ROS 2 Actions tutorials

Actions are used for tasks that take time to complete and may need feedback or be canceled, such as moving to a specific position. Actions allow continuous communication between nodes during long tasks, providing feedback or allowing users to cancel the task midway.

Example: Using Actions in ROS 2

Let's create an action server and client to move a robot to a target position.

1. Create an Action Server:

   • Open a new file simple_action_server.py and write the following code:

     import rclpy
     from rclpy.node import Node
     from example_interfaces.action import Fibonacci
     from rclpy.action import ActionServer
     
     class FibonacciActionServer(Node):
         def __init__(self):
             super().__init__('fibonacci_action_server')
             self.action_server = ActionServer(self, Fibonacci, 'fibonacci', self.execute_callback)
     
         def execute_callback(self, goal_handle):
             self.get_logger().info('Executing goal...')
             feedback_msg = Fibonacci.Feedback()
             result = Fibonacci.Result()
     
             sequence = [0, 1]
             for i in range(2, goal_handle.request.order):
                 sequence.append(sequence[i-1] + sequence[i-2])
                 feedback_msg.sequence = sequence
                 goal_handle.publish_feedback(feedback_msg)
     
             result.sequence = sequence
             goal_handle.succeed()
             return result
     
     def main(args=None):
         rclpy.init(args=args)
         node = FibonacciActionServer()
         rclpy.spin(node)
         rclpy.shutdown()
     
     if __name__ == '__main__':
         main()

2. Create an Action Client:

   • Open another file simple_action_client.py and add this code:

     import rclpy
     from rclpy.node import Node
     from example_interfaces.action import Fibonacci
     from rclpy.action import ActionClient
     
     class FibonacciActionClient(Node):
         def __init__(self):
             super().__init__('fibonacci_action_client')
             self.action_client = ActionClient(self, Fibonacci, 'fibonacci')
     
         def send_goal(self, order):
             goal_msg = Fibonacci.Goal()
             goal_msg.order = order
             self.action_client.wait_for_server()
             self._send_goal_future = self.action_client.send_goal_async(goal_msg)
     
     def main(args=None):
         rclpy.init(args=args)
         client = FibonacciActionClient()
         client.send_goal(10)
         rclpy.spin(client)
         rclpy.shutdown()
     
     if __name__ == '__main__':
         main()

3. Run the Server and Client:

   • Run the action server in one terminal:

     python3 simple_action_server.py

   • Then run the action client in another terminal:

     python3 simple_action_client.py