Lab: Introduction to ROS2

Today's Lesson

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Practice :

• Create a package
• Create a node
• Manipulate node
• Create another node
• Publisher/Receiver

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Create a package

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A package is a container for your ROS 2 code.

      $  mkdir -p ros2_ws/src
      $  cd ros2_ws/src
      $  ros2 pkg create --build-type ament_python iml_buche_test
    

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Create a node

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Create start.py and place it in ros2_ws/src/iml_buche_test/iml_buche_test/

start.py

        import rclpy
        from rclpy.node import Node

        def main(args=None):

            # initialize ROS communications
            rclpy.init(args=args)
            
            # create the node
            node = Node('start_node')
            
            # start ROS2 publishers, subscribers, services, get parameters, etc
            rclpy.spin(node)
            
            # When you request to kill the node
            rclpy.shutdown()
            
        if __name__ == '__main__':
            main()

Add an entry point in the setup.py

setup.py

    #...
    entry_points={
        'console_scripts': [
            'test_python_node = iml_buche_test.start:main'
        ],
    ...

package.xml

Add rclpy dependency <build_type>ament_python</build_type> ... <depend>rclpy</depend>

     # Compile your package
  $ cd ros2_ws
  $ colcon build

    # source your ROS2 workspace
   $ . install/setup.bash

    # run
   $ ros2 run iml_buche_test test_python_node
    

---

Manipulate node

---

    # change the node’s name from “my_node” to “another_node”
   $ ros2 run iml_buche_test test_python_node  --ros-args -r __node:=another_node

    # dans un autre terminal
   $ . install/setup.bash # source
   $ ros2 run iml_buche_test test_python_node

    # it could be nice to be able to see what’s going on
   $ ros2 node list

    # Get more details about a Node
   $ ros2 node info /another_node
    

Create another node

Create anotherNode.py and place it in ros2_ws/src/iml_buche_test/iml_buche_test/

anotherNode.py

# ROS2 Python library
           import rclpy
           from rclpy.node import Node

           class MyNode2(Node):
               def __init__(self):
                   super().__init__('my_node_name_another')
                   self.create_timer(0.2, self.timer_callback)
                   
               def timer_callback(self):
                   self.get_logger().info("Coucou -- Hello ROS2 -- this is IML ")

           def main(args=None):
               rclpy.init(args=args)
               
               node = MyNode2()
               
               rclpy.spin(node)
               rclpy.shutdown()
               
           if __name__ == '__main__':
               main()
       

Do not forget to add another entry point in setup.py

    #...
    entry_points={
        'console_scripts': [
            'test_python_node = iml_buche_test.start:main',
            'test_python_another_node = iml_buche_test.anotherNode:main'
        ],
    ...

Compile :

     $   colcon build
    

In one terminal :

          $ . install/setup.bash
          $ ros2 run iml_buche_test test_python_node
       

In another terminal :

             $ . install/setup.bash
             $ ros2 run iml_buche_test test_python_another_node
          

---

Publisher/Receiver

---

Create a new package in the workspace

           $  cd ./ros2_ws/src
    $  ros2 pkg create --build-type ament_python iml_buche_test_2
       

publisher.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, 'iml_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)


            rclpy.shutdown()


        if __name__ == '__main__':
            main()
    

receiver.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,
                    'iml_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)
            rclpy.shutdown()

        if __name__ == '__main__':
            main()
    

While running, you can display the graph

          $ rqt_graph