CodeTheRobot
Lab workspace

ROS 2 Lyrical lab 05 ยท 2 hours

Services and motion actions

Use services for short requests and actions for cancellable, feedback-producing work.

Practice this lab in your browser: Action feedback and result

Understand before building

How this part of ROS 2 works

Services fit short request-response work. Actions add goal acceptance, feedback, results, and cancellation for work that takes time.

Motion belongs in an action because an operator must be able to observe progress and request cancellation.

Before you start

  • Lab 04 complete
  • Callbacks and smart pointers

Files you will touch

  • src/robot_interfaces/action/MoveJoint.action
  • src/robot_motion/src/move_server.cpp

Step 1

Set up

# MoveJoint.action
float64 target
---
bool reached
string message
---
float64 remaining

Step 2

Create the file

using MoveJoint = robot_interfaces::action::MoveJoint;
auto server = rclcpp_action::create_server<MoveJoint>(
  this, "move_joint",
  [](const auto &, const auto &) { return rclcpp_action::GoalResponse::ACCEPT_AND_EXECUTE; },
  [](const auto) { return rclcpp_action::CancelResponse::ACCEPT; },
  [this](const auto goal_handle) { execute(goal_handle); });

What each part does

  1. 1.The action type defines goal, result, and feedback sections separated by `---`.
  2. 2.The server has callbacks for accepting goals, accepting cancellation, and starting controlled execution.

Step 3

Build, run and inspect

colcon build --packages-select robot_interfaces robot_motion
source install/setup.bash
ros2 action list -t
ros2 action send_goal /move_joint robot_interfaces/action/MoveJoint "{target: 0.5}" --feedback

Proof that it works

  • The action appears with its custom type
  • Feedback decreases toward zero
  • A cancel request stops simulated motion and returns a canceled result

Recovery notes

  • Generate interfaces in a dedicated ament_cmake package with rosidl_default_generators.
  • Never block the executor callback; run accepted goals on controlled worker logic.
C++ action server and client