Program Listing for File realtime_publisher.hpp

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// Copyright (c) 2008, Willow Garage, Inc.
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/*
 * Publishing ROS messages is difficult, as the publish function is
 * not realtime safe.  This class provides the proper locking so that
 * you can call publish in realtime and a separate (non-realtime)
 * thread will ensure that the message gets published over ROS.
 *
 * Author: Stuart Glaser
 */
#ifndef REALTIME_TOOLS__REALTIME_PUBLISHER_HPP_
#define REALTIME_TOOLS__REALTIME_PUBLISHER_HPP_

#include <atomic>
#include <chrono>
#include <condition_variable>
#include <memory>
#include <mutex>
#include <string>
#include <thread>

#include "rclcpp/publisher.hpp"

namespace realtime_tools
{
template <class MessageT>
class RealtimePublisher
{
public:
  using PublisherType = rclcpp::Publisher<MessageT>;
  using PublisherSharedPtr = typename rclcpp::Publisher<MessageT>::SharedPtr;

  using PublishedType = typename rclcpp::TypeAdapter<MessageT>::custom_type;
  using ROSMessageType = typename rclcpp::TypeAdapter<MessageT>::ros_message_type;

  RCLCPP_SMART_PTR_DEFINITIONS(RealtimePublisher<MessageT>)


  MessageT msg_;

  explicit RealtimePublisher(PublisherSharedPtr publisher)
  : publisher_(publisher), is_running_(false), keep_running_(true), turn_(State::LOOP_NOT_STARTED)
  {
    thread_ = std::thread(&RealtimePublisher::publishingLoop, this);

    // Wait for the thread to be ready before proceeding
    // This is important to ensure that the thread is properly initialized and ready to handle
    // messages before any other operations are performed on the RealtimePublisher instance.
    while (!thread_.joinable() ||
           turn_.load(std::memory_order_acquire) == State::LOOP_NOT_STARTED) {
      std::this_thread::sleep_for(std::chrono::microseconds(100));
    }
  }

  [[deprecated(
    "Use constructor with rclcpp::Publisher<T>::SharedPtr instead - this class does not make sense "
    "without a real publisher")]]
  RealtimePublisher()
  : is_running_(false), keep_running_(false), turn_(State::LOOP_NOT_STARTED)
  {
  }

  ~RealtimePublisher()
  {
    stop();
    while (is_running()) {
      std::this_thread::sleep_for(std::chrono::microseconds(100));
    }
    if (thread_.joinable()) {
      thread_.join();
    }
  }

  void stop()
  {
    keep_running_ = false;
    updated_cond_.notify_one();  // So the publishing loop can exit
  }

  bool trylock()
  {
    if (turn_.load(std::memory_order_acquire) == State::REALTIME && msg_mutex_.try_lock()) {
      return true;
    } else {
      return false;
    }
  }

  bool tryPublish(const MessageT & msg)
  {
    if (!trylock()) {
      return false;
    }

    msg_ = msg;
    unlockAndPublish();
    return true;
  }

  void unlockAndPublish()
  {
    turn_.store(State::NON_REALTIME, std::memory_order_release);
    unlock();
  }

  void lock() { msg_mutex_.lock(); }

  void unlock()
  {
    msg_mutex_.unlock();
    updated_cond_.notify_one();
  }

private:
  // non-copyable
  RealtimePublisher(const RealtimePublisher &) = delete;
  RealtimePublisher & operator=(const RealtimePublisher &) = delete;

  bool is_running() const { return is_running_; }

  void publishingLoop()
  {
    is_running_ = true;

    while (keep_running_) {
      MessageT outgoing;

      {
        turn_.store(State::REALTIME, std::memory_order_release);
        // Locks msg_ and copies it to outgoing
        std::unique_lock<std::mutex> lock_(msg_mutex_);
        updated_cond_.wait(lock_, [&] { return turn_ == State::NON_REALTIME || !keep_running_; });
        outgoing = msg_;
      }

      // Sends the outgoing message
      if (keep_running_) {
        publisher_->publish(outgoing);
      }
    }
    is_running_ = false;
  }

  PublisherSharedPtr publisher_;
  std::atomic<bool> is_running_;
  std::atomic<bool> keep_running_;

  std::thread thread_;

  std::mutex msg_mutex_;  // Protects msg_
  std::condition_variable updated_cond_;

  enum class State : int { REALTIME, NON_REALTIME, LOOP_NOT_STARTED };
  std::atomic<State> turn_;  // Who's turn is it to use msg_?
};

template <class MessageT>
using RealtimePublisherSharedPtr = std::shared_ptr<RealtimePublisher<MessageT>>;

}  // namespace realtime_tools
#endif  // REALTIME_TOOLS__REALTIME_PUBLISHER_HPP_