.observer.OnCompleted();
                s.Dispose();
            }, this);
        }
    }

    public void Dispose()
    {
        collection.CollectionChanged -= handlerDelegate;
        cancellationTokenRegistration.Dispose();
    }

    protected abstract void Handler(in NotifyCollectionChangedEventArgs<T> eventArgs);
}