Simple MQTT Client

Creating a minimalistic telemetry application.

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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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Simple MQTT Client▲

Simple MQTT Client demonstrates how to create a minimalistic client application.

To use the application, you first specify a broker, such as test.mosquitto.org or broker.hivemq.com and the port number 1883, and connect to it. You can then subscribe to a topic and send a message, which you will also receive.

Port number 1883 is not encrypted, and therefore it is suitable only for development and testing purposes. In production, always use encrypted connections.

Creating a Client▲

First, we use the QMqttClient class to create an MQTT client. The class provides properties for setting a unique client ID as well as the broker host name and port to connect to:

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    m_client = new QMqttClient(this);
    m_client-&gt;setHostname(ui-&gt;lineEditHost-&gt;text());
    m_client-&gt;setPort(static_cast&lt;quint16&gt;(ui-&gt;spinBoxPort-&gt;value()));

We do not set the client ID, and therefore it will be automatically generated for us.

Next, we connect to QMqttClient::messageReceived() to receive all messages sent to the broker:

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    connect(m_client, &amp;QMqttClient::messageReceived, this, [this](const QByteArray &amp;message, const QMqttTopicName &amp;topic) {
        const QString content = QDateTime::currentDateTime().toString()
                    + " Received Topic: "_L1
                    + topic.name()
                    + " Message: "_L1
                    + message
                    + u'\n';
        ui-&gt;editLog-&gt;insertPlainText(content);
    });

When users subscribe to topics in the client, we call QMqttClient::subscribe() on the specified topic:

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void MainWindow::on_buttonSubscribe_clicked()
{
    auto subscription = m_client-&gt;subscribe(ui-&gt;lineEditTopic-&gt;text());
    if (!subscription) {
        QMessageBox::critical(this, u"Error"_s,
                              u"Could not subscribe. Is there a valid connection?"_s);
        return;
    }

In this example, we subscribe to all topics. For more information about how to receive messages on particular topics, see the https:// example.

For an example of how to use the QMqttClient class in a Qt Quick application, see https://.

List of Files▲

Files:

List of Files▲

simpleclient/CMakeLists.txt
simpleclient/main.cpp
simpleclient/mainwindow.cpp
simpleclient/mainwindow.h
simpleclient/mainwindow.ui
simpleclient/simpleclient.pro

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