paho.mqtt.cpp/examples/async_publish_time.cpp

// async_publish_time.cpp
//
// This is a Paho MQTT C++ client, sample application.
//
// It's a fairly contrived, but useful example of an MQTT data monitor and
// publisher, using the C++ asynchronous client interface. A fairly common
// usage for MQTT applications to monitor a sensor and publish the reading
// when it changes by a "significant" amount (whatever that may be).
// This might be temperature, pressure, humidity, soil moisture, CO2 levels,
// or anything like that.
//
// Since we don't have a universal sensor to use for this example, we simply
// use time itself as out input data. We periodically "sample" the time
// value and when it changes by more than our required delta amount, we
// publish the time. In this case we use the system clock, measuring the
// time with millisecond precision.
//
// The sample demonstrates:
//  - Connecting to an MQTT server/broker
//  - Sampling a value
//  - Publishing messages using a `topic` object
//  - Last will and testament
//  - Callbacks with lambdas
//  - Using `create_options`
//  - Creating options with builder classes
//  - Offline buffering in the client
//

/*******************************************************************************
 * Copyright (c) 2019-2023 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 <atomic>
#include <chrono>
#include <cstdlib>
#include <cstring>
#include <iostream>
#include <string>
#include <thread>  // For sleep

#include "mqtt/async_client.h"

using namespace std;
using namespace std::chrono;

const std::string DFLT_SERVER_ADDRESS{"mqtt://localhost:1883"};

// The QoS for sending data
const int QOS = 1;

// How often to sample the "data"
const auto SAMPLE_PERIOD = milliseconds(5);

// How much the "data" needs to change before we publish a new value.
const int DELTA_MS = 100;

// How many to buffer while off-line
const int MAX_BUFFERED_MESSAGES = 1200;

// --------------------------------------------------------------------------
// Gets the current time as the number of milliseconds since the epoch:
// like a time_t with ms resolution.

uint64_t timestamp()
{
    auto now = system_clock::now();
    auto tse = now.time_since_epoch();
    auto msTm = duration_cast<milliseconds>(tse);
    return uint64_t(msTm.count());
}

// --------------------------------------------------------------------------

int main(int argc, char* argv[])
{
    // The server URI (address)
    string serverURI = (argc > 1) ? string(argv[1]) : DFLT_SERVER_ADDRESS;

    // The amount of time to run (in ms). Zero means "run forever".
    uint64_t trun = (argc > 2) ? stoll(argv[2]) : 0LL;

    cout << "Initializing for server '" << serverURI << "'..." << endl;

    // We configure to allow publishing to the client while off-line,
    // and that it's OK to do so before the 1st successful connection.
    auto createOpts = mqtt::create_options_builder()
                          .server_uri(serverURI)
                          .send_while_disconnected(true, true)
                          .max_buffered_messages(MAX_BUFFERED_MESSAGES)
                          .delete_oldest_messages()
                          .finalize();

    mqtt::async_client cli(createOpts);

    // Set callbacks for when connected and connection lost.

    cli.set_connected_handler([&cli](const std::string&) {
        std::cout << "*** Connected (" << timestamp() << ") ***" << std::endl;
    });

    cli.set_connection_lost_handler([&cli](const std::string&) {
        std::cout << "*** Connection Lost (" << timestamp() << ") ***" << std::endl;
    });

    auto willMsg = mqtt::message("test/events", "Time publisher disconnected", 1, true);
    auto connOpts = mqtt::connect_options_builder()
                        .clean_session()
                        .will(willMsg)
                        .automatic_reconnect(seconds(1), seconds(10))
                        .finalize();

    try {
        // Note that we start the connection, but don't wait for completion.
        // We configured to allow publishing before a successful connection.
        cout << "Starting connection..." << endl;
        cli.connect(connOpts);

        auto top = mqtt::topic(cli, "data/time", QOS);
        cout << "Publishing data..." << endl;

        while (timestamp() % DELTA_MS != 0)
            ;

        uint64_t t = timestamp(), tlast = t, tstart = t;

        top.publish(to_string(t));

        while (true) {
            this_thread::sleep_for(SAMPLE_PERIOD);

            t = timestamp();
            // cout << t << endl;
            if (abs(int(t - tlast)) >= DELTA_MS)
                top.publish(to_string(tlast = t));

            if (trun > 0 && t >= (trun + tstart))
                break;
        }

        // Disconnect
        cout << "\nDisconnecting..." << endl;
        cli.disconnect()->wait();
        cout << "  ...OK" << endl;
    }
    catch (const mqtt::exception& exc) {
        cerr << exc.what() << endl;
        return 1;
    }

    return 0;
}