Chapter 3: Building a ROS 2 Package

In the previous chapters, we learned about the fundamental communication mechanisms in ROS 2. Now, it's time to put that knowledge into practice by creating our own ROS 2 package. A package is the primary unit of software organization in ROS 2. It contains your nodes, along with other files like launch files, configuration files, and message definitions.

Creating a ROS 2 Package

To create a ROS 2 package, we use the ros2 pkg create command. This command generates a directory with the necessary files and folder structure for a ROS 2 package.

For example, to create a Python package named my_robot_pkg, you would run the following command:

ros2 pkg create --build-type ament_python my_robot_pkg

This will create a my_robot_pkg directory with the following structure:

my_robot_pkg/
├── package.xml
├── setup.py
├── setup.cfg
└── resource/
    └── my_robot_pkg
└── my_robot_pkg/
    └── __init__.py

package.xml

The package.xml file is the manifest file for your package. It contains important metadata about the package, such as its name, version, author, and dependencies.

Here is an example package.xml file:

<?xml version="1.0"?>
<package format="3">
  <name>my_robot_pkg</name>
  <version>0.0.0</version>
  <description>A simple ROS 2 package.</description>
  <maintainer email="user@example.com">Your Name</maintainer>
  <license>Apache License 2.0</license>

  <depend>rclpy</depend>
  <depend>std_msgs</depend>

  <test_depend>ament_copyright</test_depend>
  <test_depend>ament_flake8</test_depend>
  <test_depend>ament_pep257</test_depend>
  <test_depend>python3-pytest</test_depend>

  <export>
    <build_type>ament_python</build_type>
  </export>
</package>

setup.py

The setup.py file is the build script for your Python package. It tells ROS 2 how to build and install your package, including where to find your nodes.

Here is an example setup.py file:

from setuptools import setup

package_name = 'my_robot_pkg'

setup(
    name=package_name,
    version='0.0.0',
    packages=[package_name],
    data_files=[
        ('share/ament_index/resource_index/packages',
            ['resource/' + package_name]),
        ('share/' + package_name, ['package.xml']),
    ],
    install_requires=['setuptools'],
    zip_safe=True,
    maintainer='Your Name',
    maintainer_email='user@example.com',
    description='A simple ROS 2 package.',
    license='Apache License 2.0',
    tests_require=['pytest'],
    entry_points={
        'console_scripts': [
            'talker = my_robot_pkg.talker:main',
            'listener = my_robot_pkg.listener:main',
        ],
    },
)

Building a ROS 2 Package

To build your ROS 2 package, you use the colcon build command from the root of your workspace.

colcon build

This will build your package and install it into the install directory of your workspace. To use your new package, you need to source the setup.bash file in the install directory:

source install/setup.bash

Now you can run your nodes using the ros2 run command:

ros2 run my_robot_pkg talker

ros2 run my_robot_pkg listener

Congratulations! You have successfully created and built your own ROS 2 package. In the next module, we will learn how to create a digital twin of our robot and simulate it in a virtual environment.