paho.mqtt.cpp/examples/sync_consume_v5.cpp

// sync_consume_v5.cpp
//
// This is a Paho MQTT C++ client, sample application.
//
// This application is an MQTT consumer/subscriber using the C++ synchronous
// client interface, which uses the queuing API to receive messages.
//
// The sample demonstrates:
//  - Connecting to an MQTT server/broker
//  - Using a persistent (non-clean) session
//  - Subscribing to multiple topics
//  - Receiving messages through the queueing consumer API
//  - Receiving and acting upon commands via MQTT topics
//  - Auto reconnect
//  - Updating auto-reconnect data
//

/*******************************************************************************
 * Copyright (c) 2020-2024 Frank Pagliughi <fpagliughi@mindspring.com>
 *
 * All rights reserved. This program and the accompanying materials
 * are made available under the terms of the Eclipse Public License v2.0
 * and Eclipse Distribution License v1.0 which accompany this distribution.
 *
 * The Eclipse Public License is available at
 *    http://www.eclipse.org/legal/epl-v20.html
 * and the Eclipse Distribution License is available at
 *   http://www.eclipse.org/org/documents/edl-v10.php.
 *
 * Contributors:
 *    Frank Pagliughi - initial implementation and documentation
 *******************************************************************************/

#include <cctype>
#include <chrono>
#include <cstdlib>
#include <cstring>
#include <functional>
#include <iostream>
#include <string>
#include <thread>
#include <vector>

#include "mqtt/client.h"

using namespace std;
using namespace std::chrono;

const string SERVER_ADDRESS{"mqtt://localhost:1883"};
const string CLIENT_ID{"paho_cpp_sync_consume5"};

constexpr int QOS_0 = 0;
constexpr int QOS_1 = 1;

// Infinite time for session expiration
const uint32_t INFINITE = std::numeric_limits<uint32_t>::max();

/////////////////////////////////////////////////////////////////////////////

// Message table function signature
using handler_t = std::function<bool(const mqtt::message&)>;

// Handler for data messages (i.e. topic "data/#")
bool data_handler(const mqtt::message& msg)
{
    cout << msg.get_topic() << ": " << msg.to_string() << endl;
    return true;
}

// Handler for command messages (i.e. topic "command")
// Return false to exit the application
bool command_handler(const mqtt::message& msg)
{
    if (msg.to_string() == "exit") {
        cout << "Exit command received" << endl;
        return false;
    }
    return true;
}

/////////////////////////////////////////////////////////////////////////////

int main(int argc, char* argv[])
{
    mqtt::client cli(SERVER_ADDRESS, CLIENT_ID, mqtt::create_options(MQTTVERSION_5));

    auto connOpts = mqtt::connect_options_builder::v5()
                        .automatic_reconnect(seconds(2), seconds(30))
                        .clean_start(false)
                        .properties({{mqtt::property::SESSION_EXPIRY_INTERVAL, INFINITE}})
                        .finalize();

    // Dispatch table to handle incoming messages based on Subscription ID's.
    std::vector<handler_t> handler{data_handler, command_handler};

    try {
        cout << "Connecting to the MQTT server..." << flush;
        mqtt::connect_response rsp = cli.connect(connOpts);
        cout << "OK\n" << endl;

        if (!rsp.is_session_present()) {
            std::cout << "Subscribing to topics..." << std::flush;

            mqtt::subscribe_options subOpts;
            mqtt::properties props1{
                {mqtt::property::SUBSCRIPTION_IDENTIFIER, 1},
            };
            cli.subscribe("data/#", QOS_0, subOpts, props1);

            mqtt::properties props2{
                {mqtt::property::SUBSCRIPTION_IDENTIFIER, 2},
            };
            cli.subscribe("command", QOS_1, subOpts, props2);

            std::cout << "OK" << std::endl;
        }
        else {
            cout << "Session already present. Skipping subscribe." << std::endl;
        }

        // Consume messages

        while (true) {
            auto msg = cli.consume_message();

            // Note: In a real app, you'd want to do a lot more error
            // and bounds checking than this.

            if (msg) {
                // Get the subscription ID from the incoming message
                auto subId = mqtt::get<uint32_t>(
                    msg->get_properties(), mqtt::property::SUBSCRIPTION_IDENTIFIER
                );

                // Dispatch to a handler function based on the Subscription ID
                if (!(handler[subId - 1])(*msg))
                    break;
            }
            else if (!cli.is_connected()) {
                cout << "Lost connection" << endl;
                while (!cli.is_connected()) {
                    this_thread::sleep_for(milliseconds(250));
                }
                cout << "Re-established connection" << endl;
            }
        }

        // Disconnect

        cout << "\nDisconnecting from the MQTT server..." << flush;
        cli.disconnect();
        cout << "OK" << endl;
    }
    catch (const mqtt::exception& exc) {
        cerr << exc.what() << endl;
        return 1;
    }

    return 0;
}