Key Components of ROS 2 Launch Files

Launch files in ROS 2 provide a powerful way to start multiple nodes and configure various aspects of your robotic system. Here are the key components and concepts you need to understand when working with ROS 2 launch files.

• LaunchDescription
• Node
• Remappings
• Event Handlers

1. LaunchDescription

It is a container that holds all the launch actions you want to execute. Each action can be a node to be launched, a parameter setting, or other configurations.

Example

from launch import LaunchDescription
from launch_ros.actions import Node

def generate_launch_description():
    return LaunchDescription([
        Node(
            package='my_package',
            executable='my_node',
            output='screen'
        )
    ])

2. Node

The Node action specifies a ROS 2 node to be launched. It includes several parameters such as the package containing the node, the executable name, node name, output configuration, and more.

Node(
    package='my_package',
    executable='my_node',
    output='screen'
)

Key Arguments:

• package: The name of the package containing the node.
• executable: The name of the executable to run.
• name: The name of the node (optional).
• namespace: The namespace to which the node belongs (optional).
• output: Defines where to send the output of the node ('screen' or 'log').

3. Remappings

Remappings allow you to change topic, service, or action names at launch time.

Example

Node(
    package='my_package',
    executable='my_node',
    name='my_node_name',
    remappings=[
        ('/input/pose', '/turtlesim1/turtle1/pose'),
        ('/output/cmd_vel', '/turtlesim2/turtle1/cmd_vel'),
    ]
)

4. Event Handlers

Event handlers allow you to define actions based on certain events during the launch process.

Example

from launch.actions import RegisterEventHandler
from launch.event_handlers import OnProcessExit

RegisterEventHandler(
    OnProcessExit(
        target_action=my_node,
        on_exit=[EmitEvent(event=Shutdown())],
    )
)

setup.py

add the below code in setup.py

import os
from glob import glob


(os.path.join('share', package_name, 'launch'),  glob('launch/*.py'))

This is typically used in ROS2 packages for setting up the paths to launch files during the installation process, particularly in the setup.py file.

Components

1. os.path.join('share', package_name, 'launch'):

• This is constructing a path that combines various parts using the os.path.join method.

• 'share': Refers to the directory within the package's installation location where non-source files are stored (like launch files, configuration files, etc.).

• package_name: This is a variable that holds the name of your package. When installing the package, files are placed in the folder named after the package.

• 'launch': This is the directory inside the package where launch files are stored.

2. glob('launch/*.py'):

This is using Python’s glob module to find all Python files (*.py) in the launch directory.

In ROS2, this code is commonly used in the setup.py file to specify where launch files should be installed within the package directory structure. It ensures that during the installation process, the launch files are placed in the correct location, typically under share/<package_name>/launch, so that ROS2 can easily locate and access them when needed for running launch configurations.