didUnregisterListener method
Calls didStopListening when an only remaining listener is unregistered, thus making the list empty.
See also:
- didRegisterListener, which causes the listener list to become non-empty.
Implementation
void didUnregisterListener() {
assert(_listenerCounter >= 1);
_listenerCounter -= 1;
if (_listenerCounter == 0)
didStopListening();
}