CollectionSubscription

    /// <summary>
    /// Subscribes collection child item events to some specific handler.
    /// The logic of subscription and unsubscription can be complicated,
    /// considering that you need to take into account collection changes,
    /// new child items have to subscribe, while unused and removed items
    /// need to unsubscribe.
    /// </summary>
    public class CollectionSubscription<TItem, THandler>
    {
        private Action<TItem> subscribe;
        private Action<TItem> unsubscribe;
        private Action raiseEvent;

        public CollectionSubscription(Action<TItem> subscribe, Action<TItem> unsubscribe, Action raiseEvent)
        {
            this.subscribe = subscribe;
            this.unsubscribe = unsubscribe;
            this.raiseEvent = raiseEvent;
        }

        public void Subscribe(ObservableCollection<TItem> collection)
        {
            collection.CollectionChanged -= OnCollectionChanged;
            collection.CollectionChanged += OnCollectionChanged;
            foreach (var item in collection)
            {
                unsubscribe(item);
                subscribe(item);
            }
        }

        public void Unsubscribe(ObservableCollection<TItem> collection)
        {
            collection.CollectionChanged -= OnCollectionChanged;
            foreach (var item in collection)
                unsubscribe(item);
        }

        private void OnCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
        {
            if (e.Action == NotifyCollectionChangedAction.Reset)
            {
                var collection = sender as IEnumerable<TItem>;
                foreach (var item in collection)
                {
                    unsubscribe(item);
                    subscribe(item);
                }
            }

            var oldItems = e.OldItems as IEnumerable<TItem>;
            if (oldItems != null)
            {
                foreach (var oldItem in oldItems)
                    unsubscribe(oldItem);
            }

            var newItems = e.NewItems as IEnumerable<TItem>;
            if (newItems != null)
            {
                foreach (var newItem in newItems)
                {
                    unsubscribe(newItem);
                    subscribe(newItem);
                }
            }

            raiseEvent();
        }
    }