qmlRegisterSingletonType和多个顶级窗口

Question

I have a QGuiAppplication derived class (called Sy_application ) that I've wrapped with another (called Sy_application_qml , with extra some QML specific features) and then registered that as a singleton with the QML engine.

This all works fine from the first opened top-level window, but the second (and presumably any more) only gets null from the singleton. Adding some debug:

// The static factory method used in the qmlRegisterSingletonType call.
QObject* Sy_application_qml::factory( QQmlEngine* engine,
                                      QJSEngine* scriptEngine )
{
    Q_UNUSED( engine )
    Q_UNUSED( scriptEngine )

    qDebug() << "Creating";
    return new Sy_application_qml();
}

// SY_APP is the qApp macro casted to my Sy_application type.
QObject* Sy_application_qml::get()
{
    qDebug() << "Getting:" << SY_APP;
    return SY_APP;
}

// This is an example of it's use within QML
onPositiveClicked: {
    console.log( Sy_application_qml );
    Sy_application_qml.get().newProject( sampleRate.value, frameRate.value );
    close();
}

And the debug output:

// Opening first window.
Creating

// Creating second
qml: Sy_application_qml(0x1895b30)
Getting: Sy_application(0x7fff80f051c0)

// Attempting to create third
qml: Sy_application_qml(0x1895b30)
Getting: Sy_application(0x7fff80f051c0)
qrc:/qml/gui/dialogs/Sy_newProjectDialog.qml:62: TypeError: Cannot call method 'newProject' of null

As you can see the singleton is still present and the C++ Sy_application instance is being returned correctly, but is appearing as null on the QML side. Any reason why it works for one window and not another?

Answer 1

So I finally worked it out, and it had nothing to do with QML singletons or having multiple top-level windows - but it had everything to do with passing QObject pointers to the QML engine from Q_INVOKABLE methods (the Sy_application_qml::get() method in my case).

From the docs:

Objects not-created by QML have CppOwnership by default. The exception to this is objects returned from C++ method calls; in these cases, the ownership of the returned objects will be set to JavaScriptOwnerShip. Note this applies only to explicit invocations of Q_INVOKABLE methods or slots, and not to property getter invocations.

So the QML engine had deleted my Sy_application pointer. The solution was nice and simple:

QObject* Sy_application_qml::factory( QQmlEngine* engine,
                                      QJSEngine* scriptEngine )
{
    Q_UNUSED( engine )
    Q_UNUSED( scriptEngine )

    QQmlEngine::setObjectOwnership( SY_APP, QQmlEngine::CppOwnership );
    return new Sy_application_qml();
}