QML instantiates C++ objects. How do I access their methods?

Question

Here's what my main.cpp looks like:

int main(int argc, char **argv)
{
    QGuiApplication app(argc, argv);
    QCoreApplication::addLibraryPath("./");

    QQuickView view;
    view.setResizeMode(QQuickView::SizeRootObjectToView);
    view.setSource(QUrl("qrc:/myqml.qml"));
    view.show();

    return app.exec();
}

As you can see, it creates things from myqml. Well, myqml instantiates a C++ class MyClass.

How do I access this C++ methods from the object QQuickView view? For example, I'd like to do something of the type view.MyClassInstance.myMethod1()

@eyllanesc I don't have them yet, it was just an example to see if it's possible to do what I want. MyClass is simply a child of QObject and myqml is simply MyClass{} — Poperton
– Poperton, Commented Feb 18, 2019 at 1:00

eyllanesc · Accepted Answer · 2019-02-18 01:28:40Z

You want to obtain an object created in QML by C++, here it does not matter if the target has a prototype created in C ++ or not. If you want this you must obtain the object through findChild since all the objects created in QML have a kinship relationship with the window.

main.cpp

#include <QtQuick>
#include <QtGui>

class MyClass: public QObject
{
    Q_OBJECT
public:
    using QObject::QObject;
    Q_INVOKABLE void invokable(){
        qDebug()<< "invokable";
    }
    Q_SLOT void slot(){
        qDebug()<< "slot";
    }
    void function(){
        qDebug()<< "function";
    }
};

int main(int argc, char *argv[])
{
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
    qmlRegisterType<MyClass>("foo", 1, 0, "MyClass");
    QGuiApplication app(argc, argv);

    QQuickView view;
    view.setResizeMode(QQuickView::SizeRootObjectToView);
    view.setSource(QUrl(QStringLiteral("qrc:/main.qml")));
    view.show();
    if(MyClass* myclass_instance = view.findChild<MyClass *>("myclass_instance")){
        myclass_instance->invokable();
        myclass_instance->slot();
        myclass_instance->function();
    }
    return app.exec();
}
#include "main.moc"

main.qml

import QtQuick 2.9
import foo 1.0

Rectangle {
    color: "salmon"
    width: 640
    height: 480
    MyClass{
        objectName: "myclass_instance"
    }
}

But this method has several drawbacks such as who manages the life cycle of the object is QML, not C ++ so the pointer could at some point point to an unreserved address. Another disadvantage is that there is a dependency of C++ to QML since if the objectName is changed in QML the code in C ++ would have to be changed.

Another approach is to create a helper class that is exported to QML with setContextProperty() and that interacts with the MyClass object.

main.cpp

#include <QtQuick>
#include <QtGui>

class MyClass: public QObject
{
    Q_OBJECT
public:
    using QObject::QObject;
    Q_INVOKABLE void invokable(){
        qDebug()<< "invokable";
    }
    Q_SLOT void slot(){
        qDebug()<< "slot";
    }
    void function(){
        qDebug()<< "function";
    }
};

class Helper: public QObject
{
    Q_OBJECT
public:
    using QObject::QObject;
    void call_function(){
        emit called();
    }
    Q_SIGNAL void called();
};

int main(int argc, char *argv[])
{
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
    qmlRegisterType<MyClass>("foo", 1, 0, "MyClass");
    QGuiApplication app(argc, argv);

    Helper helper;

    QQuickView view;
    view.rootContext()->setContextProperty("helper", &helper);
    view.setResizeMode(QQuickView::SizeRootObjectToView);
    view.setSource(QUrl(QStringLiteral("qrc:/main.qml")));
    view.show();

    helper.call_function();
    return app.exec();
}
#include "main.moc"

main.qml

import QtQuick 2.9
import foo 1.0

Rectangle {
    color: "salmon"
    width: 640
    height: 480
    MyClass{
        id: myclass
    }
    Connections{
        target: helper
        onCalled:{
            myclass.invokable()
            myclass.slot()
        }
    }
}

The advantage of this method is that there is no dependence between the logic of C++ and QML, besides the life cycle does not generate problems since the myclass objects are not handled directly in QML. The disadvantage is that you write a little more code and you can only call the Q_INVOKABLE or Q_SLOT.

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