Passing objects from C++ to QML

[MTK358]

I am having trouble understanding how objects in QML work. I see that it's possible to register a QObject subclass so that QML can make instances of it, but is it possible to make an instance of a QObject in C++ and pass it to QML? If so, how can signal handlers be connected to it, and how is the memory managed?

This is the example I tried, but it fails with this error message:

QMetaObject::invokeMethod: No such method
QQuickWindowQmlImpl_QML_0::myCallback(QObject*)
Candidates are:
    myCallback(QVariant)

This is the C++ code:

QQmlApplicationEngine engine;
engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
QObject *rootObj = engine.rootObjects().at(0);
QObject *myObj = new QObject();
myObj->setProperty("myStringProperty", "asdfasdf");
QMetaObject::invokeMethod(rootObj, "myCallback", Q_ARG(QObject*, myObj));

And the QML code:

import QtQuick 2.4
import QtQuick.Window 2.2
Window {
    visible: true
    Rectangle {
        id: root
        anchors.fill: parent
        color: "gray"
        Text {
            id: label
            anchors.centerIn: parent
            text: "(waiting for callback)"
        }
    }
    function myCallback(obj) {
        label.text = obj.myStringProperty
    }
}

When I try replacing QObject* with QVariant in the Q_ARG, that causes a C++ compiler error.

If this is not possible, then how else can a QML user interface be made aware of dynamically added and removed entities in the C++ code, each with some properties and events associated with them, which need a representation in the UI?

[JKSH]

Hi @MTK358,

is it possible to make an instance of a QObject in C++ and pass it to QML? If so, how can signal handlers be connected to it

Yes. Set your C++ QObject as a Context Property: http://doc.qt.io/qt-5/qtqml-cppintegration-contextproperties.html

That link also shows you how to make connections.

and how is the memory managed?

Your C++ code remains responsible for deleting that QObject: http://doc.qt.io/qt-5/qqmlcontext.html#setContextProperty

[MTK358]

@JKSH said:

Yes. Set your C++ QObject as a Context Property: http://doc.qt.io/qt-5/qtqml-cppintegration-contextproperties.html

That link also shows you how to make connections.

Thank you, that's what I was looking for.

Your C++ code remains responsible for deleting that QObject: http://doc.qt.io/qt-5/qqmlcontext.html#setContextProperty

What if QML connects to a signal of a context property object, that signal has a QObject* as an argument, and the JS handler code saves it in some item's property?

Will deleting it cause a segfault in the QML interpreter? Is there a way to know that it's not referenced any more?

[GTDev]

There are several ways how to communicate between C++ and QML. The most important part is to set up your class with signals and slots to be used in QML.

A simple guide with an example project can be found here:
https://v-play.net/cross-platform-development/how-to-expose-a-qt-cpp-class-with-signals-and-slots-to-qml#how-to-communicate-between-cpp-and-qml