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Expanding on ROS 2 interfaces

Goal: Learn more ways to implement custom interfaces in ROS 2

Tutorial level: Beginner

Time: 15 minutes

Background 

In a previous tutorial, you learned how to create custom msg and srv interfaces.

While best practice is to declare interfaces in dedicated interface packages, sometimes it can be convenient to declare, create and use an interface all in one package.

Recall that interfaces can currently only be defined in CMake packages. It is possible, however, to have Python libraries and nodes in CMake packages (using ament_cmake_python), so you could define interfaces and Python nodes together in one package. We’ll use a CMake package and C++ nodes here for the sake of simplicity.

This tutorial will focus on the msg interface type, but the steps here are applicable to all interface types.

Prerequisites 

We assume you’ve reviewed the basics in the Creating custom ROS 2 msg and srv files tutorial before working through this one.

You should have ROS 2 installed, a workspace, and an understanding of creating packages.

As always, don’t forget to source ROS 2 in every new terminal you open.

Tasks 

1 Create a package 

In your workspace, create a package more_interfaces and make a folder within it for msg files:

cd ~/dev_ws/src
ros2 pkg create --build-type ament_cmake more_interfaces
mkdir more_interfaces/msg

2 Create a msg file 

Inside more_interfaces/msg, create a new file AddressBook.msg

Paste the following code to create a message meant to carry information about an individual:

bool FEMALE=true
bool MALE=false

string first_name
string last_name
bool gender
uint8 age
string address

This message is composed of 5 fields:

first_name: of type string
last_name: of type string
gender: of type bool, that can be either MALE or FEMALE
age: of type uint8
address: of type string

Notice that it’s possible to set default values for fields within the message definition. See About ROS 2 interfaces for more ways you can customize interfaces.

Next, we need to make sure that the msg file is turned into source code for C++, Python, and other languages.

2.1 Build a msg file

Open package.xml, and add the following lines:

<buildtool_depend>rosidl_default_generators</buildtool_depend>

<exec_depend>rosidl_default_runtime</exec_depend>

<member_of_group>rosidl_interface_packages</member_of_group>

Note that at build time, we need rosidl_default_generators, while at runtime, we only need rosidl_default_runtime.

Open CMakeLists.txt and add the following lines:

Find the package that generates message code from msg/srv files:

find_package(rosidl_default_generators REQUIRED)

Declare the list of messages you want to generate:

set(msg_files
  "msg/AddressBook.msg"
)

By adding the .msg files manually, we make sure that CMake knows when it has to reconfigure the project after you add other .msg files.

Generate the messages:

rosidl_generate_interfaces(${PROJECT_NAME}
  ${msg_files}
)

Also make sure you export the message runtime dependency:

ament_export_dependencies(rosidl_default_runtime)

Now you’re ready to generate source files from your msg definition. We’ll skip the compile step for now as we do it all together below in step 4.

2.2 (Extra) Set multiple interfaces

Note

You can use set to neatly list all of your interfaces:

set(msg_files
  "msg/Message1.msg"
  "msg/Message2.msg"
  # etc
  )

set(srv_files
  "srv/Service1.srv"
  "srv/Service2.srv"
   # etc
  )

And generate all lists at once like so:

rosidl_generate_interfaces(${PROJECT_NAME}
  ${msg_files}
  ${srv_files}
)

3 Use an interface from the same package 

Now we can start writing code that uses this message.

In more_interfaces/src create a file called publish_address_book.cpp and paste the following code:

#include <chrono>
#include <memory>

#include "rclcpp/rclcpp.hpp"
#include "more_interfaces/msg/address_book.hpp"

using namespace std::chrono_literals;

class AddressBookPublisher : public rclcpp::Node
{
public:
  AddressBookPublisher()
  : Node("address_book_publisher")
  {
    address_book_publisher_ =
      this->create_publisher<more_interfaces::msg::AddressBook>("address_book", 10);

    auto publish_msg = [this]() -> void {
        auto message = more_interfaces::msg::AddressBook();

        message.first_name = "John";
        message.last_name = "Doe";
        message.age = 30;
        message.gender = message.MALE;
        message.address = "unknown";

        std::cout << "Publishing Contact\nFirst:" << message.first_name <<
          "  Last:" << message.last_name << std::endl;

        this->address_book_publisher_->publish(message);
      };
    timer_ = this->create_wall_timer(1s, publish_msg);
  }

private:
  rclcpp::Publisher<more_interfaces::msg::AddressBook>::SharedPtr address_book_publisher_;
  rclcpp::TimerBase::SharedPtr timer_;
};


int main(int argc, char * argv[])
{
  rclcpp::init(argc, argv);
  rclcpp::spin(std::make_shared<AddressBookPublisher>());
  rclcpp::shutdown();

  return 0;
}

3.1 The code explained

#include "more_interfaces/msg/address_book.hpp"

Include the header of our newly created AddressBook.msg.

using namespace std::chrono_literals;

class AddressBookPublisher : public rclcpp::Node
{
public:
  AddressBookPublisher()
  : Node("address_book_publisher")
  {
    address_book_publisher_ =
      this->create_publisher<more_interfaces::msg::AddressBook>("address_book");

Create a node and an AddressBook publisher.

auto publish_msg = [this]() -> void {

Create a callback to publish the messages periodically.

auto message = more_interfaces::msg::AddressBook();

Create an AddressBook message instance that we will later publish.

message.first_name = "John";
message.last_name = "Doe";
message.age = 30;
message.gender = message.MALE;
message.address = "unknown";

Populate AddressBook fields.

std::cout << "Publishing Contact\nFirst:" << message.first_name <<
  "  Last:" << message.last_name << std::endl;

this->address_book_publisher_->publish(message);

Finally send the message periodically.

timer_ = this->create_wall_timer(1s, publish_msg);

Create a 1 second timer to call our publish_msg function every second.

3.2 Build the publisher

We need to create a new target for this node in the CMakeLists.txt:

find_package(rclcpp REQUIRED)

add_executable(publish_address_book
  src/publish_address_book.cpp
)

ament_target_dependencies(publish_address_book
  "rclcpp"
)

install(TARGETS publish_address_book
 DESTINATION lib/${PROJECT_NAME})

3.3 Link against the interface

In order to use the messages generated in the same package we need to use the following CMake code:

rosidl_target_interfaces(publish_address_book
  ${PROJECT_NAME} "rosidl_typesupport_cpp")

This finds the relevant generated C++ code from AddressBook.msg and allows your target to link against it.

You may have noticed that this step was not necessary when the interfaces being used were from a package that was built separately. This CMake code is only required when you want to use interfaces in the same package as the one in which they are used.

4 Try it out 

Return to the root of the workspace to build the package:

cd ~/dev_ws
colcon build --packages-up-to more_interfaces

Then source the workspace and run the publisher:

. install/local_setup.bash

ros2 run more_interfaces publish_address_book

You should see the publisher relaying the msg you defined, including the values you set in publish_address_book.cpp.

To confirm the message is being published on the address_book topic, open another terminal, source the workspace, and call topic echo:

. install/local_setup.bash

ros2 topic echo /address_book

We won’t create a subscriber in this tutorial, but you can try to write one yourself for practice (use Writing a simple publisher and subscriber (C++) to help).

5 (Extra) Use an existing interface definition 

Note

You can use an existing interface definition in a new interface definition. For example, let’s say there is a message named Contact.msg that belongs to an existing ROS 2 package named rosidl_tutorials_msgs. Assume that its definition is identical to our custom-made AddressBook.msg interface from earlier.

In that case you could have defined AddressBook.msg (an interface in the package with your nodes) as type Contact (an interface in a separate package). You could even define AddressBook.msg as an array of type Contact, like so:

rosidl_tutorials_msgs/Contact[] address_book

To generate this message you would need to declare a dependency on Contact.msg's package, rosidl_tutorials_msgs, in package.xml:

<build_depend>rosidl_tutorials_msgs</build_depend>

<exec_depend>rosidl_tutorials_msgs</exec_depend>

And in CMakeLists.txt:

find_package(rosidl_tutorials_msgs REQUIRED)

rosidl_generate_interfaces(${PROJECT_NAME}
  ${msg_files}
  DEPENDENCIES rosidl_tutorials_msgs
)

You would also need to include the header of Contact.msg in you publisher node in order to be able to add contacts to your address_book.

#include "rosidl_tutorials_msgs/msg/contact.hpp"

You could change the call back to something like this:

auto publish_msg = [this]() -> void {
   auto msg = std::make_shared<more_interfaces::msg::AddressBook>();
   {
     rosidl_tutorials_msgs::msg::Contact contact;
     contact.first_name = "John";
     contact.last_name = "Doe";
     contact.age = 30;
     contact.gender = contact.MALE;
     contact.address = "unknown";
     msg->address_book.push_back(contact);
   }
   {
     rosidl_tutorials_msgs::msg::Contact contact;
     contact.first_name = "Jane";
     contact.last_name = "Doe";
     contact.age = 20;
     contact.gender = contact.FEMALE;
     contact.address = "unknown";
     msg->address_book.push_back(contact);
   }

   std::cout << "Publishing address book:" << std::endl;
   for (auto contact : msg->address_book) {
     std::cout << "First:" << contact.first_name << "  Last:" << contact.last_name <<
       std::endl;
   }

   address_book_publisher_->publish(*msg);
 };

Building and running these changes would show the msg defined as expected, as well as the array of msgs defined above.

Summary 

In this tutorial, you tried out different field types for defining interfaces, then built an interface in the same package where it’s being used.

You also learned how to use another interface as a field type, as well as the package.xml, CMakeLists.txt, and #include statements necessary for utilizing that feature.

Next steps 

Next you will create a simple ROS 2 package with a custom parameter that you will learn to set from a launch file. Again, you can choose to write it in either C++ or Python.