CurrentIndex of ComboBox from QML to C++ code

Question

I would like to get the currentIndex of a ComboBox in QML.

How can I get that?

From C++, I tried to have a Q_INVOKABLE function to call from QML code when onCurrentIndexChanged but it seems I can only pass QObject* as parameters in these functions in QML. So, it seems I can not pass just a int, or I get the error

"Could not convert argument 0 at"
...
"Passing incompatible arguments to C++ functions from JavaScript is dangerous and deprecated."
"This will throw a JavaScript TypeError in future releases of Qt!"

I tried to look in internet, but I did not find anything straighforward. I can not believe there is no trivial way to get the currentIndex from the C++ code. Maybe I am using QML in a wrong way.

A link to some example would also be appreciate. The ComboBox model is defined in C++ and it is a QList<MyObject> but MyObject is NOT a QObject.

Answer 1

There are many ways of solving this. One is to register a handler class and call a slot every time you change the current index.

Example handler class:

#pragma once
#include <QObject>
#include <QDebug>

class ComboBoxHandler: public QObject {
    Q_OBJECT
public slots:
    void onItemChanged(int newValue) {
        qDebug() << "Value: " << newValue;
    }
};

Register it with the engine in your main.cpp

qmlRegisterType<ComboBoxHandler>("ComboBoxHandler", 1, 0, "ComboBoxHandler");

Use it in your QML file:

ComboBoxHandler {
    id: handler
}

ComboBox {
     textRole: "text"
     valueRole: "value"

     // When an item is selected, update the backend.
     onActivated: handler.onItemChanged(currentIndex)

     model: [
         { value: Qt.NoModifier, text: qsTr("No modifier") },
         { value: Qt.ShiftModifier, text: qsTr("Shift") },
         { value: Qt.ControlModifier, text: qsTr("Control") }
     ]
}

This is also partly the example code provided in the QML help: ComboBox docu

For more insiged and other solutions, start here: Integrate QML with C++