@classmethod
def _register_listener(cls, unbound_method, partial_receiver, mesg_filter):
    """ A helper class for creating listener decorators. """
    listeners = getattr(unbound_method, cls.LISTENERS_ATTR, None)
    if listeners is None:
        listeners = []
        setattr(unbound_method, cls.LISTENERS_ATTR, listeners)
    listeners.append((partial_receiver, mesg_filter))
