How to tell whether an Observable is "awaiting" another with RxJS (for Ajax request indicator, for example)

Question

While learning RxJS, I'm wondering:

How to create an Observable that tells whether Observable X has had value after Observable Y?

Example need would be to tell whether there is an Ajax request pending response? Like awaiting with Bacon.js.

Answer 1

While formulating the question, I found one answer:

• map (select within RxJS) another Observable (the one triggering the request) to true
• map another Observable (response one) to false
• merge

i.e.

var availabilityPending = usernameToValidate.select(function() {return true;})
         .merge(usernameAvailable.select(function() {return false;}));

Don't know if there's already a helper for this kind of behaviour?