How to perform an action after the subscriber on next callback in Rx?

Question

I'd like to be able to add an extension method like this:

IObservable<T> AfterSubscriber(this IObservable<T> observable, Action<T> action)

Where:

var subj = new Subject<int>();
subj.
    AfterSubscriber(i => Console.WriteLine("Second")).
    Subscribe(i => Console.WriteLine("First"));

subj.OnNext(1);

Produces the output:

First
Second

Any suggestions on how to implement the above method? Are there any built in methods I can use? (Assumption: synchronous/immediate subscription)

Answer 1

How about this implementation?

public static IObservable<T> AfterSubscriber<T>(
    this IObservable<T> @this, Action<T> after)
{
    if (@this == null) throw new ArgumentNullException("@this");
    if (after == null) throw new ArgumentNullException("after");
    return Observable.Create<T>(o =>
    {
        return @this.Subscribe(t =>
        {
            o.OnNext(t);
            try
            {
                after(t);
            }
            catch (Exception exception)
            {
                o.OnError(exception);
            }
        }, o.OnError, o.OnCompleted);
    });
}