Cannot add model to QML

milan

Hello, I cannot add C++ model to QML which runs in QQuickWidget. I get error while running the application. Error is : "qrc:/Gauge.qml:43: ReferenceError: DataModel is not defined". I can see the gauge when the program is run, it is only that the value is not updating the gauge.

My code is like this:

//main.cpp
int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    QQmlApplicationEngine* engine = new QQmlApplicationEngine();
    DataModel datamodel;
    engine->rootContext()->setContextProperty("DataModel", &datamodel);
    engine->load(QUrl(QStringLiteral("qrc:/Gauge.qml")));
    if (engine->rootObjects().isEmpty())
        return -1;
    MainWindow w;
    w.show();
    return app.exec();

//mainwindow.cpp
MainWindow::MainWindow(QWidget *parent) :  QMainWindow(parent), ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    ui->quickWidget->setSource(QUrl(QStringLiteral("qrc:/Gauge.qml")));
}

MainWindow::~MainWindow()
{
    delete ui;
}

//datamodel.cpp
#include "datamodel.h"
#include <iostream>
#include <QTimer>
int temp = 0;
DataModel::DataModel(QObject *parent) : QObject (parent)
{
    m_num = 10;
    qDebug() << "Creating datamodel constructing...";
    m_timer = new QTimer(this);
    connect(m_timer, SIGNAL(timeout()), this, SLOT(update()));
    m_timer->start(1000);
}

int DataModel::num() const
{
    return m_num;
}

void DataModel::update()
{
    temp += 5;
    this->setNum(temp);
    qDebug() << "Updating value:" << m_num;
}

void DataModel::setNum(int num)
{
    if (m_num == num)
        return;
    qDebug() << "Setting value:" << num;
    m_num = num;
    emit numChanged(m_num);
}

// datamodel.h
#ifndef DATAMODEL_H
#define DATAMODEL_H

#include <QObject>
#include <QTimer>
#include <QDebug>

class DataModel : public QObject
{
    Q_OBJECT
    Q_PROPERTY(int num READ num WRITE setNum NOTIFY numChanged)

public:
    explicit DataModel(QObject *parent = nullptr);

    int num() const;
signals:
    void numChanged(int num);
    void operate();

public slots:
    void setNum(int num);
    void update();

private:
    int m_num;
    QTimer* m_timer;
};
#endif // DATAMODEL_H

//Gauge.qml
  CircularGauge {
      value: DataModel.num
      anchors.centerIn: parent
      }
  }

sierdzio

Your MainWindow is creating another QML engine which does not have "DataModel" set.

milan

@sierdzio . So, how can i fix it. Do I add DataModel in mainwindow.cpp?

sierdzio

@milan said in Cannot add model to QML:

QQmlApplicationEngine* engine = new QQmlApplicationEngine();

remove the engine since you're not using it anywhere. Or remove MainWindow if you are not using that. I don't know which is true in your case.

Extract the engine from your ui->quickWidget and add the DataModel there. If you are using QQuickWidget, you can use https://doc.qt.io/qt-5/qquickwidget.html#engine.

milan

@sierdzio .Thanks for your answer, it works. The initial value of gauge is set to 10. But the timer I have in my datamodel does not update the gauge value.

sierdzio

@milan said in Cannot add model to QML:

qDebug() << "Setting value:" << num;

Do you see this message printed?

milan

@sierdzio . I see that DataModel object gets destructed after it is constructed.

sierdzio

@milan said in Cannot add model to QML:

@sierdzio . I see that DataModel object gets destructed after it is constructed.

So make sure it lives long :-)

milan

@sierdzio. Yes I did that. Is qmlregistertype better way that setContextProperty I am using now? The C++ model should update the qml. Is it possible to use qmlregistertype in this context?

sierdzio

@milan said in Cannot add model to QML:

@sierdzio. Yes I did that. Is qmlregistertype better way that setContextProperty I am using now? The C++ model should update the qml

No it is not better, it server a different purpose. It makes the type recognizable (and creatable) in QML. Since you want to use a global data model (I think), then setting the context property is a better way for your use case. Just make sure DataModel object is not destroyed while your UI is alive. You can do it, for example, by declaring it as a member variable of MainWindow class, or by creating an object on heap and assigning MainWindow as parent.

milan

@sierdzio. Thanks a lot for your answer.