Skip to content

new_interface_tutorial

Jump to bottom
marhcouto edited this page Jan 11, 2024 · 1 revision

Custom ROS 2 Interfaces Tutorial

This tutorial is meant to aid the developer create new custom interfaces in the ROS2 program for specific topics.

Links

Keywords

  • ROS - Robot Operating System
  • ROS2 - New version of ROS that will be used
  • Interface / Msg - The content which is sent throught ROS2 topics or any other type of communication channel

1. Create package

It is essential you create an independent package for the purpose of the tutorial. (Note: the project may already include one, you are not supposed to create multiple packages for new interfaces)

ros2 pkg create --build-type ament_cmake tutorial_interfaces

tutorial_interfaces is the name of the newly introduced package. Note that it is a CMake package since there is no way to generate a .msg or .srv file in a pure Python package.

It is good practice to keep .msg and .srv files in their own directories within a package. So we must create the respective folders:

mkdir msg

mkdir srv

2. Create custom definitions

In 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 num

This will be our custom message, and it transfers a single 64-bit integer called num.

There is a set of other primitive types with the respective Python & C++ conversions in the Links section.

2.1 Custom dependent definitions

If by any chance you wish to implement an interface which depends on other created interfaces, you may do it like so:

geometry_msgs/Point point

Here we are transferring a Point derived from the geometry_msgs package.

If we take a look at its definition, we conclude it is the same as writing the following piece of code:

# This contains the position of a point in free space
float64 x
float64 y
float64 z

3. Package.xml

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>

3.1 Custom dependent definitions

As you may know from other ROS2 experiments, if we use any external interface, we need to create a specific dependency for it. In the case we presented before, we would need to include the following line:

<depend>geometry_msgs</depend>

4. CMakeLists.txt

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)

# Add any new custom interface here
rosidl_generate_interfaces(${PROJECT_NAME}
  "msg/Num.msg"
)

ament_export_dependencies(rosidl_default_runtime)

4.1 Custom dependent definitions

In addition to the above lines, in case we are using dependent definitions, we must add the correspondent find_package statement and add it to the rosidl_generate_interfaces like so:

find_package(geometry_msgs REQUIRED)

# Add any new custom interface here
rosidl_generate_interfaces(${PROJECT_NAME}
  "msg/Num.msg"
  DEPENDENCIES geometry_msgs
)

5. Build the package

Now that all the parts of your custom interfaces package are in place, you can build the package:

colcon build --packages-select tutorial_interfaces

6. Confirm msg and srv creation

In a new terminal, run the following command from within your workspace to source it:

. install/setup.bash

Now you can confirm that your interface creation worked by using the command:

ros2 interface show tutorial_interfaces/msg/Num

With the desired display:

int64 num

7. Test the new interfaces

The new interfaces can be used now in a real Python/C++ package. You may do so by importing the respective .msg file:

Python:

from tutorial_interfaces.msg import Num

C++:

#include "tutorial_interfaces/msg/num.hpp"

And not forget to complete the package.xml / CMakeLists.txt files.

8. SRV files

The .srv files may be used to create custom service messages. However, the focus of this tutorial is on topic-related messaging, so we will not cover any further steps.

You may follow the first link which also guides this type of communication.

Clone this wiki locally