Trouble when using QObject::connect to connect QML signal to Cpp slot

Aymeric_Qt

Hello,

I try to create a very simple interaction between a QML item with a 'TextField' and cpp class.

QML item - WebRadioUrlSettingForm.qml

TextField {
            id: urlField
            implicitWidth: parent.width
            placeholderText: "Enter URL here..."
            onAccepted: urlForm.setWebRadioUrl(urlField.getText())
        }

Cpp class - Player.cpp

class Player : public QObject
{
    Q_OBJECT
    QML_ELEMENT
    QML_SINGLETON

public:
    explicit Player(QObject *parent = nullptr);

    //some variables and function

public slots:
    void setWebRadioUrl(QString newUrl);

The problem is when I try to connect those in the main.cpp class as it is shown in the documentation

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

    QScopedPointer<Player> player(new Player);

    QQmlApplicationEngine engine;

    QQuickView view(QUrl::fromLocalFile("WebRadioUrlSettingForm.qml"));
    QQuickItem* item = view.rootObject();

    QObject::connect(item, SIGNAL(setWebRadioUrl(QString)), player, SLOT(setWebRadioUrl(QString)));

    qmlRegisterSingletonInstance("Player", 1, 0, "Player", player.get());

    const QUrl url(u"qrc:/POCWebRadioPlayer/main.qml"_qs);
    QObject::connect(&engine, &QQmlApplicationEngine::objectCreated,
                     &app, [url](QObject *obj, const QUrl &objUrl) {
        if (!obj && url == objUrl)
            QCoreApplication::exit(-1);
    }, Qt::QueuedConnection);
    engine.load(url);

    return app.exec();
}

On this line:

QObject::connect(item, SIGNAL(setWebRadioUrl(QString)), player, SLOT(setWebRadioUrl(QString)));

I get this error:
"no matching function for call to 'QObject::connect(QQuickItem*&, const char*, QScopedPointer<Player>&, const char*)'"

Is it because I made a mistake on the connect function parameters? Or because the receiver (the 'player' class) is a scoped pointer? Or because there is two QObject::connect, which are refering to different functions in the qobject.h file, in the same function? Or something else?

Thanks for reading.

Have a nice day

GrecKo

@Aymeric_Qt Don't do that. Here's why:
https://doc.qt.io/qt-6/qtquick-bestpractices.html#interacting-with-qml-from-c
http://doc.qt.io/qt-6/qtqml-cppintegration-overview.html#interacting-with-qml-objects-from-c
https://youtu.be/vzs5VPTf4QQ?t=23m20s

Just call your C++ object slot from QML: onAccepted: Player.setWebRadioUrl(urlField.getText())

Or you could do that in your WebRadioUrlSettingForm file and do:
onSetWebRadioUrl: url => Player.setWebRadioUrl(url)

J.Hilk

@Aymeric_Qt said in Trouble when using QObject::connect to connect QML signal to Cpp slot:

QScopedPointer

thats not a valid overload for the connect statement you have to use the QObjecte class object, so get it via data()

Aymeric_Qt

Hi @J-Hilk
I've just add the .data() but I still get the same error. However it is certainly a solution to the first part of this problem.

QObject::connect(item, SIGNAL(setWebRadioUrl(QString)), player.data(), SLOT(setWebRadioUrl(QString)));

Beside I have a another question: why can't I write "QObject* item = view.rootObject();" (like they do in the documentation) ?
Because "view.rootObject()" returns a pointer to a QQuickItem which inherits from QObject ? Can it be a problem too ?

jsulm

@Aymeric_Qt said in Trouble when using QObject::connect to connect QML signal to Cpp slot:

why can't I write

Cast the pointer returned by rootObject() to QObject*

GrecKo

@Aymeric_Qt Don't do that. Here's why:
https://doc.qt.io/qt-6/qtquick-bestpractices.html#interacting-with-qml-from-c
http://doc.qt.io/qt-6/qtqml-cppintegration-overview.html#interacting-with-qml-objects-from-c
https://youtu.be/vzs5VPTf4QQ?t=23m20s

Just call your C++ object slot from QML: onAccepted: Player.setWebRadioUrl(urlField.getText())

Or you could do that in your WebRadioUrlSettingForm file and do:
onSetWebRadioUrl: url => Player.setWebRadioUrl(url)

Aymeric_Qt

@GrecKo
Thank you for your reply. I saw the same warning ("Warning: Although it is possible to access QML objects from C++ and manipulate them, it is not the recommended approach...") in this page but I thought it was only applied to this section.
Which, thinking about this now, does not make any sense because in order to connect to a QML signal I need to access the QML object by name so indeed it was not a good idea/practice (the video you linked is very clear about this and very interesting btw).

I think that calling the cpp object slot from the QML will be much easier!

Thank you all for your answers