ROS Package

This tutorial is based on the official documentation: docs.ros.org, with some tuning considering our preferences... Typically we prefer cmake method, whatever the targeted language (C++ or python).

Your first package in ROS2

You can build your first package using the ros tool, with the build-type ament_cmake, then compile and install the package. Typically a tuto_kickoff package:

ros2 pkg create --build-type ament_cmake tuto_kickoff
colcon build --packages-select tuto_kickoff

Package creation generate files identifying tuto_kickoff directory as a ROS package source. The file package.xml setup the ROS configuration, the name of the packages and its dependencies in ROS ecosystem. The file CMakeLists.txt is required to use CMake tool, a very famous cross-platform tool for automatizing building processes.

The colcon build command produces the build and the install directories with respectively, temporary files generated by the build process and the resulting built executable and resources (yes, none at this point...). To inform your terminal that some ROS resources are available here, you have to source the ROS setup file.

ls
source ./install/setup.bash

The package.xml file

As we said, package.xml is the entrance point of the package for ROS tools. It is a text file in eXtensible Markup Language format. Typically, it regroups all the dependencies, for instance rclcpp and rclpy the ROS client libraries for Cpp and Python or std_msgs. std_msgs and geometry_msgs are a ROS package defining the format of the simple and common messages allowing the communication between ROS processes (standard and geometry messages for our use).

In the package.xml file :

<package format="3">
  ...
  <depend>rclcpp</depend>
  <depend>rclpy</depend>
  <depend>std_msgs</depend>
  <depend>geometry_msgs</depend>
  ...
</package>

Those lines inform ROS to make the targeted resources available at build and execution time. For information about package.xml you can refer to the official specifications on www.ros.org.

The CMakeList.txt

The CMakeList.txt provide instructions on how to build libraries and programs. Generally, the file starts with a project name and the importation of dependencies. The dependencies must be installed, reachable on the machine and with the appropriate version number. Then its define how to build new resources (typically, libraries and programs/executable). And finally the element to install and where.

For instance, all launch file included in the launchs directory can be installed with the cmake command:

# Install launch files.
install(DIRECTORY
  launch
  DESTINATION share/${PROJECT_NAME}/
)

Most of the primitive (find_package, add_executable, install) and macros (PROJECT_NAME, REQUIRED, ... ) are CMake primitives and macros. The ament tools provides some of primitive dedicated to ROS build automation.

Python scripts with Cmake

Scripts are short executable code file, defining a ROS Node (in our context).

The simplest way to include Python-based ROS node depending on a specific Python Package is to use install instruction in CMakeLists.txt. Considering that your work is in a scripts directory and knowing that the ROS destination to make the resources available for ros2 run is lib (...), the install instructions would look-like:

# Python scripts
install( PROGRAMS scripts/myNode DESTINATION lib/${PROJECT_NAME})

Naturally, it supposes that you have myNode script file aside. A minimal myNode can look-like :

#!/usr/bin/python3
import rclpy                 # core ROS2 client python library
from rclpy.node import Node  # To manipulate ROS Nodes

def main():
    rclpy.init()     # Initialize ROS2 client
    myNode= Node('blanc_node') # Create a Node, with a name         

    # Start the ros infinite loop with myNode.
    rclpy.spin( myNode )

    # At the end, destroy the node explicitly.
    myNode.destroy_node()

    # and shut the light down.
    rclpy.shutdown()

    print("tuto_move :: STOP.")

# If the file is executed as a script (ie. not imported).
if __name__ == '__main__':
    # call main() function
    main()

ATTENTION, The #!/usr/bin/python3 in the first line is mandatory. It indicate the interpreter processing the script. In fact, you have a executable program named python3 in the location /usr/bin.

Run your node:

Build your packages, from your workspace directory, and update your shell environment with source.

colcon build
source install/setup.bash

Normally, at this point a new package tuto_kickoff with myNode is reachable by ros2.

ros2 run tuto_kickoff myNode

The executed myNode script is the one in install/tuto_kickoff/lib. Any modification in your script files would require a new colcon build for propagation in ros ecosystem.

Exercice:

1. Add the node developed into the tutorial "move the robot" into your package tuto_kickoff as a move_tbots node. At the end, you will be capable of starting it with the command: ros2 run tuto_kickoff move_tbot.

2. Add a launch file starting a gazebo simulation with the move_tbot node. At the end, you will be capable of launching gazebo, a robot, your control node with the command: ros2 launch tuto_kickoff sim_move_launch.yaml.