Exercise 3.2 BB8 cicle moving service

this question came up doing exercise 3.2. Which is creating a moving bb8 in a circle using a service.
When i want to move bb8 using a simple topic, the publisher has to run in a loop and constantly publish in order to move the robot. If no loop is used the robot won’t move.
But if the publisher is called in callback function of a service no loop is needed, even though the callback function runs only once when the service is called by a client.
I don’t understand why the service can do this without constantly publishing into the topic?

Hi @sebastian.grede,

I think the post below might help: