How to register C++ function to scxml ecmascript d

This question is deprecated

I have since converted my state machines to use an extra state to call the C++ function on enter and I trigger sendEvent when the function returns a value.

Old question is as follows:

I have a simple SCXML model designed in Qt. It is lacking some functionality such as listening to pub/sub queues, for which I want to register some functions into the QScxmlStateMachine instance so I can call them from the ECMAScript portion of the state machine. I took a look at the QScxmlEcmaScriptDataModel, but I don't understand how I can modify the evaluate functions to register callbacks. Is it possible to register C++ functions and callbacks to Qt scxml state machines? If so, how?

Additional information:

I created a class called RegisterJs, this is registerjs.h

#ifndef REGISTERJS_H
#define REGISTERJS_H

#include <QObject>
#include <QString>
#include <QQmlEngine>

class RegisterJS : public QObject
{
    Q_OBJECT
public:
    explicit RegisterJS(QObject *parent = nullptr);
    Q_INVOKABLE void showMessage(QString message);
signals:

public slots:
};

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

    RegisterJS *singletonClass = new RegisterJS();
    return singletonClass;
}

#endif // REGISTERJS_H

This is registerjs.cpp

#include "registerjs.h"
#include <QMessageBox>

RegisterJS::RegisterJS(QObject *parent) : QObject(parent)
{

}

void RegisterJS::showMessage(QString message){
    QMessageBox::information(0, message, message);
}

This is how I register the class:

qmlRegisterSingletonType<RegisterJS>("in.nurett", 1, 0, "RegisterJS", registerJSProvider);

This is the content of testflow.js:

function testShowMessage() { RegisterJS.showMessage("test"); }

This is the relevant content from testflow.scxml:

<scxml xmlns="http://www.w3.org/2005/07/scxml" version="1.0" binding="early" xmlns:qt="http://www.qt.io/2015/02/scxml-ext" name="teststate" qt:editorversion="4.6.2" initial="initialize">
    <state id="initialize">
        <onentry>
            <script>testShowMessage();</script>
        </onentry>
    </state>
    <script src=":testflow.js"/>

But the information message doesn't show up.