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Exposing C++ values to QML issue



  • Hello guys,

    I have some strange issues with my actual project i'm sitting on. Maybe some of you could help me :) I've already read some blogs but unfortunately none of them could help me.
    Actually the issue should be quite easy but I didn't get my mistake. I want to send the value of my AlertMessage class to the QML and display it. That's all. Unfortunately, I'm getting weird float values instead of the right double values.
    Here are my classes. It would be awesome if someone could help me :D

    main.cpp

    int main(int argc, char *argv[])
    {
        QGuiApplication app(argc, argv);
    
        Subscriber subber(targetDir, "");
    
        qmlRegisterType<AlertMessage>("io.qt.alertmessage", 1, 0, "AlertMessage");
    
        QQmlApplicationEngine engine;
        engine.load(QUrl(QStringLiteral("PassengerUI/HeadUnit.qml")));
        qDebug() << "--- Passenger UI started ---";
    
        return app.exec();
    }
    

    To be more precise I've created an instance of my Subscriber class ('subber' above) which initiate an instance of my MessageHandler class ('m_handler' below) calling another class to read some local json files. At the end I'm storing my collected information in an instance of my AlertMessage class which then should be sent to my qml.

    subscriber.cpp

    void Subscriber::readFromJsonFile(QString filePath)
    {
        if (filePath.isEmpty())
        {
            filePath = QDir::currentPath();
        }
    
        QString useCase;
        QFile file;
        file.setFileName(filePath + "/sub_message.json");
        file.open(QIODevice::ReadOnly | QIODevice::Text);
        useCase = file.readAll();
        file.close();
    
        QJsonDocument useCaseDoc = QJsonDocument::fromJson(useCase.toUtf8());
        QJsonObject useCaseObj = useCaseDoc.object();
        m_handler->processReceivedMessages(useCaseObj);
    }
    

    messagehandler.cpp

    void MessageHandler::processReceivedMessages(QJsonObject jsonObject)
    {
        m_alertMessage->readDataFromJSONFormat(jsonObject);
        qDebug() << m_alertMessage->printout();
    }
    

    alertmessage.h

    #ifndef ALERTMESSAGE_H
    #define ALERTMESSAGE_H
    
    #include <QString>
    #include <QJsonObject>
    #include <QObject>
    
    class AlertMessage : public QObject
    {
        Q_OBJECT
        Q_PROPERTY(QString messageType      READ messageType        WRITE setMessageType        NOTIFY messageTypeChanged)
        Q_PROPERTY(QString sender           READ sender             WRITE setSender             NOTIFY senderChanged)
        Q_PROPERTY(QString alertType        READ alertType          WRITE setAlertType          NOTIFY alertTypeChanged)
        Q_PROPERTY(QString id               READ id                 WRITE setID                 NOTIFY idChanged)
        Q_PROPERTY(QString origin           READ origin             WRITE setOrigin             NOTIFY originChanged)
        Q_PROPERTY(QString dataSource       READ dataSource         WRITE setDataSource         NOTIFY dataSourceChanged)
        Q_PROPERTY(double distanceToEvent   READ distanceToEvent    WRITE setDistanceToEvent    NOTIFY distanceToEventChanged)
        Q_PROPERTY(double severity          READ severity           WRITE setSeverity           NOTIFY severityChanged)
        Q_PROPERTY(double azimuth           READ azimuth            WRITE setAzimuth            NOTIFY azimuthChanged)
        Q_PROPERTY(double timeToEvent       READ timeToEvent        WRITE setTimeToEvent        NOTIFY timeToEventChanged)
        Q_PROPERTY(int detailLevel          READ detailLevel        WRITE setDetailLevel        NOTIFY detailLevelChanged)
    
    public:
        explicit AlertMessage(QObject *parent = nullptr);
        AlertMessage(QJsonObject &jsonObject);
        AlertMessage(std::string &JsonInfo);
    
        void cleanMessage();
    
        void readDataFromJSONFormat(QJsonObject &jsonObject);
        void readDataFromJSONFormat(std::string &json);
    
        QString messageType() const;
        void setMessageType(const QString &messageType);
    
        QString sender() const;
        void setSender(const QString &sender);
    
        QString alertType() const;
        void setAlertType(const QString &alertType);
    
        QString id() const;
        void setID(const QString &id);
    
        QString origin() const;
        void setOrigin(const QString &origin);
    
        QString dataSource() const;
        void setDataSource(const QString &dataSource);
    
        double distanceToEvent() const;
        void setDistanceToEvent(const double &distance);
    
        double severity() const;
        void setSeverity(const double &severity);
    
        double azimuth() const;
        void setAzimuth(const double &azimuth);
    
        double timeToEvent() const;
        void setTimeToEvent(const double &time);
    
        int detailLevel() const;
        void setDetailLevel(const int &level);
    
        void setAlertReceived(const bool &fcw, const bool &rww, const bool &eebl);
    
        QString printout();
    
    signals:
        void messageTypeChanged();
        void senderChanged();
        void alertTypeChanged();
        void idChanged();
        void originChanged();
        void dataSourceChanged();
        void distanceToEventChanged();
        void severityChanged();
        void azimuthChanged();
        void timeToEventChanged();
        void detailLevelChanged();
    
    private:
    
        QString m_messageType;
        QString m_sender;
        QString m_alertType;
        QString m_id;
        QString m_origin;
        QString m_dataSource;
        double m_distanceToEvent;
        double m_severity;
        double m_azimuth;
        double m_timeToEvent;
        int m_detailLevel;
    
        bool m_fcwAvailable;
        bool m_rwwAvailable;
        bool m_eeblAvailable;
    };
    
    #endif // ALERTMESSAGE_H
    
    

    alertmessage.cpp

    #include <QJsonObject>
    #include <QJsonDocument>
    #include <QDebug>
    
    #include "alertmessage.h"
    
    AlertMessage::AlertMessage(QObject *parent) :
        QObject(parent)
    {
    }
    
    AlertMessage::AlertMessage(QJsonObject &jsonObject):
        AlertMessage()
    {
        readDataFromJSONFormat(jsonObject);
    }
    
    AlertMessage::~AlertMessage()
    {
        cleanMessage();
    }
    
    void AlertMessage::cleanMessage()
    {
        m_messageType = "";
        m_sender = "";
        m_alertType = "";
        m_id = "";
        m_origin = "";
        m_dataSource = "";
    
        m_distanceToEvent = 0;
        m_severity = 0;
        m_azimuth = 0;
        m_timeToEvent = 0;
        m_detailLevel = 0;
        m_fcwAvailable = false;
        m_rwwAvailable = false;
        m_eeblAvailable = false;
    }
    
    void AlertMessage::readDataFromJSONFormat(QJsonObject &jsonObject)
    {
        if (jsonObject.count() <= 0)
        {
            qDebug() << "AlertMessage::Wrong JSON-Array.";
            return;
        }
        cleanMessage();
    
        setMessageType(jsonObject["type"].toString());
        setSender(jsonObject["sender"].toString());
    
        QJsonValue valueData = jsonObject.value(QString("data"));
        QJsonObject dataObject = valueData.toObject();
    
        if (!dataObject.isEmpty())
        {
            setAlertType(dataObject["alert"].toString());
            setID(dataObject["id"].toString());
            setOrigin(dataObject["origin"].toString());
            setDataSource(dataObject["data_source"].toString());
            setDistanceToEvent(dataObject["distance_to_event"].toDouble());
            setSeverity(dataObject["severity"].toDouble());
            setAzimuth(dataObject["azimuth"].toDouble());
            setTimeToEvent(dataObject["time_to_event"].toDouble());
    
            QJsonValue detailValue = dataObject.value(QString("detail"));
            QJsonObject detailObject = detailValue.toObject();
    
            if (!detailObject.isEmpty())
            {
                 setDetailLevel(detailObject["level"].toInt());
            }
        }
    }
    
    QString AlertMessage::messageType() const
    {
        return m_messageType;
    }
    
    void AlertMessage::setMessageType(const QString &messageType)
    {
        if (m_messageType != messageType)
        {
            m_messageType = messageType;
            emit messageTypeChanged();
        }
        return;
    }
    
    

    Do not wonder. I've created for every variable an own getter and setter method like above. I posted only one example to keep my post clear :D
    On the C++ side everything seems to work fine. The data was stored successful to my alertmessage instance. I've debugged the values and got this.
    "** AlertMessage begin**\nMessageType = ALERT\nSender = \nAlertType = FCW\nID = \nOrigin = Cohda_TA\nDataSource = \nDistance_To_Event = 55.2\nSeverity = 9\nAzimuth = 25.5434\nTime_To_Event = 20.3\nDetailLevel = 0\nFCWAvailable = 0\nRWWAvailable = 0\nEEBLAvailable = 0\n** AlertMessage end **\n"

    But on the QML side there are some strange output:
    qml: QML:1.6911919422062236e-306, ... and so on.

    I hope you can help me.

    Best regards,
    Thanni


  • Lifetime Qt Champion

    Since you don't initialize your members I would guess "if (!dataObject.isEmpty())" is false so your values will not be set so you see the uninitialized data in your qml



  • No, I just checked the value. dataObject is not empty and it will execute the setter methods.


  • Lifetime Qt Champion

    Please properly initialize the members in the ctor and see if the 'strange' values disappear.



  • I initialized all members but I still didn't get any values displayed on the qml side ...

    AlertMessage::AlertMessage(QObject *parent) :
        QObject(parent),
        m_messageType(""),
        m_sender(""),
        m_alertType(""),
        m_id(""),
        m_origin(""),
        m_dataSource(""),
        m_distanceToEvent(0),
        m_severity(0),
        m_azimuth(0),
        m_timeToEvent(0)
    {
    }
    


  • QML, what QML? Please show the QML. Preferably a minimally compileable example.



  • Here is a snippet of my QML code

    import QtQuick 2.0
    import io.qt.alertmessage 1.0
    
    import "../BasicItems"
    import "../ModellingItems"
    
    import "../../WarningCalc.js" as WarningCalcer
    
    Rectangle
    {
        id: informationCellArea
    
        property int informationTextHeaderSize: 10
        property int informationTextSize: 17
        property int itemHeight: parent.height/4
        property int headerToTextMargin: 10
    
        property string senderString
        property string distanceString
        property string timeString
        property string descriptionString
        property string trafficSignString
        property int severityInt
    
        color: "transparent"
        radius: 10
    
        AlertMessage
        {
            id: alertMessage
        }
    
        Item
        {
            id: distanceArea
            width: parent.width/2
            height: itemHeight
            anchors.left: parent.left
            anchors.bottom: descriptionUseCaseArea.top
    
            HMIText
            {
                id: distanceTextHeader
                text: "Distance to event"
                fontSize: informationTextHeaderSize
                anchors.centerIn: parent
            }
    
            HMIText
            {
                id: distanceText
                text:  alertMessage.distanceToEvent + " m"
                fontSize: informationTextSize
                useFontBold: true
                anchors.top: distanceTextHeader.bottom
                anchors.topMargin: headerToTextMargin
                anchors.horizontalCenter: parent.horizontalCenter
    
                onTextChanged: {
                    console.log("QML:" + distanceString);
                }
            }
        }
    }
    
    

  • Lifetime Qt Champion

    @Thanni123 said in Exposing C++ values to QML issue:

    get any values displayed on the qml side ...

    You don't see any double value or you don't see the strange values anymore? Please initialize one string to something other than an empty string.



  • What is in readDataFromJSONFormat and printout?
    So far I cannot see any real issues.



  • Well if I'm initializing the constructor with some default value, it display the value I've created. But if I try to set the values with the json file the value mentioned above appear or the value equals 0.



  • @fcarney I've posted the method declaration above and the printout method is only a debug method to display if the values were set.

    I think I did a mistake during initializing an object of my AlertMessage class but I didn't get my fault yet... It must be some stupid mistake I've done.



  • @Thanni123 said in Exposing C++ values to QML issue:

    But if I try to set the values with the json file the value mentioned above appear or the value equals 0.

    The problem is in readDataFromJSONFormat and/or printout. If you want help you need to show what is in those. So far nothing you have posted shows any problems.



  • Hey, I don't get your point exactly but here is the source code of your request for both methods including the output for the printout mehtod.

    void AlertMessage::readDataFromJSONFormat(const QJsonObject &jsonObject)
    {
        if (jsonObject.count() <= 0)
        {
            qDebug() << "AlertMessage::Wrong JSON-Array.";
            return;
        }
        cleanMessage();
    
        setMessageType(jsonObject["type"].toString());
        setSender(jsonObject["sender"].toString());
    
        QJsonValue valueData = jsonObject.value(QString("data"));
        QJsonObject dataObject = valueData.toObject();
    
        if (!dataObject.isEmpty())
        {
            setAlertType(dataObject["alert"].toString());
            setID(dataObject["id"].toString());
            setOrigin(dataObject["origin"].toString());
            setDataSource(dataObject["data_source"].toString());
            setDistanceToEvent(dataObject["distance_to_event"].toDouble());
            setSeverity(dataObject["severity"].toDouble());
            setAzimuth(dataObject["azimuth"].toDouble());
            setTimeToEvent(dataObject["time_to_event"].toDouble());
    
            QJsonValue detailValue = dataObject.value(QString("detail"));
            QJsonObject detailObject = detailValue.toObject();
    
            if (!detailObject.isEmpty())
            {
                 setDetailLevel(detailObject["level"].toInt());
            }
        }
    }
    
    QString AlertMessage::printout()
    {
        QString print = "** AlertMessage begin**\n";
        print += "MessageType       = " + QString("%1").arg(this->messageType()) + "\n";
        print += "Sender            = " + QString("%1").arg(this->sender()) + "\n";
        print += "AlertType         = " + QString("%1").arg(this->alertType()) + "\n";
        print += "ID                = " + QString("%1").arg(this->id()) + "\n";
        print += "Origin            = " + QString("%1").arg(this->origin()) + "\n";
        print += "DataSource        = " + QString("%1").arg(this->dataSource()) + "\n";
        print += "Distance_To_Event = " + QString("%1").arg(this->distanceToEvent()) + "\n";
        print += "Severity          = " + QString("%1").arg(this->severity()) + "\n";
        print += "Azimuth           = " + QString("%1").arg(this->azimuth()) + "\n";
        print += "Time_To_Event     = " + QString("%1").arg(this->timeToEvent()) + "\n";
        print += "DetailLevel       = " + QString("%1").arg(this->detailLevel()) + "\n";
        print += "FCWAvailable      = " + QString("%1").arg(this->fcwReceived()) + "\n";
        print += "RWWAvailable      = " + QString("%1").arg(this->rwwReceived()) + "\n";
        print += "EEBLAvailable     = " + QString("%1").arg(this->eeblReceived()) + "\n";
    
        print += "** AlertMessage end **\n";
    
        return print;
    }
    

    The output for my printout method:

    QDateTime(2020-02-18 00:15:02.786 Mitteleuropäische Zeit Qt::LocalTime)
    "** AlertMessage begin**\nMessageType       = ALERT\nSender            = \nAlertType         = FCW\nID                = \nOrigin            = Cohda_TA\nDataSource        = \nDistance_To_Event = 55.2\nSeverity          = 9\nAzimuth           = 25.5434\nTime_To_Event     = 20.3\nDetailLevel       = 0\nFCWAvailable      = 0\nRWWAvailable      = 0\nEEBLAvailable     = 0\n** AlertMessage end **\n"
    

    Hope this helps somehow :)



  • I cannot see anything wrong in this code. All I can think is do a make clean, qmake, make project and see if this gets any better. I don't see any variables such as references exceeding their lifetime or anything. Sorry I cannot be more helpful. This should just work.



  • I think I figured out the issue but I don't know how to solve it. It's relying on this call in the main.cpp.

    main.cpp

    qmlRegisterType<AlertMessage>("io.qt.alertmessage", 1, 0, "AlertMessage");
    

    This method uses the standard constructor of my AlertMessage class without the QJsonObject as a parameter. I need to define my custom constructor but I get stuck with some errors ...
    If I don't use my custom constructor it will always initiliaze without any values set.



  • @Thanni123 said in Exposing C++ values to QML issue:

    If I don't use my custom constructor it will always initiliaze without any values set.

    This is exactly what @Christian-Ehrlicher has ask you to do: always initialize your member variables in constructor this is for all constructors.
    So if you add a call to clearMessage() in your default constructor, you will probably solve your issue.


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