Unable to create qml component with C++ property passed as parameter

[Cyrille de Brebisson]

Hello,

I have a C++ signal that passes a C++ object (type registered with QML), and is caught by a QML component.
This QML component,creates another QML component which must have this C++ object as a property.
However, I am unable to make it work :-(

I have tried 2 different things which you can see in the code bellow (I am not showing the C++ as it is insubstancial for the moment)

So, what is the proper way to "connect" a QML component with an existing C++ object?

ApplicationWindow {
visible: true; width: 640; height: 480

Component { // Test version 1: Does not work as the "text" property initialization generates an error
    id: myTabButton
    TabButton {
        property HPACDevice device
        id: button
        text: device.value.toString() // causes: "TypeError: Cannot read property 'value' of null"
    }
}

Component { // Test version 2: Does "work", but does not do what I want as I end up with 2 instances of HPACDevice
    id: myTabButton2
    TabButton {
        HPACDevice
        { id: device }
        id: button
        text: device.value.toString() // works, but I end up with 2 HPACDevice being instanciated!
    }
}

HPACgroup {
    id: group
    onNewDevice: { // respond to: void HPACgroup::newDevice(HPACDevice &device);
        myTabButton.createObject(row, {"device": device}); // test 1. Causes error
        myTabButton2.createObject(row, {"device": device}); // test 2: cause 2 instances of HPACDevice
    }
}

Column {
    id: row
    Button { // Just a button that tells the C++ group to add a device. In "real life", the devices will be created in reaction to OS events (USB Plugged detected in C++)
        width: 100; height: 100; text: "add"
        onClicked: group.addDevice() // Causes the C++ group create another "device".
    }
}

}

Thanks,
Cyrille

[ambershark]

Here is how you create a C++ object and then use it in QML.

You don't need to register the object either.

#pragma once
 
#include <QObject>
 
class Foo : public QObject
{
   Q_OBJECT
 
public:
   explicit Foo(QObject *parent = nullptr) : QObject(parent) { }
   Q_INVOKABLE QString hello()
   {
      return "hello world";
   }
};

#include <QQmlApplicationEngine>
#include <QQmlComponent>
#include "foo.h"
 
int main(int ac, char **av)
{
    QApplication app(ac, av);
     
    // load qml engine and component
    auto eng = new QQmlApplicationEngine();
    auto comp = new QQmlComponent(eng, QUrl("qrc:///qml/main.qml"));
    auto obj = comp->create();
    if (!obj)
        return;
 
    // No registration required for this ...
    auto foo = new Foo();
    eng->rootContext()->setContextProperty("foo", foo);
 
    return app.exec();
}

And here is the QML:

import QtQuick 2.11
import com.ambershark.example.foo 1.0
 
Rectangle {
    width: 300
    height: 300
 
    Text {
       text: qsTr("Click me")
       anchors.centerIn: parent
 
       MouseArea {
          anchors.fill: parent
          onClicked: { console.log(foo.hello())
       }
    }
}

[Cyrille de Brebisson]

Hello,

Thanks, this is actually valuable information as I was trying to do something similar!

However, it does not what I was trying to achieve here.
What I was trying to achieve was the creation of a QML object in response to the creation of a C++ object, with said C++ object being a "typed property" of the QML object, not a global variable (of course, multiple such objects will be instantiated)

One of the reason why I would like it to be a registered property is so that, in QML, I get dynamic completion and other development niceties.

Cyrille