ROS2-5: Interfaces

ROS2 interfaces are the messages and services you use with your topics and services.

ROS2 - Msg and Srv

• use msg primitive types to create a message definition
• create a message definition using other message definitions

Topic:

• Name: /number_count
• Msg definition: example_interfaces/msg/int64

Service:

• Name: /reset_number_count
• Srv definition: example_interfaces/srv/SetBool

Create and Build Your First Custom Msg

cd /ros2_ws/src
ros2 pkg create my_robot_interfaces 
cd my_robot_interfaces/
rm -rf include/
rm -rf src/
mkdir msg

In package.xml

  <!-- functionalities to build interfaces -->
  <build_depend>rosidl_default_generators</build_depend>
  <exec_depend>rosidl_default_runtime</exec_depend>
  <member_of_group>rosidl_interface_packages</member_of_group>

In CMakeLists.txt

# find dependencies
find_package(ament_cmake REQUIRED)
find_package(rosidl_default_generators REQUIRED)

rosidl_generate_interfaces(${PROJECT_NAME}
  "msg/HardwareStatus.msg"
)

ament_export_dependencies(rosidl_default_runtime)
ament_package()

Then start build a first msg

cd msg/

# use uppercase for new word, no msg inside the name
touch HardwareStatus.msg

In HardwareStatus.msg

int64 temperature
bool are_motors_ready
string debug_message

Then build the package

cd /ros2_ws
colcon build --packages-select my_robot_interfaces

test

source ~/.bashrc
ros2 interface show my_robot_interfaces/msg/HardwareStatus

Use Your Custom Msg in a Python Node

add installed custom msg to the dependencies

in package.xml

  <depend>rclpy</depend>
  <depend>example_interfaces</depend>
  <depend>my_robot_interfaces</depend>

add python autocomplete path in settings.json in VScode and restart it

"python.autoComplete.extraPaths": [
    "~/ros2_ws/install/my_robot_interfaces/local/lib/python3.10/dist-packages/my_robot_interfaces/"
]

create a publisher that use this custom msg

Create new python file in my_py_pkg:

touch hw_status_publisher.py
chmod +x hw_status_publisher.py

#!/usr/bin/env python3
import rclpy
from rclpy.node import Node

from my_robot_interfaces.msg import HardwareStatus


class HardwareStatusPublisherNode(Node):
    def __init__(self):
        super().__init__("hardware_status_publisher")
        self.hw_status_publisher_ = self.create_publisher(
            msg_type=HardwareStatus, topic="hardware_status", qos_profile=10)
        self.get_logger().info("Hardware status publisher has been started!")

    def publish_hw_status(self):
        msg = HardwareStatus()
        msg.temperature = 45
        msg.are_motors_ready = True
        msg.debug_message = "Nothing special here"
        self.hw_status_publisher_.publish(msg)


def main(args=None):
    rclpy.init(args=args)
    node = HardwareStatusPublisherNode(Node)
    rclpy.spin(node)
    rclpy.shutdown()


if __name__ == "__main__":
    main()

In setup.py

entry_points={
    'console_scripts': [
        "py_node = my_py_pkg.my_first_node:main",  # executable
        "robot_news_station = my_py_pkg.robot_news_station:main",
        "smartphone = my_py_pkg.smartphone:main",
        "add_two_ints_server = my_py_pkg.add_two_ints_server:main",
        "add_two_ints_client = my_py_pkg.add_two_ints_client:main",
        "hw_status_publisher = my_py_pkg.hw_status_publisher:main"
    ],
},

build package, run the node, test the result

# terminal 1
cd /ros2_ws
colcon build --packages-select my_py_pkg --symlink-install
ros2 run my_py_pkg hw_status_publisher

# terminal 2
source ~/.bashrc
ros2 node list
ros2 topic list
ros2 topic echo /hardware_status

useful commands for debug:

ros interface package list
ros2 interface package sensor_msgs

# find node name
ros2 node list 

# find interface name
ros2 node info /hardware_status_publisher

# find msg
ros2 interface show my_robot_interfaces/msg/HardwareStatus


ros2 topic list
ros2 topic info /hardware_status
ros2 interface show my_robot_interfaces/msg/HardwareStatus


ros2 service list
ros2 service type /add_two_ints
ros2 interface show example_interfaces/srv/AddTwoInts