Topics

Nodes communicate with each other using ROS 2 communication features. There are three types of communication: topics, services, and actions. We will discover all three of them, starting with topics.

Here, we will make some basic discoveries to get an idea of what a ROS 2 topic is, and you’ll learn much more about them, including how to write code for topics, in Chapter 5.

Running a topic publisher and subscriber

Stop all running nodes (Ctrl + C), and let’s come back to our first example.

In Terminal 1, input the following:

$ ros2 run demo_nodes_cpp talker

In Terminal 2, input the following:

$ ros2 run demo_nodes_cpp listener

In Terminal 3, input the following:

$ rqt_graph

If needed, refresh the view a few times, select Nodes/Topics (all), and you should get the same visual as in Figure 3.1.

In the middle, you will see a /chatter box. This box represents a ROS 2 topic. What you can also see is that the talker node is sending something...