MQTT Subscriptions

Creating a application that communicates with an MQTT broker.

MQTT Subscriptions shows how to create an application that communicates with an MQTT broker. A new dialog opens for each subscription, where you can see the messages on the subscribed topics.

Creating a Client

We use the QMqttClient class to create an MQTT client and to set the broker host name and port to use for the connection:

     m_client = new QMqttClient(this);
     m_client->setHostname(ui->lineEditHost->text());
     m_client->setPort(static_cast<quint16>(ui->spinBoxPort->value()));

Subscribing to Topics

When users subscribe to topics in the client, a new subscription object is created:

 void MainWindow::on_buttonSubscribe_clicked()
 {
     auto subscription = m_client->subscribe(ui->lineEditTopic->text(),
                                             static_cast<quint8>(ui->spinQoS->text().toUInt()));
     if (!subscription) {
         QMessageBox::critical(this, u"Error"_s,
                               u"Could not subscribe. Is there a valid connection?"_s);
         return;
     }
     auto subWindow = new SubscriptionWindow(subscription);
     subWindow->setWindowTitle(subscription->topic().filter());
     subWindow->show();
 }

We use the QMqttSubscription class to store the topic, state, and QoS level of a subscription:

 SubscriptionWindow::SubscriptionWindow(QMqttSubscription *sub, QWidget *parent) :
     QWidget(parent),
     ui(new Ui::SubscriptionWindow),
     m_sub(sub)
 {
     ui->setupUi(this);

     ui->labelSub->setText(m_sub->topic().filter());
     ui->labelQoS->setText(QString::number(m_sub->qos()));
     updateStatus(m_sub->state());
     connect(m_sub, &QMqttSubscription::messageReceived, this, &SubscriptionWindow::updateMessage);
     connect(m_sub, &QMqttSubscription::stateChanged, this, &SubscriptionWindow::updateStatus);
     connect(m_sub, &QMqttSubscription::qosChanged, [this](quint8 qos) {
         ui->labelQoS->setText(QString::number(qos));
     });
     connect(ui->pushButton, &QAbstractButton::clicked, m_sub, &QMqttSubscription::unsubscribe);
 }

The QoS level can be set separately for a message and for a subscription. The QoS level set for a subscription determines the minimum QoS level. If a message is sent with a higher QoS level, the broker increases the QoS of that message to the higher level. For example, if client A subscribed to topic with QoS 1, and client B publishes a message on the topic, with QoS 0, the broker will automatically increase the QoS of the message to 1. If client B publishes a message on the topic with the QoS 2, the broker will send it with QoS 2.

Receiving Messages

When the client receives a message, the QMqttMessage class is used to store the actual message payload:

 void SubscriptionWindow::updateMessage(const QMqttMessage &msg)
 {
     ui->listWidget->addItem(msg.payload());
 }

Files:

Contents

MQTT Subscriptions

Creating a Client

Subscribing to Topics

Receiving Messages