ROS2-3: Topics

Topics are used for data streams.

ROS2 - Topics

• A topic is a named bus over which nodes exchange messages
  • unidirectional data stream (publisher/subscriber)
  • anonymous
  • a topic has a message type
  • can be written in Python, C++, … directly inside ROS nodes
  • a node can have many publishers/subscribers for many different topics

Python Publisher

cd ros2_ws/src/my_py_pkg/my_py_pkg

touch robot_news_station.py


chmod +x robot_news_station.py

In robot_news_station.py

#!/usr/bin/env python3
import rclpy
from rclpy.node import Node

from example_interfaces.msg import String


class RobotNewsStationNode(Node):
    def __init__(self):
        super().__init__("robot_news_station")
        self.publisher_ = self.create_publisher(String, "robot_news", 10)
        self.timer_ = self.create_timer(
            timer_period_sec=0.5, callback=self.publish_news)
        self.get_logger().info("Robot News Station has been started")

    def publish_news(self):
        msg = String()
        msg.data = "Hello"
        self.publisher_.publish(msg)


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


if __name__ == "__main__":
    main()

In setup.py

entry_points={
     'console_scripts': [
         "py_node = my_py_pkg.my_first_node:main", # executable
         "robot_news_station = my_py_pkg.robot_news_station:main"
     ],
 },

In package.xml

  <depend>rclpy</depend>
  <depend>example_interfaces</depend>

Then

# terminal 1
colcon build --package-select my_py_pkg --symlink-install
source ~/.bashrc
ros2 run my_py_pkg robot_news_station

# terminal 2
ros2 topic list
ros2 topic echo /robot_news

Python Subscriber

In smartphone.py

#!/usr/bin/env python3
import rclpy
from rclpy.node import Node
from example_interfaces.msg import String


class SmartphoneNode(Node):
    def __init__(self):
        super().__init__("smartphone")
        self.publisher_ = self.create_subscription(
            msg_type=String, topic="robot_news", callback=self.callback_robot_news, qos_profile=10)
        self.get_logger().info("Smartphone has been started")

    def callback_robot_news(self, msg):
        self.get_logger().info(msg.data)


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


if __name__ == "__main__":
    main()

In setup.py

entry_points={
     'console_scripts': [
         "py_node = my_py_pkg.my_first_node:main",  # executable
         "robot_news_station = my_py_pkg.robot_news_station:main",
         "smartphone = my_py_pkg.smartphone:main",
     ],
 },

Then

# terminal 1
colcon build --package-select my_py_pkg --symlink-install
source ~/.bashrc
ros2 run my_py_pkg robot_news_station

# terminal 2
ros2 run my_py_pkg smartphone

Debug ROS2 Topics with Command Line Tools

# double tabs after typing:
ros2 topic

# show the info 
ros2 topic info /robot_news

# message type info
ros2 interface show example_interfaces/msg/String

# monitor publish frequency
ros2 topic hz /robot_news 

# bandwidth
ros2 topic bw /robot_news

# publish in 10hz with customised message
ros2 topic pub -r 10 /robot_news example_interfaces/msg/String "{data: hello from terminal}"

Remap a Topic at Runtime

ros2 run my_py_pkg robot_news_station --ros-args -r __node:=my_station -r robot_news:=my_news

ros2 run my_py_pkg smartphone --ros-args -r __node:=my_station -r robot_news:=my_news