Simple MQTT Client Example

 // Copyright (C) 2017 The Qt Company Ltd.
 // SPDX-License-Identifier: LicenseRef-Qt-Commercial OR BSD-3-Clause

 #include "mainwindow.h"
 #include "ui_mainwindow.h"

 #include <QtCore/QDateTime>
 #include <QtMqtt/QMqttClient>
 #include <QtWidgets/QMessageBox>

 MainWindow::MainWindow(QWidget *parent) :
     QMainWindow(parent),
     ui(new Ui::MainWindow)
 {
     ui->setupUi(this);

     m_client = new QMqttClient(this);
     m_client->setHostname(ui->lineEditHost->text());
     m_client->setPort(ui->spinBoxPort->value());

     connect(m_client, &QMqttClient::stateChanged, this, &MainWindow::updateLogStateChange);
     connect(m_client, &QMqttClient::disconnected, this, &MainWindow::brokerDisconnected);

     connect(m_client, &QMqttClient::messageReceived, this, [this](const QByteArray &message, const QMqttTopicName &topic) {
         const QString content = QDateTime::currentDateTime().toString()
                     + QLatin1String(" Received Topic: ")
                     + topic.name()
                     + QLatin1String(" Message: ")
                     + message
                     + QLatin1Char('\n');
         ui->editLog->insertPlainText(content);
     });

     connect(m_client, &QMqttClient::pingResponseReceived, this, [this]() {
         const QString content = QDateTime::currentDateTime().toString()
                     + QLatin1String(" PingResponse")
                     + QLatin1Char('\n');
         ui->editLog->insertPlainText(content);
     });

     connect(ui->lineEditHost, &QLineEdit::textChanged, m_client, &QMqttClient::setHostname);
     connect(ui->spinBoxPort, QOverload<int>::of(&QSpinBox::valueChanged), this, &MainWindow::setClientPort);
     updateLogStateChange();
 }

 MainWindow::~MainWindow()
 {
     delete ui;
 }

 void MainWindow::on_buttonConnect_clicked()
 {
     if (m_client->state() == QMqttClient::Disconnected) {
         ui->lineEditHost->setEnabled(false);
         ui->spinBoxPort->setEnabled(false);
         ui->buttonConnect->setText(tr("Disconnect"));
         m_client->connectToHost();
     } else {
         ui->lineEditHost->setEnabled(true);
         ui->spinBoxPort->setEnabled(true);
         ui->buttonConnect->setText(tr("Connect"));
         m_client->disconnectFromHost();
     }
 }

 void MainWindow::on_buttonQuit_clicked()
 {
     QApplication::quit();
 }

 void MainWindow::updateLogStateChange()
 {
     const QString content = QDateTime::currentDateTime().toString()
                     + QLatin1String(": State Change")
                     + QString::number(m_client->state())
                     + QLatin1Char('\n');
     ui->editLog->insertPlainText(content);
 }

 void MainWindow::brokerDisconnected()
 {
     ui->lineEditHost->setEnabled(true);
     ui->spinBoxPort->setEnabled(true);
     ui->buttonConnect->setText(tr("Connect"));
 }

 void MainWindow::setClientPort(int p)
 {
     m_client->setPort(p);
 }

 void MainWindow::on_buttonPublish_clicked()
 {
     if (m_client->publish(ui->lineEditTopic->text(), ui->lineEditMessage->text().toUtf8()) == -1)
         QMessageBox::critical(this, QLatin1String("Error"), QLatin1String("Could not publish message"));
 }

 void MainWindow::on_buttonSubscribe_clicked()
 {
     auto subscription = m_client->subscribe(ui->lineEditTopic->text());
     if (!subscription) {
         QMessageBox::critical(this, QLatin1String("Error"), QLatin1String("Could not subscribe. Is there a valid connection?"));
         return;
     }
 }