Getting Started with ROS on Embedded Systems - User Guide - C++ - Publishers and subscribers

Getting Started with ROS on Embedded Systems
ROS on Embedded Systems Basics
Getting Started
Installation Configuration Setting up environment Execution
C++ User Guide
Package set up Initialization Topics Messages Names Publishers and subscribers Launch files Parameters
Examples
Basic pipelines V4L2 capture node Turn detector node Proximity detector node
Performance
Xavier TX1 TX2 Nano
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Introduction

This wiki is a small introduction to the Publishers and Subscriber model that ROS is based upon. ROS2 uses a topic's name and the message type to bind n publishers and n subscribers.

Publishers

In order to publish messages, you will need a node handle, a message type and create a rclcpp::Publisher<TYPE> using the rclcpp::Node::create_publisher function. Like the following example:

rclcpp::Node pub_node = new rclcpp::Node(NONE_NAME);
rclcpp::Publisher<std_msgs::msg::String>::SharedPtr publisher_;
publisher_ = pub_node->create_publisher<TYPE>(TOPIC_NAME, N);

• N is the max message queue.
• TOPIC_NAME is the name of the topic to publish to.
• TYPE is the type of the messages on the topic, like std_msgs::msg::String
• NONE_NAME the name of the node

Subscribers

Similar to publishers, a ROS subscriber needs a node handle and with the subscribe function it will create a new rclcpp::Subscription, we also need a callback method, like the following example:

class MinimalSubscriber : public rclcpp::Node
{
  public:
    MinimalSubscriber(std::string channel_name)
    : Node("minimal_subscriber"), channel_name_(channel_name)
    {

      subscription_ = this->create_subscription<std_msgs::msg::String>(
      channel_name_, 10, std::bind(&MinimalSubscriber::topic_callback, this, _1));
    }

  private:
    void topic_callback(const std_msgs::msg::String msg) const
    {
      RCLCPP_INFO(this->get_logger(), "Received message");
    }
    rclcpp::Subscription<std_msgs::msg::String>::SharedPtr subscription_;
    std::string channel_name_;
};

The assigned callback will be executed each time there is a new message on the topic.