Try to understand this
ROS2 (and more precisely DDS) uses UDP multicast to communicate the metadata allowing different node to discover each other and establish communication.
We will be using the same python package that we created earlier, cd into it.
cd ros2_ws/src/my_py_pkg/my_py_pkg
Let us create a new file using
touch robot_news_station.py
Then, concert it into an executable file using
chmod +x robot_news_station.py
After creating this executable file, let us write our first Node inside it. You may use the Node template that I have mentioned in the previous section of this Notion Blog.
The updated node would look like this.
#!/usr/bin/env python3
import rclpy
from rclpy.node import Node
class RobotNewsStationNode(Node): # define Class of Node
def __init__(self):
super().__init__("robot_news_station") # node name, same as file name
def main(args=None):
rclpy.init(args=args)
node = RobotNewsStationNode() # modify as above
rclpy.spin(node)
rclpy.shutdown()
if __name__ == "__main__":
main()
After doing this, go to your setup.py file inside your package and add another line of code to your console_scripts array to convert this file into an executable. Somewhat similar to this.
That’s it for the setup file of the package.
Now, let us write our publisher!
#!/usr/bin/env python3
import rclpy
from rclpy.node import Node
class RobotNewsStationNode(Node):
def __init__(self):
super().__init__("robot_news_station")
# we need to define a topic (common interface, all nodes can find + msg type)
self.publisher_ = self.create_publisher()
def main(args=None):
rclpy.init(args=args)
node = RobotNewsStationNode()
rclpy.spin(node)
rclpy.shutdown()
if __name__ == "__main__":
main()
And we need to give three arguments. The first one is the message type. So you've seen that to define a topic, you need a name, okay, which is a common interface that all nodes can find and a message type. All nodes using the topic as publisher or subscriber will need to use the same message type for the communication to be successful.
