multithreaded_executor.cpp

// Copyright 2020 Open Source Robotics Foundation, Inc.
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

#include <chrono>
#include <functional>
#include <memory>
#include <string>
#include <thread>

#include "rclcpp/rclcpp.hpp"
#include "std_msgs/msg/string.hpp"

using namespace std::chrono_literals;

/**
 * A small convenience function for converting a thread ID to a string
 **/
std::string string_thread_id()
{
  auto hashed = std::hash<std::thread::id>()(std::this_thread::get_id());
  return std::to_string(hashed);
}

/* For this example, we will be creating a publishing node like the one in minimal_publisher.
 * We will have a single subscriber node running 2 threads. Each thread loops at different speeds, and
 * just repeats what it sees from the publisher to the screen.
 */

class PublisherNode : public rclcpp::Node
{
public:
  PublisherNode()
  : Node("PublisherNode"), count_(0)
  {
    publisher_ = this->create_publisher<std_msgs::msg::String>("topic", 10);
    auto timer_callback =
      [this]() -> void {
        auto message = std_msgs::msg::String();
        message.data = "Hello World! " + std::to_string(this->count_++);

        // Extract current thread
        auto curr_thread = string_thread_id();

        // Prep display message
        RCLCPP_INFO(
          this->get_logger(), "\n<<THREAD %s>> Publishing '%s'",
          curr_thread.c_str(), message.data.c_str());
        this->publisher_->publish(message);
      };
    timer_ = this->create_wall_timer(500ms, timer_callback);
  }

private:
  rclcpp::TimerBase::SharedPtr timer_;
  rclcpp::Publisher<std_msgs::msg::String>::SharedPtr publisher_;
  size_t count_;
};

class DualThreadedNode : public rclcpp::Node
{
public:
  DualThreadedNode()
  : Node("DualThreadedNode")
  {
    /* These define the callback groups
     * They don't really do much on their own, but they have to exist in order to
     * assign callbacks to them. They're also what the executor looks for when trying to run multiple threads
     */
    callback_group_subscriber1_ = this->create_callback_group(
      rclcpp::CallbackGroupType::MutuallyExclusive);
    callback_group_subscriber2_ = this->create_callback_group(
      rclcpp::CallbackGroupType::MutuallyExclusive);

    // Each of these callback groups is basically a thread
    // Everything assigned to one of them gets bundled into the same thread
    auto sub1_opt = rclcpp::SubscriptionOptions();
    sub1_opt.callback_group = callback_group_subscriber1_;
    auto sub2_opt = rclcpp::SubscriptionOptions();
    sub2_opt.callback_group = callback_group_subscriber2_;

    subscription1_ = this->create_subscription<std_msgs::msg::String>(
      "topic",
      rclcpp::QoS(10),
      // std::bind is sort of C++'s way of passing a function
      // If you're used to function-passing, skip these comments
      std::bind(
        &DualThreadedNode::subscriber1_cb,  // First parameter is a reference to the function
        this,                               // What the function should be bound to
        std::placeholders::_1),             // At this point we're not positive of all the
                                            // parameters being passed
                                            // So we just put a generic placeholder
                                            // into the binder
                                            // (since we know we need ONE parameter)
      sub1_opt);                  // This is where we set the callback group.
                                  // This subscription will run with callback group subscriber1

    subscription2_ = this->create_subscription<std_msgs::msg::String>(
      "topic",
      rclcpp::QoS(10),
      std::bind(
        &DualThreadedNode::subscriber2_cb,
        this,
        std::placeholders::_1),
      sub2_opt);
  }

private:
  /**
   * Simple function for generating a timestamp
   * Used for somewhat ineffectually demonstrating that the multithreading doesn't cripple performace
   */
  std::string timing_string()
  {
    rclcpp::Time time = this->now();
    return std::to_string(time.nanoseconds());
  }

  /**
   * Every time the Publisher publishes something, all subscribers to the topic get poked
   * This function gets called when Subscriber1 is poked (due to the std::bind we used when defining it)
   */
  void subscriber1_cb(const std_msgs::msg::String::ConstSharedPtr msg)
  {
    auto message_received_at = timing_string();

    // Extract current thread
    RCLCPP_INFO(
      this->get_logger(), "THREAD %s => Heard '%s' at %s",
      string_thread_id().c_str(), msg->data.c_str(), message_received_at.c_str());
  }

  /**
   * This function gets called when Subscriber2 is poked
   * Since it's running on a separate thread than Subscriber 1, it will run at (more-or-less) the same time!
   */
  void subscriber2_cb(const std_msgs::msg::String::ConstSharedPtr msg)
  {
    auto message_received_at = timing_string();

    // Prep display message
    RCLCPP_INFO(
      this->get_logger(), "THREAD %s => Heard '%s' at %s",
      string_thread_id().c_str(), msg->data.c_str(), message_received_at.c_str());
  }

  rclcpp::CallbackGroup::SharedPtr callback_group_subscriber1_;
  rclcpp::CallbackGroup::SharedPtr callback_group_subscriber2_;
  rclcpp::Subscription<std_msgs::msg::String>::SharedPtr subscription1_;
  rclcpp::Subscription<std_msgs::msg::String>::SharedPtr subscription2_;
};

int main(int argc, char * argv[])
{
  rclcpp::init(argc, argv);

  // You MUST use the MultiThreadedExecutor to use, well, multiple threads
  rclcpp::executors::MultiThreadedExecutor executor;
  auto pubnode = std::make_shared<PublisherNode>();
  auto subnode = std::make_shared<DualThreadedNode>();  // This contains BOTH subscriber callbacks.
                                                        // They will still run on different threads
                                                        // One Node. Two callbacks. Two Threads
  executor.add_node(pubnode);
  executor.add_node(subnode);
  executor.spin();
  rclcpp::shutdown();
  return 0;
}