QMqtt5 extracting response topic from received message

lukutis222

Hello. In QMqtt5 some updates were introduced. One of those updates is response topic and correlation data.

Whenever server receives a message on a subscribed topic, there should be a way to extract its response topic (if the message has one) and then publish an acknowledgment message on that response topic.

I have read simmilar QT forum thread regarding simmillar topic:
https://forum.qt.io/topic/104756/mqtt-v5-request-response-example/7

In QT documentation, I have found the following:
https://doc.qt.io/qt-6/qmqttpublishproperties.html

One of the public functions that this class contains is the following:

I am not fully understanding how can I use this class in my code. How can I retrieve what is the response topic?

In my code, I have MQTT_message_handler that is being called everytime a message that I am subscribed to is received:


void MainWindow::Mqtt_message_handler(QString topic,QString message){
    qDebug("MQTT handler");
    
        qDebug("Topic received = %s ",topic.toStdString().c_str());
        qDebug("Message received = %s ",message.toStdString().c_str());
        QMqttPublishProperties test;
        qDebug("response topic = %s",test.responseTopic().toStdString().c_str());
        qDebug("corellation data = %u",test.correlationData().toUInt());
        qDebug("\n");
    
    heck if subscribed topic has a response topic
    auto response_topic = QMqttPublishProperties::ResponseTopic();

    //LAST WILL RECEIVED
    if(strcmp(topic.toStdString().c_str(),LAST_WILL_T.toStdString().c_str())==0){
        ui->cell1_edit->setText(message);
    }

    //TEST TOPIC RECEIVED
    if(strcmp(topic.toStdString().c_str(),TEST_T.toStdString().c_str())==0){
        qDebug("Test topic message received");
    }
}

I am then monitoring the application output. I can see the following being printed when I receive a message that I am subscribed to:

MQTT handler
Topic received = /topic/led 
Message received = Enable 
response topic = 
corellation data = 0

For some reason the response topic is empty and corellation data is 0. However, on my mqtt client device I have set the following publish properties:

static esp_mqtt5_publish_property_config_t publish_property_toggle_led = {
    .payload_format_indicator = 1,
    .message_expiry_interval = 1000,
    .topic_alias = 0,
    .response_topic = "/topic/test/response",
    .correlation_data = "5",
    .correlation_data_len = 1,
};

I would very much appreciate if someone could clarify to me how to correctly retrieve the response topic.