Difference between spin and rate.sleep in ROS

Question

I am new to ROS and trying to understand this powerful tool. I am confused between the spin and rate.sleep functions. Could anyone help me with the difference between these two functions and when to use each one?

Answer 1

ros::spin() and ros::spinOnce() are responsible to handle communication events, e.g. arriving messages. If you are subscribing to messages, services, or actions, you must call spin to process the events.

While ros::spinOnce() handles the events and returns immediately, ros::spin() blocks until ROS invokes a shutdown. So, ros::spinOnce() gives you more control if needed. More on that matter here: Callbacks and Spinning.

rate.sleep() on the other hand is merely a thread sleep with a duration defined by a frequency. Here is an example

ros::Rate rate(24.);
while(ros::ok())
{
    rate.sleep();
}

This loop will be executed 24 times a second or less, depends what you do inside the loop. A ros::Rate object keeps track of how much time since the last rate.sleep() was executed and sleep for the correct amount of time to hit the 24 Hz mark. See ros::Rate::sleep() API.

The equivalent way in the time domain is ros::Duration::sleep()

ros::Duration duration(1./24.);
while(ros::ok())
{
    duration.sleep();
}

Which one you use is just a matter of convenience.