Observer pattern in API definition using Franca IDL

I am researching Franca as IDL for automatic API generation. http://ift.tt/2hLaNwI

In my current API, the observer pattern is widely used. Listener classes and callbacks are defined. I do not find a way to actually model that in Franca IDL.

Is it broadcasts the way to model them? If so, aren't the broadcasts supposed to model interaction between a server and a client?