Example of a Package for ROS2 with Important Basic Principles

The Python Programm/Script

The following source-code tries to be a basic example of a python-file in a ROS2-package. It would be stored in a workspace-folder such as ros2_ws/src/<package_name>/<package_name>/MyFirstPythonROSProgram.py [yes, there are 2 times folders with the <package_name>]

 
#!/usr/bin/env python # this specifies the interpreter of this script

# ros-core-library for standard ros2-functionality (this is the one for python)
import rclpy
# use ROS-nodes
from rclpy.node import Node

# create a Node "HeartbeatNode", that inherits all functionality from "Node"
class HeartbeatNode(Node):
    def __init__(self, rover_name, timer_period=0.2):
        # call super() in the constructor to initialize the Node object
        # the parameter we pass is the node name
        self._rover_name = rover_name
        super().__init__(self._rover_name)
        # create a timer sending two parameters:
        # - the duration between two callbacks (0.2 seconds)
        # - the timer function (timer_callback)
        self.create_timer(timer_period, self.timer_callback)
        
    def timer_callback(self):
        ros_time_stamp = self.get_clock().now()
        self.get_logger().info(self._rover_name +" is alive..."+str(ros_time_stamp))

# define the Standard-MAIN
def main(args=None):
    # initialize the ROS2 communication
    rclpy.init(args=args)
    # declare the node constructor
    node = HeartbeatNode(rover_name="mars_rover_1", timer_period=1.0)
    # keeps the node alive, waits for a request to kill the node (ctrl+c)
    # The spin() function is used to tell ROS2 to process and manage incoming callbacks within the given ROS2 node.
   # spin() acts as as a highly efficient INFINITE WHILE LOOP
    rclpy.spin(node)
    # shutdown the ROS2 communication
    rclpy.shutdown()

# define another MAIN2
def main2(args=None):
    # initialize the ROS2 communication
    rclpy.init(args=args)
    # declare the node constructor
    node = HeartbeatNode(rover_name="mars_rover_2", timer_period=1.0)
    # keeps the node alive, waits for a request to kill the node (ctrl+c)
    # The spin() function is used to tell ROS2 to process and manage incoming callbacks within the given ROS2 node.
   # spin() acts as as a highly efficient INFINITE WHILE LOOP
    rclpy.spin(node)
    # shutdown the ROS2 communication
    rclpy.shutdown()


# define another main_shutdown
def main_shutdown(args=None):

    # initialize ROS2-communication and systems
    rclpy.init(args=args) 

    # print message
    print("Shutting down Mars rover 1...")

    # close all connections
    rclpy.shutdown()


# execute the main()-method above
if __name__ == '__main__':
    main() # call the main function

Setup.py

This file is automatically created. We have to edit the fields marked with ######## text... ########

 

from setuptools import find_packages, setup
######## input the following two imports ######## 
import os
from glob import glob

package_name = 'mars_rover_systems'

setup(
    name=package_name,
    version='0.0.0',
    packages=find_packages(exclude=['test']),
    ######## edit the last line of data_fields so that ros2 finds the launch-files ########
    data_files=[
        ('share/ament_index/resource_index/packages',
            ['resource/' + package_name]),
        ('share/' + package_name, ['package.xml']),
        (os.path.join('share', package_name), glob('launch/*.launch.py'))
    ],
    install_requires=['setuptools'],
    zip_safe=True,
    maintainer='user',
    maintainer_email='user@todo.todo',
    description='TODO: Package description',
    license='TODO: License declaration',
    tests_require=['pytest'],
    ######## edit the executables here to enable ros2 to find them ######## 
    entry_points={
        'console_scripts': [
            'heartbeat_executable = mars_rover_systems.heartbeat:main',
            'heartbeat_executable2 = mars_rover_systems.heartbeat:main2',
            'heartbeat_shutdown = mars_rover_systems.heartbeat:main_shutdown',
        ],
    },
)