Publishers and Subscriber Activity

Introduction

Basics

Create a node that accomplishes the following:

Publishes on turtle1/cmd_vel and causes the turtle to move forward in the x direction
Subscribes to turtle1/pose and, when the turtle's x coordinate is greater less than 2 or greater than 7 reverses the turtle direction

Advanced

Make the x span where the turtle turns around customizable via parameters called xmin and xmax.
Run two copies of your node and the turtlesim in two separate namespaces with two separate xmin and xmax values.

Sample Code

import rclpy
from rclpy.node import Node
from geometry_msgs.msg import Twist, Vector3


class TurtleAct(Node):
    """ Move the turtle back and forth """
    def __init__(self):
        super().__init__("turtleact")
        self.timer = self.create_timer(1, self.timer_callback)

        # Notice that we are using a relative name "cmd_vel" here
        self.pub = self.create_publisher(Twist, "cmd_vel", 10)

    def timer_callback(self):
        """ This timer callback is called at a fixed frequency """
        self.get_logger().info("Timer!")
        self.pub.publish(Twist(linear=Vector3(x=1.0, y=1.0, z=1.0),
                               angular=Vector3(x=2.0, y=2.0, z=2.0)))

def turtleact_node(args=None):
    """ The node's entry point """
    rclpy.init(args=args)
    node = TurtleAct()
    rclpy.spin(node)
    rclpy.shutdown()

Testing

Run the turtlesim_node and your node and observe whether the expected behavior occurs

Reference

ROS API (and references therein).

Introduction

Basics

Advanced

Sample Code

Testing

Reference

Tutorials