CompleteAfter

static class ObservableExtensions
{
    public static IObservable<TSource> CompleteAfter<TSource>(this IObservable<TSource> source, Func<TSource, bool> predicate)
    {
        return new AnonymousObservable<TSource>(observer =>
        {
            var complete = false;
            return source.Subscribe(
                x =>
                {
                    if (!complete)
                    {
                        try
                        {
                            complete = predicate(x);
                        }
                        catch (Exception exception)
                        {
                            observer.OnError(exception);
                            return;
                        }

                        observer.OnNext(x);

                        if (complete)
                            observer.OnCompleted();
                    }
                },
                observer.OnError,
                observer.OnCompleted);
        });
    }
}