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Chapter 4: Creating Your First Node

Creating a Package​

cd ~/ros2_ws/src
ros2 pkg create --build-type ament_python my_robot_controller

Publisher Node​

my_robot_controller/publisher_node.py:

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

class PublisherNode(Node):
    def __init__(self):
        super().__init__('publisher_node')
        self.publisher = self.create_publisher(String, 'robot_news', 10)
        self.timer = self.create_timer(1.0, self.publish_news)

    def publish_news(self):
        msg = String()
        msg.data = f'Robot update'
        self.publisher.publish(msg)
        self.get_logger().info(f'Publishing: "{msg.data}"')

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

Subscriber Node​

my_robot_controller/subscriber_node.py:

class SubscriberNode(Node):
    def __init__(self):
        super().__init__('subscriber_node')
        self.subscription = self.create_subscription(
            String, 'robot_news', self.callback, 10)

    def callback(self, msg):
        self.get_logger().info(f'Received: "{msg.data}"')

Setup & Run​

Edit setup.py:

entry_points={
    'console_scripts': [
        'publisher = my_robot_controller.publisher_node:main',
        'subscriber = my_robot_controller.subscriber_node:main',
    ],
},

Build and run:

colcon build --packages-select my_robot_controller
source install/setup.bash
ros2 run my_robot_controller publisher

Exercise: Create temperature sensor publisher/subscriber pair.

Next: Chapter 5