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7. ROS 2 Topic Communication

1. Introduction to Topic Communication​

Topic communication is the most frequently used communication method in ROS 2. A publisher publishes data on a specified topic, and subscribers who subscribe to that topic receive the data.

Topic communication is based on the publish/subscribe model.

Topic Communication Model

Topic data transmission is a process where data is transmitted from one node to another. The object sending data is called a publisher, and the object receiving data is called a subscriber. Each topic must have a name, and the transmitted data must have a fixed data type.

Next, we will explain how to implement topic communication between nodes using Python.

2. Create a New Package​

  • Switch to the src directory of the workspace\
  • Create a new pkg_topic package
ros2 pkg create pkg_topic --build-type ament_python --dependencies rclpy --node-name publisher_demo

After executing the above command, the pkg_topic package will be created along with a publisher_demo node and the relevant configuration files.

Package Structure

3. Publisher Implementation​

3.1 Create a Publisher​

Edit publisher_demo.py and add the following code:

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

class Topic_Pub(Node):
    def __init__(self, name):
        super().__init__(name)
        self.pub = self.create_publisher(String, "/topic_demo", 1)
        self.timer = self.create_timer(1, self.pub_msg)

    def pub_msg(self):
        msg = String()
        msg.data = "Hi, I send a message."
        self.pub.publish(msg)

def main():
    rclpy.init()
    pub_demo = Topic_Pub("publisher_node")
    rclpy.spin(pub_demo)
    pub_demo.destroy_node()
    rclpy.shutdown()

3.2 Editing the Configuration File​

Config File

3.3 Compiling the Package​

colcon build --packages-select pkg_topic

Refresh the workspace:

source install/setup.bash

3.4 Running the Program​

ros2 run pkg_topic publisher_demo

Check published topics:

ros2 topic list

Topic List

View topic data:

ros2 topic echo /topic_demo

Topic Echo Output

4. Subscriber Implementation​

4.1 Creating a Subscriber​

Create a new file subscriber_demo.py in the same directory as publisher_demo.py.

Subscriber File

Edit the file with the following code:

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

class Topic_Sub(Node):
    def __init__(self, name):
        super().__init__(name)
        self.sub = self.create_subscription(String, "/topic_demo", self.sub_callback, 1)

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

def main():
    rclpy.init()
    sub_demo = Topic_Sub("subscriber_node")
    rclpy.spin(sub_demo)
    sub_demo.destroy_node()
    rclpy.shutdown()

4.2 Editing the Configuration File​

Subscriber Config

4.3 Compile the Workspace​

colcon build