setContextProperty : integrating c++ with qml

[Anas_Deshmukh]

Hi all ,
i am using Q_Property and setContextProperty to capture changes in C++ class variable into Qml file.
Qt 5.12, Linux + Windows

problem statement : i dont want to call "setContextProperty" for every object created.
if i have to monitor 20 object it leads me to call "setContextProperty" 20 times.

any wy that i capture c++ object in qml , apart from using setContext property, cant proceed with singletone approach or qml registertype.

any other good practice to achieve the same.

// tempraturegugage.h

#include <QObject>

class TempratureGuage : public QObject
{
    Q_OBJECT
    Q_PROPERTY(int temprature READ temprature WRITE setTemprature NOTIFY tempratureChanged)

public:
    TempratureGuage();

    void setTemprature(const int &t) {
        if (t != m_temprature) {
            m_temprature = t;
            emit tempratureChanged();
        }
    }
    int temprature() const {
        return m_temprature;
    }
signals:
    void tempratureChanged();
private:
    int m_temprature;
};

// tempratureguage.cpp

#include "tempratureguage.h"

TempratureGuage::TempratureGuage()
    : m_temprature(0)
{
}

// main.cpp
#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QQmlContext>
#include "tempratureguage.h"

int main(int argc, char *argv[])
{
#if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
#endif

    QGuiApplication app(argc, argv);

    QQmlApplicationEngine engine;

    QQmlContext* context = new QQmlContext(engine.rootContext());

// problem statement : i dont want to use setContext property for every object i created.
    TempratureGuage obj1;
    context->setContextProperty("obj1", &obj1);

    TempratureGuage obj2;
    context->setContextProperty("obj2", &obj2);

    TempratureGuage obj3;
    context->setContextProperty("obj3", &obj3);

    const QUrl url(QStringLiteral("qrc:/main.qml"));
    QObject::connect(&engine, &QQmlApplicationEngine::objectCreated,
                     &app, [url](QObject *obj, const QUrl &objUrl) {
        if (!obj && url == objUrl)
            QCoreApplication::exit(-1);
    }, Qt::QueuedConnection);
    engine.load(url);

    return app.exec();
}

//// main.qml

import QtQuick 2.12
import QtQuick.Window 2.12
import QtQuick.Layouts 1.15


Window {
    id : root
    width: 640
    height: 480
    visible: true
    title: qsTr("Hello World")

    property int  temp1: obj1.tempratureChanged
    property int  temp2: obj2.tempratureChanged
    property int  temp3: obj3.tempratureChanged

    ColumnLayout{
        spacing: 2

        Rectangle {
            Text {
                id: t1
                text:root.temp1
            }
        }

        Rectangle {
            Text {
                id: t2
                text: root.temp2
            }
        }

        Rectangle {
            Text {
                id: t3
                text: root.temp3
            }
        }
    }
}

[eyllanesc]

@Anas_Deshmukh

If you don't want to export object by object then use a qml registertype and then create the objects in QML. setContextProperty is one of the many options that Qt offers for a good integration between C ++ and QML, it will depend on your requirements if you choose one or the other, read https://doc.qt.io/qt-5/qtqml-cppintegration-overview.html.

[Anas_Deshmukh]

@eyllanesc creating object in qml is not suitable to my application at this moment.

[DBoosalis]

@Anas_Deshmukh

If your reason for not creating them in QML is that you want a handle to the objects. You could always pass them back to your C++ code though an Q_INVOCABLE method. Consider:

Controller : public QObject
{
Q_INVOKABLE void addGuage(TempGauge *);

private:
QVector<TemperatureGauge *> m_gauges;
};