Avalonia/src/Avalonia.Base/Collections/NotifyCollectionChangedExtensions.cs

// Copyright (c) The Avalonia Project. All rights reserved.
// Licensed under the MIT license. See licence.md file in the project root for full license information.

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

namespace Avalonia.Collections
{
    public static class NotifyCollectionChangedExtensions
    {
        /// <summary>
        /// Gets a weak observable for the CollectionChanged event.
        /// </summary>
        /// <param name="collection">The collection.</param>
        /// <returns>An observable.</returns>
        public static IObservable<NotifyCollectionChangedEventArgs> GetWeakCollectionChangedObservable(
            this INotifyCollectionChanged collection)
        {
            Contract.Requires<ArgumentNullException>(collection != null);

            return new WeakCollectionChangedObservable(new WeakReference<INotifyCollectionChanged>(collection));
        }

        /// <summary>
        /// Subscribes to the CollectionChanged event using a weak subscription.
        /// </summary>
        /// <param name="collection">The collection.</param>
        /// <param name="handler">
        /// An action called when the collection event is raised.
        /// </param>
        /// <returns>A disposable used to terminate the subscription.</returns>
        public static IDisposable WeakSubscribe(
            this INotifyCollectionChanged collection, 
            NotifyCollectionChangedEventHandler handler)
        {
            Contract.Requires<ArgumentNullException>(collection != null);
            Contract.Requires<ArgumentNullException>(handler != null);

            return collection.GetWeakCollectionChangedObservable()
                .Subscribe(e => handler(collection, e));
        }

        /// <summary>
        /// Subscribes to the CollectionChanged event using a weak subscription.
        /// </summary>
        /// <param name="collection">The collection.</param>
        /// <param name="handler">
        /// An action called when the collection event is raised.
        /// </param>
        /// <returns>A disposable used to terminate the subscription.</returns>
        public static IDisposable WeakSubscribe(
            this INotifyCollectionChanged collection,
            Action<NotifyCollectionChangedEventArgs> handler)
        {
            Contract.Requires<ArgumentNullException>(collection != null);
            Contract.Requires<ArgumentNullException>(handler != null);

            return collection.GetWeakCollectionChangedObservable().Subscribe(handler);
        }

        private class WeakCollectionChangedObservable : LightweightObservableBase<NotifyCollectionChangedEventArgs>,
            IWeakSubscriber<NotifyCollectionChangedEventArgs>
        {
            private WeakReference<INotifyCollectionChanged> _sourceReference;

            public WeakCollectionChangedObservable(WeakReference<INotifyCollectionChanged> source)
            {
                _sourceReference = source;
            }

            public void OnEvent(object sender, NotifyCollectionChangedEventArgs e)
            {
                PublishNext(e);
            }

            protected override void Initialize()
            {
                if (_sourceReference.TryGetTarget(out INotifyCollectionChanged instance))
                {
                    WeakSubscriptionManager.Subscribe(
                    instance,
                    nameof(instance.CollectionChanged),
                    this);
                }
            }

            protected override void Deinitialize()
            {
                if (_sourceReference.TryGetTarget(out INotifyCollectionChanged instance))
                {
                    WeakSubscriptionManager.Unsubscribe(
                        instance,
                        nameof(instance.CollectionChanged),
                        this);
                }
            }
        }
    }
}