What is the most formal way to invoke C++ code in response to a QML button?

Question

Using qt 5.5, qt quick controls 1.4 and the below qt creator boilerplate code: what is the most FORMAL way to invoke C++ code in response to a button (just debug text to screen)?

// main cpp
#include <QApplication>
#include <QQmlApplicationEngine>

int main(int argc, char *argv[])
{
     QApplication app(argc, argv);

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

     return app.exec();
}

and the QML file inside the qml.qrc:

import QtQuick 2.5
import QtQuick.Controls 1.4

ApplicationWindow {
    visible: true
    width: 640
    height: 480
    title: qsTr("Hello World")


    Button {
        id: add
        x: 248
        y: 222
        text: qsTr("add")
    }
}

I am aware of this as a possible answer but it looks a very complicated way to just hook a button to a code! If this is The Formal way to use Qt 5.5 and QML then this should be the answer.

Answer 1

As you can see in the documentation, you have many options:

• The class can be registered as an instantiable QML type. This was the option proposed by @BaCaRoZzo
• The class can be registered as a Singleton Type
• An instance of the class can be embedded into QML code as a context property or context object
• The Qt QML module also provides ways to do the reverse and manipulate QML objects from C++ code. This was the option proposed by @hyde

In your case, I'd prefer the last option because it requires fewer lines of code.

Example:

main.cpp

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

int main(int argc, char *argv[])
{
     QGuiApplication app(argc, argv);

     QQmlApplicationEngine engine;
     engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
     QObject *item = engine.rootObjects().first();

     MyClass myClass;
     QObject::connect(item, SIGNAL(qmlSignal(QString)),
                      &myClass, SLOT(cppSlot(QString)));

     return app.exec();
}

main.qml

import QtQuick 2.5
import QtQuick.Controls 1.4

ApplicationWindow {
    visible: true
    width: 640
    height: 480
    title: qsTr("Hello World")

    signal qmlSignal(string msg)

    Button {
        id: add
        x: 248
        y: 222
        text: qsTr("add")
        onClicked: qmlSignal(text)
    }
}

myclass.h

#ifndef MYCLASS_H
#define MYCLASS_H

#include <QObject>
#include <QDebug>

class MyClass : public QObject
{
    Q_OBJECT
public slots:
    void cppSlot(const QString &msg) {
        qDebug() << "Called the C++ slot with message:" << msg;
    }
};

#endif // MYCLASS_H