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: