mqtt5/example/receiver.cpp

//
// Copyright (c) 2023-2025 Ivica Siladic, Bruno Iljazovic, Korina Simicevic
//
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE or copy at http://www.boost.org/LICENSE_1_0.txt)
//

#include <boost/asio/use_awaitable.hpp>
#ifdef BOOST_ASIO_HAS_CO_AWAIT

//[receiver

#include <boost/mqtt5/logger.hpp>
#include <boost/mqtt5/mqtt_client.hpp>
#include <boost/mqtt5/reason_codes.hpp>
#include <boost/mqtt5/types.hpp>

#include <boost/asio/as_tuple.hpp>
#include <boost/asio/co_spawn.hpp>
#include <boost/asio/deferred.hpp>
#include <boost/asio/detached.hpp>
#include <boost/asio/io_context.hpp>
#include <boost/asio/ip/tcp.hpp>
#include <boost/asio/signal_set.hpp>

#include <iostream>
#include <string>

struct config {
    std::string brokers = "broker.hivemq.com";
    uint16_t port = 1883;
    std::string client_id = "boost_mqtt5_tester";
};

// Modified completion token that will prevent co_await from throwing exceptions.
constexpr auto use_nothrow_awaitable = boost::asio::as_tuple(boost::asio::deferred);

// client_type with logging enabled
using client_type = boost::mqtt5::mqtt_client<
    boost::asio::ip::tcp::socket, std::monostate /* TlsContext */, boost::mqtt5::logger
>;

// client_type without logging
//using client_type = boost::mqtt5::mqtt_client<boost::asio::ip::tcp::socket>;

boost::asio::awaitable<bool> subscribe(client_type& client) {
    // Configure the request to subscribe to a Topic.
    boost::mqtt5::subscribe_topic sub_topic = boost::mqtt5::subscribe_topic{
        "test" /* topic */,
        boost::mqtt5::subscribe_options {
            boost::mqtt5::qos_e::exactly_once, // All messages will arrive at QoS 2.
            boost::mqtt5::no_local_e::no, // Forward message from Clients with same ID.
            boost::mqtt5::retain_as_published_e::retain, // Keep the original RETAIN flag.
            boost::mqtt5::retain_handling_e::send // Send retained messages when the subscription is established.
        }
    };

    // Subscribe to a single Topic.
    auto&& [ec, sub_codes, sub_props] = co_await client.async_subscribe(
        sub_topic, boost::mqtt5::subscribe_props {}, use_nothrow_awaitable
    );
    // Note: you can subscribe to multiple Topics in one mqtt_client::async_subscribe call.

    // An error can occur as a result of:
        //  a) wrong subscribe parameters
        //  b) mqtt_client::cancel is called while the Client is in the process of subscribing
    if (ec)
        std::cout << "Subscribe error occurred: " << ec.message() << std::endl;
    else
        std::cout << "Result of subscribe request: " << sub_codes[0].message() << std::endl;

    co_return !ec && !sub_codes[0]; // True if the subscription was successfully established.
}

boost::asio::awaitable<void> subscribe_and_receive(
    const config& cfg, client_type& client
) {
    // Configure the Client.
    // It is mandatory to call brokers() and async_run() to configure the Brokers to connect to and start the Client.
    client.brokers(cfg.brokers, cfg.port) // Broker that we want to connect to.
        .credentials(cfg.client_id) // Set the Client Identifier. (optional)
        .async_run(boost::asio::detached); // Start the client.

    // Before attempting to receive an Application Message from the Topic we just subscribed to,
    // it is advisable to verify that the subscription succeeded.
    // It is not recommended to call mqtt_client::async_receive if you do not have any
    // subscription established as the corresponding handler will never be invoked.
    if (!(co_await subscribe(client)))
        co_return;

    for (;;) {
        // Receive an Appplication Message from the subscribed Topic(s).
        auto&& [ec, topic, payload, publish_props] = co_await client.async_receive(use_nothrow_awaitable);

        if (ec == boost::mqtt5::client::error::session_expired) {
            // The Client has reconnected, and the prior session has expired.
            // As a result, any previous subscriptions have been lost and must be reinstated.
            if (co_await subscribe(client))
                continue;
            else
                break;
        } else if (ec)
            break;

        std::cout << "Received message from the Broker" << std::endl;
        std::cout << "\t topic: " << topic << std::endl;
        std::cout << "\t payload: " << payload << std::endl;
    }

    co_return;
}

int main(int argc, char** argv) {
    config cfg;

    if (argc == 4) {
        cfg.brokers = argv[1];
        cfg.port = uint16_t(std::stoi(argv[2]));
        cfg.client_id = argv[3];
    }

    // Initialise execution context.
    boost::asio::io_context ioc;

    // Initialise the Client to connect to the Broker over TCP.
    client_type client(ioc, {} /* tls_context */, boost::mqtt5::logger(boost::mqtt5::log_level::info));

    // Set up signals to stop the program on demand.
    boost::asio::signal_set signals(ioc, SIGINT, SIGTERM);
    signals.async_wait([&client](boost::mqtt5::error_code /* ec */, int /* signal */) {
        // After we are done with publishing all the messages, cancel the timer and the Client.
        // Alternatively, use mqtt_client::async_disconnect.
        client.cancel();
    });

    // Spawn the coroutine.
    co_spawn(
        ioc,
        subscribe_and_receive(cfg, client),
        [](std::exception_ptr e) {
            if (e)
                std::rethrow_exception(e);
        }
    );

    // Start the execution.
    ioc.run();
}

//]

#else

#include <iostream>

int main() {
    std::cout << "This example requires C++20 standard to compile and run" << std::endl;
}

#endif