Create publisher and subscriber node

Writing a simple publisher and subscriber (Python)

you will create nodes that pass information in the form of string messages to each other over a topic. The example used here is a simple “talker” and “listener” system; one node publishes data and the other subscribes to the topic so it can receive that data.

The code used in these examples can be found here.

• Create a package

cd ros2_ws/src
ros2 pkg create --build-type ament_python --license Apache-2.0 py_pubsub

• Write the publisher node

cd py_pubsub/py_pubsub
wget https://raw.githubusercontent.com/ros2/examples/humble/rclpy/topics/minimal_publisher/examples_rclpy_minimal_publisher/publisher_member_function.py

publisher_member_function.py

import rclpy
from rclpy.node import Node
 
from std_msgs.msg import String
 
 
class MinimalPublisher(Node):
 
    def __init__(self):
        super().__init__('minimal_publisher')
        self.publisher_ = self.create_publisher(String, 'topic', 10)
        timer_period = 0.5  # seconds
        self.timer = self.create_timer(timer_period, self.timer_callback)
        self.i = 0
 
    def timer_callback(self):
        msg = String()
        msg.data = 'Hello World: %d' % self.i
        self.publisher_.publish(msg)
        self.get_logger().info('Publishing: "%s"' % msg.data)
        self.i += 1
 
 
def main(args=None):
    rclpy.init(args=args)
 
    minimal_publisher = MinimalPublisher()
 
    rclpy.spin(minimal_publisher)
 
    # Destroy the node explicitly
    # (optional - otherwise it will be done automatically
    # when the garbage collector destroys the node object)
    minimal_publisher.destroy_node()
    rclpy.shutdown()
 
 
if __name__ == '__main__':
    main()

• Add dependencies
• Open the package.xml file to add the dependencies
  • In this step, you will declare that your package depends on rclpy and std_msgs to run.

cd ..  # Navigate to the parent directory where your package.xml is located
nano package.xml  # Open the file in nano editor
# Add the following lines
<exec_depend>rclpy</exec_depend>
<exec_depend>std_msgs</exec_depend>

• Add an entry point
  • Add 'talker = py_pubsub.publisher_member_function:main' within the console_scripts brackets

setup.py

from setuptools import find_packages, setup
 
package_name = 'py_pubsub'
 
setup(
    name=package_name,
    version='0.0.0',
    packages=find_packages(exclude=['test']),
    data_files=[
        ('share/ament_index/resource_index/packages',
            ['resource/' + package_name]),
        ('share/' + package_name, ['package.xml']),
    ],
    install_requires=['setuptools'],
    zip_safe=True,
    maintainer='chxtio',
    maintainer_email='chxtio@gmail.com',
    description='TODO: Package description',
    license='Apache-2.0',
    tests_require=['pytest'],
    entry_points={
        'console_scripts': [
                'talker = py_pubsub.publisher_member_function:main',
        ],
    },
)

• The contents of the setup.cfg file should be correctly populated automatically
  • This tells setuptools to put your executables in lib, because ros2 run will look for them there.

[develop]
script_dir=$base/lib/py_pubsub
[install]
install_scripts=$base/lib/py_pubsub

• Write the subscriber node
  • The subscriber’s callback gets called as soon as it receives a message

cd py_pubsub
wget https://raw.githubusercontent.com/ros2/examples/humble/rclpy/topics/minimal_subscriber/examples_rclpy_minimal_subscriber/subscriber_member_function.py

subscriber_member_function.py

import rclpy
from rclpy.node import Node
 
from std_msgs.msg import String
 
 
class MinimalSubscriber(Node):
 
    def __init__(self):
        super().__init__('minimal_subscriber')
        self.subscription = self.create_subscription(
            String,
            'topic',
            self.listener_callback,
            10)
        self.subscription  # prevent unused variable warning
 
    def listener_callback(self, msg):
        self.get_logger().info('I heard: "%s"' % msg.data)
 
 
def main(args=None):
    rclpy.init(args=args)
 
    minimal_subscriber = MinimalSubscriber()
 
    rclpy.spin(minimal_subscriber)
 
    # Destroy the node explicitly
    # (optional - otherwise it will be done automatically
    # when the garbage collector destroys the node object)
    minimal_subscriber.destroy_node()
    rclpy.shutdown()
 
 
if __name__ == '__main__':
    main()

• Add the entry point for the subscriber node below the publisher’s entry point

entry_points={
        'console_scripts': [
                'talker = py_pubsub.publisher_member_function:main',
                'listener = py_pubsub.subscriber_member_function:main',
        ],
},

• Check for missing dependencies before building

rosdep install -i --from-path src --rosdistro humble -y

• Build

# cd into ros2_ws
colcon build --packages-select py_pubsub

• Run the talker node in a new terminal

source install/setup.bash
ros2 run py_pubsub talker

[INFO] [minimal_publisher]: Publishing: "Hello World: 0"
[INFO] [minimal_publisher]: Publishing: "Hello World: 1"
[INFO] [minimal_publisher]: Publishing: "Hello World: 2"
[INFO] [minimal_publisher]: Publishing: "Hello World: 3"
[INFO] [minimal_publisher]: Publishing: "Hello World: 4"
...

• Run the listener node in another terminal

source install/setup.bash
ros2 run py_pubsub listener

[INFO] [minimal_subscriber]: I heard: "Hello World: 10"
[INFO] [minimal_subscriber]: I heard: "Hello World: 11"
[INFO] [minimal_subscriber]: I heard: "Hello World: 12"
[INFO] [minimal_subscriber]: I heard: "Hello World: 13"
[INFO] [minimal_subscriber]: I heard: "Hello World: 14"

• Visualize running nodes and communication between them https://roboticsbackend.com/rqt-graph-visualize-and-debug-your-ros-graph/

rqt_graph

Recap: Useful commands

Example- displaying topic info

• List all active nodes

ros2 node list  

/minimal_publisher  
/minimal_subscriber  

• List all active topics

ros2 topic list  

/parameter_events  
/rosout  
/topic  

ros2 topic info <topic_name>  

• Print out messages published to a topic

ros2 topic echo <topic_name>  

data: 'Hello World: 682'  
---  
data: 'Hello World: 683'  
---  
data: 'Hello World: 684'  
---  
data: 'Hello World: 685'  

• Example:

ros2 topic info /topic  

Type: std_msgs/msg/String  
Publisher count: 1  
Subscription count: 0