public static class PropertyChangedExtensions
{
    public static IObservable<PropertyChangedEventArgs> ObserveIndexer(this INotifyPropertyChanged self, string indexerName = "Item")
    {
        return Observable.FromEvent<PropertyChangedEventHandler, PropertyChangedEventArgs>(
            h => (s, e) => h(e),
            h => self.PropertyChanged += h,
            h => self.PropertyChanged -= h)
            .Where(e => e.PropertyName == indexerName);
    }
}