Qt: I defined and registered a QML singleton in C++. How do I access this singleton from C++?

[Stefan Monov76]

I had no problems with defining and registering a QML singleton in C++, and using that singleton from QML. Now, however, I want to call methods on that singleton from C++ as well. How do I get access to the single instance that the singleton has?

For now I've done the following hack:

FileIO* fileIO;

static QObject* fileIOSingletonTypeProvider(QQmlEngine *engine, QJSEngine *scriptEngine) {
    Q_UNUSED(engine)
    Q_UNUSED(scriptEngine)

    FileIO* instantiatedObj = new FileIO();
    ::fileIO = instantiatedObj;
    return instantiatedObj;
}

// ...

qmlRegisterSingletonType<FileIO>("FileIO", 1, 0, "FileIO", fileIOSingletonTypeProvider);

So I store a global pointer to it at the point of instantiation.

This works. Is there a proper way to do it, though?

[p3c0]

@Stefan-Monov76 Another way would be to assign it to QtObject in QML and the access this property in C++. Something like:

//QML
property QtObject obj: MySingleTonObject 

//C++
QQmlApplicationEngine engine;
engine.load(QUrl(QStringLiteral("qrc:///main.qml")));
QObject *obj = qvariant_cast<QObject *>(engine.rootObjects().first()->property("obj"));

[Schluchti]

@Stefan-Monov76 you could also use something like that:

class FileIO: public QObject{
    Q_OBJECT
public:
    //singleton type provider function for Qt Quick
    static QObject* fileIOSingletonTypeProvider(QQmlEngine *engine, QJSEngine *scriptEngine);
    //singleton object provider for C++
    static FileIO* instance();
    void myFancyFunction();
};

QObject* FileIO::fileIOSingletonTypeProvider(QQmlEngine *engine, QJSEngine *scriptEngine){
    Q_UNUSED(engine)
    Q_UNUSED(scriptEngine)
    return FileIO::instance();
}

FileIO* FileIO::instance() {
    static FileIO* fileIO = new FileIO();
    return fileIO;
}

void FileIO::myFancyFunction(){
   //do something
}

In that case you could get the singleton, with:

FileIO::instance()->myFancyFunction();