A Minimal Action Client

The Code

Review the code (including the annotations) and then take a copy of it.

camera_sweep_action_client.py

#! /usr/bin/env python3

import rclpy
from rclpy.node import Node
from rclpy.action import ActionClient # (1)!

from tuos_interfaces.action import CameraSweep # (2)!

class CameraSweepActionClient(Node):

    def __init__(self):
        super().__init__("camera_sweep_action_client") # (3)!
        self.actionclient = ActionClient(
            node=self, 
            action_type=CameraSweep, 
            action_name="camera_sweep"
        ) # (4)!

        self.declare_parameters(
            namespace='',
            parameters=[
                ('goal_images', 0),
                ('goal_angle', 0)
            ]
        ) # (5)!

    def send_goal(self): # (6)!
        images = self.get_parameter(
            'goal_images' 
        ).get_parameter_value().integer_value 
        angle = self.get_parameter(
            'goal_angle'
        ).get_parameter_value().integer_value

        goal = CameraSweep.Goal() # (7)!
        goal.sweep_angle = float(angle)
        goal.image_count = images

        self.actionclient.wait_for_server() # (8)!

        # send the goal to the action server:
        return self.actionclient.send_goal_async(goal)

def main(args=None): # (9)!
    rclpy.init(args=args)
    action_client = CameraSweepActionClient()
    future = action_client.send_goal()
    rclpy.spin_until_future_complete(action_client, future)

if __name__ == '__main__':
    main()

1. As you know by now, in order to develop ROS nodes using Python we need to import the rclpy client library, and the Node class to base our node upon. In addition, here we're also importing an ActionClient class too.

2. We know that the /camera_sweep Action server uses the CameraSweep action interface from the tuos_interfaces package, so we import that here too (which we use to make a call to the server).

3. Standard practice when we initialise ROS nodes: we must give them a name

4. Here, we instantiate an ActionClient class object. In doing this we define the node to add the action client too (in our case self, i.e. our CameraSweepActionClient class). We then also define the interface type used by the server (CameraSweep), and the name of the action that we want to call (action_name="camera_sweep").

5. Here we're declaring two ROS parameters: goal_images and goal_angle.

   We'll use these to set goals for the action server at runtime.

   By default, these values are set to 0, so if we don't explicitly set values for these two parameters then they will remain at 0!

   Question

   How do we set values for parameters at runtime (i.e. when we execute this node using ros2 run)?

   Recall how we did this in Part 4.

6. Here we define a class method to construct and deliver a goal to the server.

7. As we know from earlier, a CameraSweep.Goal() contains two parameters that we can assign values to: sweep_angle and image_count.

   As above, the values assigned to these are derived from two ROS parameters: goal_angle and goal_images.

   Remember

   By default, both parameters will have a value of 0 unless we explicitly assign a value to them (see above)!

   How do we assign values to these parameters at runtime? Recall how we did this in Part 4.

8. The goal is sent to the server using the send_goal_async() method, which returns a future: i.e. something that will happen in the future, that we can wait on. This future is returned once the goal parameters have been accepted by the server, not when the action server has actually completed its job.

9. In our main method we initialise rclpy and our CameraSweepActionClient class (nothing new here), but then we call the send_goal() method of our class (as discussed above), which returns a future. We can then use the rclpy.spin_until_future_complete() method to spin up our node only until this future object has finished.

Package Dependencies

The action client has two key dependencies, so we need to modify the package.xml file (below the <exec_depend>rclpy</exec_depend> line) to include these:

package.xml

<exec_depend>action_msgs</exec_depend>
<exec_depend>tuos_interfaces</exec_depend>