What is the difference between callback_group and callback_event when creating a publisher?
The documentation of the Node class defines the create_publisher function as follows:
create_publisher(msg_type, topic, qos_profile, *, callback_group=None, event_callbacks=None)
Then, it defines the callback_group as follows: The callback group for the publisher’s event handlers. If None, then the node’s default callback group is used.
Also defines the callback_event as: User-defined callbacks for middleware events.
I understand that the callback_event is a function created by the user. However, I am not sure what the calback_group does.