def METHOD_NAME(self, connection_observer):
    """
    Subscribe connection observer instance (commands and events).
    :param connection_observer: Command or event.
    :return: None
    """
    if connection_observer not in self._connection_observers:
        self._connection_observers.append(connection_observer)
        self.subscribe(
            observer=connection_observer.data_received,
            connection_closed_handler=connection_observer.connection_closed_handler
        )