MQTT Subscriptions Example

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(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(), ui->spinQoS->text().toUInt());
    if (!subscription) {
        QMessageBox::critical(this, QLatin1String("Error"), QLatin1String("Could not subscribe. Is there a valid connection?"));
        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:

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