Launch System

The launch system in Python ROS Engine provides a way to manage multiple nodes and monitor the overall system status. It allows you to start, stop, and monitor collections of nodes as a single unit.

LaunchSystem Class

The LaunchSystem class is the main interface for managing multiple nodes.

Methods

__init__()

Initialize the launch system.

add_node(node_name, node_class, *args, **kwargs)

Add a node to the launch system.

  • node_name: Name to register the node under
  • node_class: The node class to instantiate
  • *args: Positional arguments for node initialization
  • **kwargs: Keyword arguments for node initialization

add_node_from_file(node_name, file_path, class_name, *args, **kwargs)

Add a node to the launch system from a Python file.

  • node_name: Name to register the node under
  • file_path: Path to the Python file containing the node class
  • class_name: Name of the node class in the file
  • *args: Positional arguments for node initialization
  • **kwargs: Keyword arguments for node initialization

remove_node(node_name)

Remove a node from the launch system.

  • node_name: Name of the node to remove

start()

Start all nodes in the launch system.

shutdown()

Shutdown all nodes in the launch system.

get_system_status()

Get the status of the entire system as a dictionary.

Print the status of the entire system in a human-readable format.

LaunchDescription Class

The LaunchDescription class provides a declarative way to describe a launch configuration.

Methods

__init__(launch_system=None)

Initialize a launch description.

  • launch_system: Optional LaunchSystem to use (creates a new one if not provided)

add_node(node_class, *args, **kwargs)

Add a node action to the launch description.

  • node_class: The node class to instantiate
  • *args: Positional arguments for node initialization
  • **kwargs: Keyword arguments for node initialization

add_node_from_file(file_path, class_name, *args, **kwargs)

Add a node action from a Python file to the launch description.

  • file_path: Path to the Python file containing the node class
  • class_name: Name of the node class in the file
  • *args: Positional arguments for node initialization
  • **kwargs: Keyword arguments for node initialization

execute()

Execute all actions in the launch description and return the launch system.

Usage Examples

Programmatic Usage

from pyros2 import LaunchSystem
from pyros2.message import String

class MyPublisherNode:
    def __init__(self):
        from pyros2 import Node
        self.node = Node("my_publisher")
        self.publisher = self.node.create_publisher(String, "/my_topic")

    def publish_message(self):
        msg = String()
        msg.data = "Hello World!"
        self.publisher.publish(msg)

# Create launch system
launch_system = LaunchSystem()

# Add nodes
launch_system.add_node("publisher", MyPublisherNode)

# Print system status
launch_system.print_system_status()

# Start the system
launch_system.start()

Declarative Usage

from pyros2 import LaunchDescription
from pyros2.message import String

class MyPublisherNode:
    def __init__(self):
        from pyros2 import Node
        self.node = Node("my_publisher")
        self.publisher = self.node.create_publisher(String, "/my_topic")

class MySubscriberNode:
    def __init__(self):
        from pyros2 import Node
        self.node = Node("my_subscriber")
        self.subscriber = self.node.create_subscription(
            String, "/my_topic", self.callback
        )

    def callback(self, msg):
        print(f"Received: {msg.data}")

# Create launch description
launch_description = LaunchDescription()

# Add nodes
launch_description.add_node(MyPublisherNode)
launch_description.add_node(MySubscriberNode)

# Execute the launch description
launch_system = launch_description.execute()

# Print system status
launch_system.print_system_status()

# Start the system
launch_system.start()

Launch File Usage

You can also create launch files that can be executed from the command line:

# launch_my_system.py
from pyros2 import LaunchDescription
from my_nodes import MyPublisherNode, MySubscriberNode

def generate_launch_description():
    launch_description = LaunchDescription()

    # Add nodes
    launch_description.add_node(MyPublisherNode)
    launch_description.add_node(MySubscriberNode)

    return launch_description

Then run it from the command line: 

python -m pyros2.launch_cli launch_my_system.py

To just check the system status without starting the nodes: 

python -m pyros2.launch_cli launch_my_system.py --status