Qt Quick Subscription

Use Qt Quick Controls to create an application that can subscribe to MQTT topics.

Qt Quick Subscription 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

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

QmlMqttSubscription::QmlMqttSubscription(QMqttSubscription *s, QmlMqttClient *c)
    : sub(s)
    , client(c)
    connect(sub, &QMqttSubscription::messageReceived, this, &QmlMqttSubscription::handleMessage);
    m_topic = sub->topic();

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);

Use the subscribe() function to create a subscription object:

QmlMqttSubscription* QmlMqttClient::subscribe(const QString &topic)
    auto sub = m_client.subscribe(topic, 0);
    auto result = new QmlMqttSubscription(sub, this);
    return result;

Use a 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, load the QML type Main from the module subscription:

    QGuiApplication app(argc, argv);
    QQmlApplicationEngine engine;

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

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

Now use the MqttClient type in the Main.qml file to create an MQTT client:

    MqttClient {
        id: client
        hostname: hostnameField.text
        port: portField.text


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