Learn ROS-ROS2 with Real Code Examples

import rclpy
from rclpy.node import Node
from std_msgs.msg import String

class MinimalPublisher(Node):
    def __init__(self):
        super().__init__('minimal_publisher')
        self.publisher_ = self.create_publisher(String, 'topic', 10)
        self.timer = self.create_timer(1.0, self.timer_callback)

    def timer_callback(self):
        msg = String()
        msg.data = 'Hello ROS2'
        self.publisher_.publish(msg)
        self.get_logger().info('Publishing: %s' % msg.data)


def main(args=None):
    rclpy.init(args=args)
    node = MinimalPublisher()
    rclpy.spin(node)
    node.destroy_node()
    rclpy.shutdown()

if __name__ == '__main__':
    main()

A simple ROS2 publisher node that sends messages to a topic.

#include <rclcpp/rclcpp.hpp>
#include <std_msgs/msg/string.hpp>

class MinimalSubscriber : public rclcpp::Node {
public:
    MinimalSubscriber() : Node("minimal_subscriber") {
        subscription_ = this->create_subscription<std_msgs::msg::String>(
        "topic", 10,
        std::bind(&MinimalSubscriber::topic_callback, this, std::placeholders::_1));
    }

private:
    void topic_callback(const std_msgs::msg::String::SharedPtr msg) const {
        RCLCPP_INFO(this->get_logger(), "I heard: '%s'", msg->data.c_str());
    }

    rclcpp::Subscription<std_msgs::msg::String>::SharedPtr subscription_;
};

int main(int argc, char * argv[]) {
    rclcpp::init(argc, argv);
    rclcpp::spin(std::make_shared<MinimalSubscriber>());
    rclcpp::shutdown();
    return 0;
}

A simple ROS2 subscriber node that listens to messages from a topic.

import rclpy
from rclpy.node import Node
from example_interfaces.srv import AddTwoInts

class MinimalService(Node):
    def __init__(self):
        super().__init__('minimal_service')
        self.srv = self.create_service(AddTwoInts, 'add_two_ints', self.add_two_ints_callback)

    def add_two_ints_callback(self, request, response):
        response.sum = request.a + request.b
        self.get_logger().info('Incoming request: a=%d, b=%d' % (request.a, request.b))
        return response


def main(args=None):
    rclpy.init(args=args)
    node = MinimalService()
    rclpy.spin(node)
    node.destroy_node()
    rclpy.shutdown()

if __name__ == '__main__':
    main()

A ROS2 service that responds to requests with a greeting message.