QNmeaPositionInfoSource with QBuffer IODevice, no updates received

Tilmanns

Hey everyone,

so, I have a embedded device sending NMEA sentences over an mqtt broker. I am trying to write an application that uses those NMEA sentences to visualize position measurements with corresponding accuracy estimates.

I got the broker working in Qt and am receiving multiple NMEA messages per second, but I am not receiving any signal of positionUpdated. I am using a QBuffer as a QIODevice for my QNmeaPositionInfoSource.
NMEA messages are of types: $GNGGA $GNRMC $GNGLL $GNVTG and $GNGBS $GNGST
The whole setup looks like this:

// mqttpositionclient.h
#ifndef MQTTPOSITIONCLIENT_H
#define MQTTPOSITIONCLIENT_H

#include <QNmeaPositionInfoSource  >
#include <QQmlEngine>
#include <QMqttClient>
#include <QBuffer>
class MqttPositionClient : public QNmeaPositionInfoSource
{
    Q_OBJECT
    QML_ELEMENT
public:
    MqttPositionClient();

    void subscribe();
signals:
    void msgReceived(QByteArray msg, const QGeoPositionInfo& info);
private:
    void read_mqtt_msg(const QByteArray &message, const QMqttTopicName &topic );

    QBuffer m_buffer;
    QMqttClient m_client;
};

#endif // MQTTPOSITIONCLIENT_H

and


#include <QMessageBox>
#include <QTimer>
MqttPositionClient::MqttPositionClient() : QNmeaPositionInfoSource (RealTimeMode), m_stream(&m_buffer), m_source(QNmeaPositionInfoSource::RealTimeMode) {

    m_client.setProtocolVersion(QMqttClient::MQTT_5_0);
    QString user_name = "censored";
    QString passwd = "censored";
    QString host_name = "censored.com";
    quint16 port = 8883;
    m_client.setUsername(user_name);
    m_client.setPassword(passwd);
    m_client.setPort(port);
    m_client.setHostname(host_name);
    m_client.setClientId("position_debugger");

    QSslConfiguration ssl_cfg;
    ssl_cfg.setProtocol(QSsl::TlsV1_3);

    m_client.connectToHostEncrypted(ssl_cfg);

    QTimer::singleShot(std::chrono::seconds(2), [this](){
        qInfo() << "subscribing";
        this->subscribe();
    });

    m_buffer.open(QIODeviceBase::ReadWrite);
    setDevice(&m_buffer);

    QObject::connect(&m_client, &QMqttClient::messageReceived,
                     this, &MqttPositionClient::read_mqtt_msg);

    QObject::connect(&m_source, &QNmeaPositionInfoSource::positionUpdated, [](const QGeoPositionInfo& info){
        qInfo() << "update received internally";
    });

    setUpdateInterval(1000);
    startUpdates();
}


void MqttPositionClient::read_mqtt_msg(const QByteArray &message, const QMqttTopicName &topic ){
    auto msgtype = message.first(6);
    qInfo() << msgtype;

    // to my understanding QIODevice::readyRead() should be emitted here? 
    // this signal is required according to the documentation [here](https://doc.qt.io/qt-6/qnmeapositioninfosource.html#setDevice)
    m_buffer.write(message);
}


void MqttPositionClient::subscribe(){

    QMqttTopicFilter topic("censored");
    auto subscription = m_client.subscribe(topic, 0);
    if (!subscription) {
        QMessageBox::information(nullptr, "mqtt subscription", "failed");
    }
}

I confirmed that read_mqtt_msg() is called repeatedly.

When I use QNmeaPositionInfoSource::parsePosInfoFromNmeaData() I can decode some of the messages, however looking at the sourcecode at the QPositioning Repository, it seems like QNmeaPositionInfoSource takes multiple messages into account to generate its position and I'd rather use that implementation.

Does anyone know what I am missing? I am also open for suggestions of different approaches to achieve what i'm going for.

Thanks!