Creating ROS2 Action in C++

Below are the steps to create a ROS2 action. Notice that we are going to have two packages involved: 1) a package called “my_package” which contains the action file, and 2) a package called “action_nodes_cpp” which contains the action server and client nodes. Separating the package which contains the action file(s) like this, in general, is a good practice.

Step 1: Creating an action file.

Create a folder called “action” (if you still don’t have it) inside a package folder called “my_package”. And then make the action file (.action) called “ActionFile.action”. For example, if the action is to compute Fibonacci numbers, you may call it “Fibonacci.action” which looks like below:

#goal definition
int32 order
---
#result definition
int32[] sequence
---
#feedback
int32[] partial_sequence

The lines of three dashes above separate between the request variable(s), the result variable(s), and the feedback variable(s).

Step 2: Add the following in package.xml

rosidl_default_generators
action_msgs
rosidl_interface_packages

Step 3: Add the following in CMakeLists.txt, before ament_package().

find_package(rosidl_default_generators REQUIRED)

rosidl_generate_interfaces(${PROJECT_NAME} "action/ActionFile.action")

add_library(action_server SHARED src/my_action_server.cpp)
target_include_directories(action_server PRIVATE
  &
  &)
target_compile_definitions(action_server
  PRIVATE "ACTION_NODES_CPP_BUILDING_DLL")
ament_target_dependencies(action_server
  "my_package"
  "rclcpp"
  "rclcpp_action"
  "rclcpp_components")
rclcpp_components_register_node(action_server PLUGIN "action_nodes_cpp::MyActionServer" EXECUTABLE my_action_server)
install(TARGETS
  action_server
  ARCHIVE DESTINATION lib
  LIBRARY DESTINATION lib
  RUNTIME DESTINATION bin)
  
add_library(action_client SHARED src/my_action_client.cpp)
target_include_directories(action_client PRIVATE
  &
  &)
target_compile_definitions(action_client
  PRIVATE "ACTION_NODES_CPP_BUILDING_DLL")
ament_target_dependencies(action_client
  "my_package"
  "rclcpp"
  "rclcpp_action"
  "rclcpp_components")
rclcpp_components_register_node(action_client PLUGIN "action_nodes_cpp::MyActionClient" EXECUTABLE my_action_client)
install(TARGETS
  action_client
  ARCHIVE DESTINATION lib
  LIBRARY DESTINATION lib
  RUNTIME DESTINATION bin)

Step 4: Write the “action_server” node.

Write the action_server node in a file namely “my_action_server.cpp”, inside “src” folder of the package.

Step 5: Write the “action_client” node.

Write the action_client node in a file namely “my_action_client.cpp”, inside “src” folder of the package.

Example of action_server and action_client nodes in C++ can be found here: https://docs.ros.org/en/foxy/Tutorials/Actions/Writing-a-Cpp-Action-Server-Client.html#actionscpp

Step 6: Build the package by using “colcon build” command.

Running the action

Type the following in the terminal to run the “action_server” node:

ros2 run action_nodes_cpp my_action_server

Type the following in the terminal to run the “action_client” node:

ros2 run action_nodes_cpp my_action_client

To show the stream of the action feedback:

ros2 topic echo /action_name/feedback

Official documentation:

• https://docs.ros.org/en/foxy/Tutorials/Actions/Creating-an-Action.html
• https://docs.ros.org/en/foxy/Tutorials/Actions/Writing-a-Cpp-Action-Server-Client.html#actionscpp