User Code (Python)

This section assumes that the user has learned the basics of Python and ROS 2. For learning ROS 2, please refer to Tutorials | ROS 2 Documentation.

Among the ROS packages included in a Tobas project created with Setup Assistant (example: tobas_f450.TBS), the user Python package (example: tobas_f450_user_py) is a Python package that the user can edit freely. It includes the following three launch files.

• common.launch.py: Launched in both real hardware and simulation.
• real.launch.py: Launched only on real hardware.
• gazebo.launch.py: Launched only during simulation.

As an example, let's create a Python node that checks the GNSS status and outputs a message when 3D positioning is available. Please edit tobas_f450_user_py/tobas_f450_user_py/user_node.py as follows.

import rclpy
from rclpy.node import Node
from rclpy.qos import QoSProfile, ReliabilityPolicy, DurabilityPolicy

from tobas_msgs.msg import Message
from tobas_msgs.msg import Gnss


class GnssStateCheckerNode(Node):
    def __init__(self) -> None:
        super().__init__("gnss_state_checker")

        qos = QoSProfile(depth=1)
        qos.reliability = ReliabilityPolicy.BEST_EFFORT
        qos.durability = DurabilityPolicy.VOLATILE

        self._message_pub = self.create_publisher(Message, "message", qos)
        self._gnss_sub = self.create_subscription(Gnss, "gnss", self._gnss_callback, qos)

    def _gnss_callback(self, gnss: Gnss) -> None:
        message = Message()
        message.header.stamp = gnss.header.stamp
        message.name = self.get_name()

        if gnss.fix_type == Gnss.FIX_3D:
            message.level = Message.LEVEL_INFO
            message.message = "GNSS 3D Fix"
            self._message_pub.publish(message)


def main(args=None) -> None:
    rclpy.init(args=args)
    node = GnssStateCheckerNode()
    rclpy.spin(node)


if __name__ == "__main__":
    main()

Configure this node to start automatically. Uncomment the add_action section in tobas_f450_user_py/launch/common.launch.py.

# Do not delete or rename this file because it is executed in tobas_f450_config/common_interface.launch.py.

from launch import LaunchDescription
from launch_ros.actions import Node


def generate_launch_description():
    ld = LaunchDescription()

    # Please add the nodes that run both on real hardware and in simulation.

    ld.add_action(
        Node(
            package="tobas_f450_user_py",
            executable="user_node",
        )
    )

    return ld

When you start the simulation from the GCS, a message will be displayed in the console of Control System.

For details on the API, see ROS API.