Quick MQTT Example

Registering https:// to QML and using it in a Qt Quick user interface.

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The Qt Company Ltd.

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Publié le 27 décembre 2024 - Mis à jour le 27 décembre 2024

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Quick MQTT Example▲

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

https:// 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 https:// class as a base class:

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QmlMqttClient::QmlMqttClient(QObject *parent)
    : QMqttClient(parent)
{
}

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

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QmlMqttSubscription* QmlMqttClient::subscribe(const QString &amp;topic)
{
    auto sub = QMqttClient::subscribe(topic, 0);
    auto result = new QmlMqttSubscription(sub, this);
    return result;
}

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

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

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

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

Registering Classes in QML▲

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

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

    QQmlApplicationEngine engine;

    qmlRegisterType&lt;QmlMqttClient&gt;("MqttClient", 1, 0, "MqttClient");

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

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    qmlRegisterUncreatableType&lt;QmlMqttSubscription&gt;("MqttClient", 1, 0, "MqttSubscription", QLatin1String("Subscriptions are read-only"));

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

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

List of Files▲

Files:

List of Files▲

quicksubscription/CMakeLists.txt
quicksubscription/main.cpp
quicksubscription/main.qml
quicksubscription/qml.qrc
quicksubscription/qmlmqttclient.cpp
quicksubscription/qmlmqttclient.h
quicksubscription/quicksubscription.pro

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