Goal: Define custom interface files (.msg and .srv) and use them with Python and C++ nodes.
Tutorial level: Beginner
Time: 20 minutes
Contents
In previous tutorials you utilized message and service interfaces to learn about :ref:`topics <ROS2Topics>`, :ref:`services <ROS2Services>`, and simple publisher/subscriber (:ref:`C++<CppPubSub>`/:ref:`Python<PyPubSub>`) and service/client (:ref:`C++<CppSrvCli>`/:ref:`Python<PySrvCli>`) nodes. The interfaces you used were predefined in those cases.
While it’s good practice to use predefined interface definitions, you will probably need to define your own messages and services sometimes as well. This tutorial will introduce you to the simplest method of creating custom interface definitions.
You should have a :ref:`ROS 2 workspace <ROS2Workspace>`.
This tutorial also uses the packages created in the publisher/subscriber (:ref:`C++ <CppPubSub>` and :ref:`Python<PyPubSub>`) and service/client (:ref:`C++ <CppSrvCli>` and :ref:`Python<PySrvCli>`) tutorials to try out the new custom messages.
For this tutorial you will be creating custom .msg and .srv files in their own package, and then utilizing them in a separate package.
Both packages should be in the same workspace.
Since we will use the pub/sub and service/client packages created in earlier tutorials, make sure you are in the same workspace as those packages (dev_ws/src), and then run the following command to create a new package:
ros2 pkg create --build-type ament_cmake tutorial_interfacestutorial_interfaces is the name of the new package.
Note that it is a CMake package; there currently isn’t a way to generate a .msg or .srv file in a pure Python package.
You can create a custom interface in a CMake package, and then use it in a Python node, which will be covered in the last section.
It is good practice to keep .msg and .srv files in their own directories within a package.
Create the directories in dev_ws/src/tutorial_interfaces:
mkdir msg
mkdir srvIn the tutorial_interfaces/msg directory you just created, make a new file called Num.msg with one line of code declaring its data structure:
int64 numThis is your custom message that transfers a single 64-bit integer called num.
Back in the tutorial_interfaces/srv directory you just created, make a new file called AddThreeInts.srv with the following request and response structure:
int64 a
int64 b
int64 c
---
int64 sumThis is your custom service that requests three integers named a, b, and c, and responds with an integer called sum.
To convert the interfaces you defined into language-specific code (like C++ and Python) so that they can be used in those languages, add the following lines to CMakeLists.txt:
find_package(rosidl_default_generators REQUIRED)
rosidl_generate_interfaces(${PROJECT_NAME}
"msg/Num.msg"
"srv/AddThreeInts.srv"
)Because the interfaces rely on rosidl_default_generators for generating language-specific code, you need to declare a dependency on it.
Add the following lines to package.xml
<build_depend>rosidl_default_generators</build_depend>
<exec_depend>rosidl_default_runtime</exec_depend>
<member_of_group>rosidl_interface_packages</member_of_group>Now that all the parts of your custom interfaces package are in place, you can build the package.
In the root of your workspace (~/dev_ws), run the following command:
colcon build --packages-select tutorial_interfacesNow the interfaces will be discoverable by other ROS 2 packages.
In a new terminal, run the following command from within your workspace (dev_ws) to source it:
.. tabs::
.. group-tab:: Linux
.. code-block:: console
. install/setup.bash
.. group-tab:: macOS
.. code-block:: console
. install/setup.bash
.. group-tab:: Windows
.. code-block:: console
call install/setup.bat
Now you can confirm that your interface creation worked by using the ros2 interface show command:
ros2 msg show tutorial_interfaces/msg/Numshould return:
int64 numAnd
ros2 srv show tutorial_interfaces/srv/AddThreeIntsshould return:
int64 a
int64 b
int64 c
---
int64 sumFor this step you can use the packages you created in previous tutorials.
A few simple modifications to the nodes, CMakeLists and package files will allow you to use your new interfaces.
With some slight modifications to the publisher/subscriber package created in a previous tutorial (:ref:`C++ <CppPubSub>` or :ref:`Python <PyPubSub>`), you can see Num.msg in action.
Since you’ll be changing the standard string msg to a numerical one, the output will be slightly different.
Publisher:
.. tabs::
.. group-tab:: C++
.. code-block:: c++
#include <chrono>
#include <memory>
#include "rclcpp/rclcpp.hpp"
#include "tutorial_interfaces/msg/num.hpp" // CHANGE
using namespace std::chrono_literals;
class MinimalPublisher : public rclcpp::Node
{
public:
MinimalPublisher()
: Node("minimal_publisher"), count_(0)
{
publisher_ = this->create_publisher<tutorial_interfaces::msg::Num>("topic", 10); // CHANGE
timer_ = this->create_wall_timer(
500ms, std::bind(&MinimalPublisher::timer_callback, this));
}
private:
void timer_callback()
{
auto message = tutorial_interfaces::msg::Num(); // CHANGE
message.num = this->count_++; // CHANGE
RCLCPP_INFO(this->get_logger(), "Publishing: '%d'", message.num); // CHANGE
publisher_->publish(message);
}
rclcpp::TimerBase::SharedPtr timer_;
rclcpp::Publisher<tutorial_interfaces::msg::Num>::SharedPtr publisher_; // CHANGE
size_t count_;
};
int main(int argc, char * argv[])
{
rclcpp::init(argc, argv);
rclcpp::spin(std::make_shared<MinimalPublisher>());
rclcpp::shutdown();
return 0;
}
.. group-tab:: Python
.. code-block:: python
import rclpy
from rclpy.node import Node
from tutorial_interfaces.msg import Num # CHANGE
class MinimalPublisher(Node):
def __init__(self):
super().__init__('minimal_publisher')
self.publisher_ = self.create_publisher(Num, 'topic', 10) # CHANGE
timer_period = 0.5
self.timer = self.create_timer(timer_period, self.timer_callback)
self.i = 0
def timer_callback(self):
msg = Num() # CHANGE
msg.num = self.i # CHANGE
self.publisher_.publish(msg)
self.get_logger().info('Publishing: "%d"' % msg.num) # CHANGE
self.i += 1
def main(args=None):
rclpy.init(args=args)
minimal_publisher = MinimalPublisher()
rclpy.spin(minimal_publisher)
minimal_publisher.destroy_node()
rclpy.shutdown()
if __name__ == '__main__':
main()
Subscriber:
.. tabs::
.. group-tab:: C++
.. code-block:: c++
#include <memory>
#include "rclcpp/rclcpp.hpp"
#include "tutorial_interfaces/msg/num.hpp" // CHANGE
using std::placeholders::_1;
class MinimalSubscriber : public rclcpp::Node
{
public:
MinimalSubscriber()
: Node("minimal_subscriber")
{
subscription_ = this->create_subscription<tutorial_interfaces::msg::Num>( // CHANGE
"topic", 10, std::bind(&MinimalSubscriber::topic_callback, this, _1));
}
private:
void topic_callback(const tutorial_interfaces::msg::Num::SharedPtr msg) const // CHANGE
{
RCLCPP_INFO(this->get_logger(), "I heard: '%d'", msg->num); // CHANGE
}
rclcpp::Subscription<tutorial_interfaces::msg::Num>::SharedPtr subscription_; // CHANGE
};
int main(int argc, char * argv[])
{
rclcpp::init(argc, argv);
rclcpp::spin(std::make_shared<MinimalSubscriber>());
rclcpp::shutdown();
return 0;
}
.. group-tab:: Python
.. code-block:: python
import rclpy
from rclpy.node import Node
from tutorial_interfaces.msg import Num # CHANGE
class MinimalSubscriber(Node):
def __init__(self):
super().__init__('minimal_subscriber')
self.subscription = self.create_subscription(
Num, # CHANGE
'topic',
self.listener_callback,
10)
self.subscription
def listener_callback(self, msg):
self.get_logger().info('I heard: "%d"' % msg.num) # CHANGE
def main(args=None):
rclpy.init(args=args)
minimal_subscriber = MinimalSubscriber()
rclpy.spin(minimal_subscriber)
minimal_subscriber.destroy_node()
rclpy.shutdown()
if __name__ == '__main__':
main()
CMakeLists.txt:
Add the following lines (C++ only):
...
find_package(ament_cmake REQUIRED)
find_package(rclcpp REQUIRED)
find_package(tutorial_interfaces REQUIRED) # CHANGE
add_executable(talker src/publisher_member_function.cpp)
ament_target_dependencies(talker rclcpp tutorial_interfaces) # CHANGE
add_executable(listener src/subscriber_member_function.cpp)
ament_target_dependencies(listener rclcpp tutorial_interfaces) # CHANGE
install(TARGETS
talker
listener
DESTINATION lib/${PROJECT_NAME})
ament_package()package.xml:
Add the following line:
.. tabs::
.. group-tab:: C++
.. code-block:: c++
<depend>tutorial_interfaces</depend>
.. group-tab:: Python
.. code-block:: python
<exec_depend>tutorial_interfaces</exec_depend>
After making the above edits and saving all the changes, build the package:
.. tabs::
.. group-tab:: C++
.. code-block:: console
colcon build --packages-select cpp_pubsub
.. group-tab:: Python
.. code-block:: console
colcon build --packages-select py_pubsub
Then open two new terminals, source dev_ws in each, and run:
.. tabs::
.. group-tab:: C++
.. code-block:: console
ros2 run cpp_pubsub talker
.. code-block:: console
ros2 run cpp_pubsub listener
.. group-tab:: Python
.. code-block:: console
ros2 run py_pubsub talker
.. code-block:: console
ros2 run py_pubsub listener
Since Num.msg relays only an integer, the talker should only be publishing integer values, as opposed to the string it published previously:
[INFO] [minimal_publisher]: Publishing: '0'
[INFO] [minimal_publisher]: Publishing: '1'
[INFO] [minimal_publisher]: Publishing: '2'With some slight modifications to the service/client package created in a previous tutorial (:ref:`C++ <CppSrvCli>` or :ref:`Python <PySrvCli>`), you can see AddThreeInts.srv in action.
Since you’ll be changing the original two integer request srv to a three integer request srv, the output will be slightly different.
Service:
.. tabs::
.. group-tab:: C++
.. code-block:: c++
#include "rclcpp/rclcpp.hpp"
#include "tutorial_interfaces/srv/add_three_ints.hpp" // CHANGE
#include <memory>
void add(const std::shared_ptr<tutorial_interfaces::srv::AddThreeInts::Request> request, // CHANGE
std::shared_ptr<tutorial_interfaces::srv::AddThreeInts::Response> response) // CHANGE
{
response->sum = request->a + request->b + request->c; // CHANGE
RCLCPP_INFO(rclcpp::get_logger("rclcpp"), "Incoming request\na: %ld" " b: %ld" " c: %ld", // CHANGE
request->a, request->b, request->c); // CHANGE
RCLCPP_INFO(rclcpp::get_logger("rclcpp"), "sending back response: [%ld]", (long int)response->sum);
}
int main(int argc, char **argv)
{
rclcpp::init(argc, argv);
std::shared_ptr<rclcpp::Node> node = rclcpp::Node::make_shared("add_three_ints_server"); // CHANGE
rclcpp::Service<tutorial_interfaces::srv::AddThreeInts>::SharedPtr service = // CHANGE
node->create_service<tutorial_interfaces::srv::AddThreeInts>("add_three_ints", &add); // CHANGE
RCLCPP_INFO(rclcpp::get_logger("rclcpp"), "Ready to add three ints."); // CHANGE
rclcpp::spin(node);
rclcpp::shutdown();
}
.. group-tab:: Python
.. code-block:: python
from tutorial_interfaces.srv import AddThreeInts # CHANGE
import rclpy
from rclpy.node import Node
class MinimalService(Node):
def __init__(self):
super().__init__('minimal_service')
self.srv = self.create_service(AddThreeInts, 'add_three_ints', self.add_three_ints_callback) # CHANGE
def add_three_ints_callback(self, request, response):
response.sum = request.a + request.b + request.c # CHANGE
self.get_logger().info('Incoming request\na: %d b: %d c: %d' % (request.a, request.b, request.c)) # CHANGE
return response
def main(args=None):
rclpy.init(args=args)
minimal_service = MinimalService()
rclpy.spin(minimal_service)
rclpy.shutdown()
if __name__ == '__main__':
main()
Client:
.. tabs::
.. group-tab:: C++
.. code-block:: c++
#include "rclcpp/rclcpp.hpp"
#include "tutorial_interfaces/srv/add_three_ints.hpp" // CHANGE
#include <chrono>
#include <cstdlib>
#include <memory>
using namespace std::chrono_literals;
int main(int argc, char **argv)
{
rclcpp::init(argc, argv);
if (argc != 4) { // CHANGE
RCLCPP_INFO(rclcpp::get_logger("rclcpp"), "usage: add_three_ints_client X Y Z"); // CHANGE
return 1;
}
std::shared_ptr<rclcpp::Node> node = rclcpp::Node::make_shared("add_three_ints_client"); // CHANGE
rclcpp::Client<tutorial_interfaces::srv::AddThreeInts>::SharedPtr client = // CHANGE
node->create_client<tutorial_interfaces::srv::AddThreeInts>("add_three_ints"); // CHANGE
auto request = std::make_shared<tutorial_interfaces::srv::AddThreeInts::Request>(); // CHANGE
request->a = atoll(argv[1]);
request->b = atoll(argv[2]);
request->c = atoll(argv[3]); // CHANGE
while (!client->wait_for_service(1s)) {
if (!rclcpp::ok()) {
RCLCPP_ERROR(rclcpp::get_logger("rclcpp"), "Interrupted while waiting for the service. Exiting.");
return 0;
}
RCLCPP_INFO(rclcpp::get_logger("rclcpp"), "service not available, waiting again...");
}
auto result = client->async_send_request(request);
// Wait for the result.
if (rclcpp::spin_until_future_complete(node, result) ==
rclcpp::executor::FutureReturnCode::SUCCESS)
{
RCLCPP_INFO(rclcpp::get_logger("rclcpp"), "Sum: %ld", result.get()->sum);
} else {
RCLCPP_ERROR(rclcpp::get_logger("rclcpp"), "Failed to call service add_three_ints"); // CHANGE
}
rclcpp::shutdown();
return 0;
}
.. group-tab:: Python
.. code-block:: python
from tutorial_interfaces.srv import AddThreeInts # CHANGE
import sys
import rclpy
from rclpy.node import Node
class MinimalClientAsync(Node):
def __init__(self):
super().__init__('minimal_client_async')
self.cli = self.create_client(AddThreeInts, 'add_three_ints') # CHANGE
while not self.cli.wait_for_service(timeout_sec=1.0):
self.get_logger().info('service not available, waiting again...')
self.req = AddThreeInts.Request() # CHANGE
def send_request(self):
self.req.a = int(sys.argv[1])
self.req.b = int(sys.argv[2])
self.req.c = int(sys.argv[3]) # CHANGE
self.future = self.cli.call_async(self.req)
def main(args=None):
rclpy.init(args=args)
minimal_client = MinimalClientAsync()
minimal_client.send_request()
while rclpy.ok():
rclpy.spin_once(minimal_client)
if minimal_client.future.done():
try:
response = minimal_client.future.result()
except Exception as e:
minimal_client.get_logger().info(
'Service call failed %r' % (e,))
else:
minimal_client.get_logger().info(
'Result of add_three_ints: for %d + %d + %d = %d' % # CHANGE
(minimal_client.req.a, minimal_client.req.b, minimal_client.req.c, response.sum)) # CHANGE
break
minimal_client.destroy_node()
rclpy.shutdown()
if __name__ == '__main__':
main()
CMakeLists.txt:
Add the following lines (C++ only):
...
find_package(ament_cmake REQUIRED)
find_package(rclcpp REQUIRED)
find_package(tutorial_interfaces REQUIRED) # CHANGE
add_executable(server src/add_two_ints_server.cpp)
ament_target_dependencies(server
rclcpp tutorial_interfaces) #CHANGE
add_executable(client src/add_two_ints_client.cpp)
ament_target_dependencies(client
rclcpp tutorial_interfaces) #CHANGE
install(TARGETS
server
client
DESTINATION lib/${PROJECT_NAME})
ament_package()package.xml:
Add the following line:
.. tabs::
.. group-tab:: C++
.. code-block:: c++
<depend>tutorial_interfaces</depend>
.. group-tab:: Python
.. code-block:: python
<exec_depend>tutorial_interfaces</exec_depend>
After making the above edits and saving all the changes, build the package:
.. tabs::
.. group-tab:: C++
.. code-block:: console
colcon build --packages-select cpp_srvcli
.. group-tab:: Python
.. code-block:: console
colcon build --packages-select py_srvcli
Then open two new terminals, source dev_ws in each, and run:
.. tabs::
.. group-tab:: C++
.. code-block:: console
ros2 run cpp_srvcli server
.. code-block:: console
ros2 run cpp_srvcli client 2 3 1
.. group-tab:: Python
.. code-block:: console
ros2 run py_srvcli service
.. code-block:: console
ros2 run py_srvcli client 2 3 1
In this tutorial, you learned how to create custom interfaces in their own package and how to utilize those interfaces from within other packages.
This is a simple method of interface creation and utilization. You can learn more about interfaces :ref:`here <InterfaceConcept>`.
The :ref:`next tutorial <SinglePkgInterface>` covers more ways to use interfaces in ROS 2.