Build a Subscriber Node

Create:

robotnix_pubsub/robotnix_pubsub/listener.py

with:

import rclpy
from rclpy.node import Node
from std_msgs.msg import String


class RobotnixListener(Node):
    def __init__(self):
        super().__init__('robotnix_listener')
        self.subscription = self.create_subscription(
            String,
            'student_chatter',
            self.receive_message,
            10,
        )

    def receive_message(self, msg):
        self.get_logger().info(f'I heard: {msg.data}')


def main(args=None):
    rclpy.init(args=args)
    node = RobotnixListener()
    rclpy.spin(node)
    node.destroy_node()
    rclpy.shutdown()

Compare to the publisher

Both nodes:

• inherit from Node;
• initialize through rclpy;
• use std_msgs.msg.String;
• spin to process ROS work.

The key difference is the endpoint:

talker creates publisher
listener creates subscription

They must agree on topic name and message type.

Callback behavior

The subscriber does not need a timer for incoming messages. Its callback runs when matching messages arrive.

Checkpoint

Change only the listener's topic name on paper to wrong_topic and predict what the running graph would show. Save that failure idea for the diagnosis lesson.