How to structure code inside a node

edit

Hi, I'm learning ROS and I was wondering how should I write the code inside my nodes.

Let's take this example: The node should control the movement of a robot (publishing a velocity command) but at the same time check received messages from a sensor.

I also need to set the velocity gradually I can easily achieve this using a loop. But this avoid my node from calling the ros::Spin() and so the callback updating my global variables (or class members) is never called, right?

So I think I am supposed to make the code inside an infinite loop which calls ros::spinOnce() and write the code to control the robot in such a way that it doesn't block the loop?

I'm not sure I have been very clear, but I hope someone can point me in the right direction, thanks.

EDIT = I thought about something that could be smart in my case (mainly for readibility reasons) The switch/case. (I need to perform multiple action and I can't execute the next until i finished the previous or the sensor detect something. See:

 while(nh.ok())
    {
        switch(STATUS)
        {
        case 1:
            inc_vel();
            if (object detected) //global variable updated by the sensor callback
            {
                stop();
                STATUS++;
            }
            break;
        case 2:
            //do something else, for example turn around and change path
            break;
        }
        ros::spinOnce();
    }