Accessing QMLEngine /rootObject properties in .cpp files other than main.cpp - c++

Question

I have two radio buttons in one panel defined in one .qml file.
I need to access the property whether it is checked or not in another QML file or in .cpp file of some c++ class.
I am able to do it in main.cpp
using these lines below
QQmlApplicationEngine engine;
engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
if(engine.rootObjects().isEmpty())
return -1;
// Step 1: get access to the root object
QObject *rootObject = engine.rootObjects().first();
QObject *qmlObject_serial_radio = rootObject->findChild<QObject*>("serial_radio");
QObject *qmlObject_tcpip_radio = rootObject->findChild<QObject*>("tcpip_radio");
// Step 2a: set or get the desired property value for the root object
qDebug() << qmlObject_serial_radio->property("checked");
qDebug() << qmlObject_tcpip_radio->property("checked");
But I want to do the same in some other .cpp file.
how to do it?

Answer 1

Instancing QML objects in C++ can be dangerous since the life cycle of the items is not handled directly in C++ so QML could eliminate them without notifying it as for example the StackView where Pages are created and deleted.
A better approach is to export the C ++ object to the QML:
main.cpp
#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QQmlContext>
#include <QDebug>
class Helper: public QObject{
Q_OBJECT
Q_PROPERTY(bool isSerialEnabled READ isSerialEnabled WRITE setIsSerialEnabled NOTIFY isSerialEnabledChanged)
Q_PROPERTY(bool isTcpIpEnabled READ isTcpIpEnabled WRITE setIsTcpIpEnabled NOTIFY isTcpIpEnabledChanged)
public:
using QObject::QObject;
bool isSerialEnabled() const{
return mIsSerialEnabled;
}
void setIsSerialEnabled(bool isSerialEnabled){
if(mIsSerialEnabled == isSerialEnabled) return;
mIsSerialEnabled = isSerialEnabled;
emit isSerialEnabledChanged(mIsSerialEnabled);
}
bool isTcpIpEnabled () const{
return mIsTcpIpEnabled ;
}
void setIsTcpIpEnabled (bool isTcpIpEnabled ){
if(mIsTcpIpEnabled == isTcpIpEnabled ) return;
mIsTcpIpEnabled = isTcpIpEnabled ;
emit isTcpIpEnabledChanged(mIsTcpIpEnabled );
}
signals:
void isSerialEnabledChanged(bool);
void isTcpIpEnabledChanged(bool);
private:
bool mIsSerialEnabled;
bool mIsTcpIpEnabled ;
};
int main(int argc, char *argv[])
{
QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
QGuiApplication app(argc, argv);
Helper helper;
// check changes
QObject::connect(&helper, &Helper::isSerialEnabledChanged, [](bool isSerialEnabled){
qDebug()<<"isSerialEnabled"<<isSerialEnabled;
});
QObject::connect(&helper, &Helper::isTcpIpEnabledChanged, [](bool isSerialEnabled){
qDebug()<<"isTcpIpEnabledChanged"<<isSerialEnabled;
});
QQmlApplicationEngine engine;
engine.rootContext()->setContextProperty("helper", &helper);
engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
if (engine.rootObjects().isEmpty())
return -1;
return app.exec();
}
#include "main.moc"
main.qml
RadioButton {
text: "Serial"
onCheckedChanged: helper.isSerialEnabled = checked
}
RadioButton {
text: "tcpip"
onCheckedChanged: helper.isTcpIpEnabled = checked
}