Quick MQTT Example

Registering QMqttClient to QML and using it in a Qt Quick user interface.

Quick MQTT demonstrates how to register QMqttClient as a QML type and use it in a Qt Quick application.

Qt MQTT does not provide a QML API in its current version. However, you can make the C++ classes of the module available to QML.

Creating a Client

We create a QmlMqttClient class with the QMqttClient class as a base class:

QmlMqttClient::QmlMqttClient(QObject *parent)
    : QObject(parent)
{
    connect(&m_client, &QMqttClient::hostnameChanged, this, &QmlMqttClient::hostnameChanged);
    connect(&m_client, &QMqttClient::portChanged, this, &QmlMqttClient::portChanged);
    connect(&m_client, &QMqttClient::stateChanged, this, &QmlMqttClient::stateChanged);
}

We use the subscribe() function to create a subscription object:

void QmlMqttClient::connectToHost()
{
    m_client.connectToHost();
}

We connect to QMqttSubscription::messageReceived( ) to receive all messages sent to the broker:

void QmlMqttClient::disconnectFromHost()
{
    m_client.disconnectFromHost();
}

We use an QMqttMessage object to store the payload of a received message:

void QmlMqttSubscription::handleMessage(const QMqttMessage &qmsg)
{
    emit messageReceived(qmsg.payload());
}

Registering Classes in QML

In the main.cpp file, we register the QmlMqttClient class as a QML type, MqttClient:

int main(int argc, char *argv[])
{
    QGuiApplication app(argc, argv);
    QQmlApplicationEngine engine;

    QObject::connect(
            &engine, &QQmlApplicationEngine::objectCreationFailed, &app,
            []() { QCoreApplication::exit(EXIT_FAILURE); }, Qt::QueuedConnection);

    engine.loadFromModule(u"subscription"_s, u"Main"_s);

    return QGuiApplication::exec();
}

In addition, we register the QmlMqttSubscription class as an uncreatable type:

We can now use the MqttClient type in the main.qml file to create an MQTT client:

Files:

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