Reading value from QML in C++

[Raad]

I am trying to read an int value from my QML side on c++ but this value remains 0 and doesn't update on c++ side while console.log() clearly shows me that it increments after each click . What am I doing wrong here?

.qml file

FocusScope {

        property alias myrecordint: startbutton.recordint

        Rectangle {
            id: buttonPaneShadow
            width: bottomColumn.width + 16
            height: parent.height
            anchors.top: parent.top
            anchors.right: parent.right
            color: "white"

            Column {
                anchors {
                    right: parent.right
                    top: parent.top
                    margins: 8
                }

                id: buttonsColumn
                anchors.rightMargin: 20
                anchors.topMargin: 20
                spacing: 12

                CameraButton {
                    id: startbutton
                    property int recordint:0
                    text: "Record"
                    visible: camera.videoRecorder.recorderStatus == CameraRecorder.LoadedStatus
                    onClicked:
                    {
                        recordint=recordint+1
                        camera.videoRecorder.record()
                        console.log(recordint)

                    }
                }
           }
      }
 }

.cpp file

QQmlEngine engine;
QQmlComponent component(&engine, QUrl("qrc:/VideoCaptureControls.qml"));
QObject *object = component.create();
qDebug() << "Property value:" << QQmlProperty::read(object, "myrecordint").toInt();

[J.Hilk]

hi @Raad

You're actually not supposed to manipulate the Properties this way.
Take a look at this topic, on how you should do it https://forum.qt.io/topic/72407/connect-qml-signal-with-c-slot

or this basic example, if you really want to listen directly to signals from your main.cpp:

//main.qml
import QtQuick 2.11
import QtQuick.Window 2.2
import QtQuick.Controls 2.4

Window {
    visible: true
    width: 640
    height: 640

    property int counter: 0
    onCounterChanged: changed(counter)

    signal changed(int counter)

    Timer{
        running: true
        interval: 1000
        repeat: true
        onTriggered: counter++
    }
}

//main.cpp
#include <QGuiApplication>
#include <QQmlApplicationEngine>

#include "QQmlContext"
#include <QDebug>
#include "backend.h"

int main(int argc, char *argv[])
{
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
    QGuiApplication app(argc, argv);
    QQmlApplicationEngine engine;

    const QUrl url(QStringLiteral("qrc:/main.qml"));
    engine.load(url);

    QObject *rootItem = engine.rootObjects().first();

    Backend b;
    QObject::connect(rootItem, SIGNAL(changed(int)), &b, SLOT(onValueChanged(int)));

    return app.exec();
}

//backend.h
#ifndef BACKEND_H
#define BACKEND_H

#include <QObject>
#include <QDebug>

class Backend : public QObject
{
    Q_OBJECT
public:
    explicit Backend(QObject *parent = nullptr): QObject(parent) {}

signals:

public slots:
    void onValueChanged(int value){qDebug() << value;}
};

#endif // BACKEND_H

[J.Hilk]

hi @Raad

You're actually not supposed to manipulate the Properties this way.
Take a look at this topic, on how you should do it https://forum.qt.io/topic/72407/connect-qml-signal-with-c-slot

or this basic example, if you really want to listen directly to signals from your main.cpp:

//main.qml
import QtQuick 2.11
import QtQuick.Window 2.2
import QtQuick.Controls 2.4

Window {
    visible: true
    width: 640
    height: 640

    property int counter: 0
    onCounterChanged: changed(counter)

    signal changed(int counter)

    Timer{
        running: true
        interval: 1000
        repeat: true
        onTriggered: counter++
    }
}

//main.cpp
#include <QGuiApplication>
#include <QQmlApplicationEngine>

#include "QQmlContext"
#include <QDebug>
#include "backend.h"

int main(int argc, char *argv[])
{
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
    QGuiApplication app(argc, argv);
    QQmlApplicationEngine engine;

    const QUrl url(QStringLiteral("qrc:/main.qml"));
    engine.load(url);

    QObject *rootItem = engine.rootObjects().first();

    Backend b;
    QObject::connect(rootItem, SIGNAL(changed(int)), &b, SLOT(onValueChanged(int)));

    return app.exec();
}

//backend.h
#ifndef BACKEND_H
#define BACKEND_H

#include <QObject>
#include <QDebug>

class Backend : public QObject
{
    Q_OBJECT
public:
    explicit Backend(QObject *parent = nullptr): QObject(parent) {}

signals:

public slots:
    void onValueChanged(int value){qDebug() << value;}
};

#endif // BACKEND_H

[Raad]

@J-Hilk Thank you so much for your answer, link and example! but in this case I'm not trying to connect signal and slots between QML and c++. just need to pass the most recent value of QML object to c++. If I'm wrong please correct me because I don't know much about QML :)

[J.Hilk]

@Raad

just need to pass the most recent value of QML object to c++

But that's what signal and slots are for. To pass data from one object to an other, (void if it's just a signal without data 😉 )

Currently this, from your opening post:

QQmlEngine engine;
QQmlComponent component(&engine, QUrl("qrc:/VideoCaptureControls.qml"));
QObject *object = component.create();
qDebug() << "Property value:" << QQmlProperty::read(object, "myrecordint").toInt();

only reads the property once, just after creation. You'll have to regularly call QQmlProperty::read(object, "myrecordint").toInt(); if you want new data without signals

via a QTimer for example

[Raad]

@J-Hilk Thank you so much for your detailed response! now it is all clear to me!!!