1. override
void didStartListening()

Called when the number of listeners changes from zero to one.

Source 

@override
void didStartListening() {
  first.addListener(_maybeNotifyListeners);
  first.addStatusListener(_maybeNotifyStatusListeners);
  next.addListener(_maybeNotifyListeners);
  next.addStatusListener(_maybeNotifyStatusListeners);
}