Access to an After Loaded View From Qml To C++

[yohannvf]

Hello ! I'm new in Qt Devlopment and I have a short question that certainly won't take long time to resolve for such specialists ;)

I made a Loader to display a splashscreen that will load the main.qml thereafter like this:

@
QDeclarativeView view;
view.setSource(QUrl("MainAppLoader.qml"));
view.setResizeMode(QDeclarativeView::SizeRootObjectToView);
view.showFullScreen();@

I would like to connect some stuff to a child of my PageStackWindows in main.qml : mainMenu.qml

The problem if I do that :

@ QObject rootObject = dynamic_cast<QObject>(view.rootObject());
QObject mainMenu = rootObject->findChild<QObject>("mainMenu");
QObject::connect(&myClass, SIGNAL(data(QVariant)), mainMenu, SLOT(updateData(QVariant)));@

I get this error: QObject::connect: Cannot connect <MyClass>::data(QVariant) to (null)::updateData(QVariant)

Yes mainMenu is null. Of course I put: objectName: "mainMenu"

In my MainAppLoader.qml I have this that load the main.qml :

@Loader {
id: mainLoader
width: parent.width
height: parent.height
}

    Timer {
        id: firstPhaseTimer
        interval: 700
        running: true
        repeat: false

        onTriggered: {
            if (!mainLoader.Loading) {
                // Start to load the main application
                mainLoader.source = Qt.resolvedUrl("main.qml");
                secondPhaseTimer.start();
            }
        }
    }@

Do you know how I could get / use the reference of my main.qml / MainMenu.qml freshly created to connect what I want from cpp ?

If I do the same without my splashScreenLoader, it works.

Thanks !

[sierdzio]

You are probably asking for connection too early. Either emit a signal to C++ once loading is ready, or make the connection in QML.

[yohannvf]

You say it's possible to connect a freshly created QML View with CPP in QML ?

In other words, I just need to warn Cpp with a signal when my main.qml is created mainLoader.source = Qt.resolvedUrl("main.qml");
to tell him to make the rootObject->findChild<QObject*>("mainMenu"); and connect it ? that's all ?

I'l try tomorow, thank you for the quick anwser !

[sierdzio]

I think so. To be honest I did not take time to read through and fully understand your post, I just gave a first impression.

The definite point is that object needs to be ready before meta objects can be connected.

BTW. you don't need the dynamic_cast line, root object should have ::findChild() available.

[feldifux]

As I see it, you have the following options:

1.) Call a C++ slot after the Loader finished, as sierdzio suggested
You can call a C++ function from QML by registering the C++ object as a context property with @QDeclarativeContext::setContextProperty()@ and then call the slot function of this object. This is useful if the object you are calling only exists once, i.e. a Singleton.

2.) By registering the C++ item as a QML item. You can do that with @qmlRegisterType<CppClassName>("NamespaceForImport", 1, 0 "QMLItemName")@

In that way, you could also directly access the signal from QML, e.g. if the signal is called @dataChanged(QVariant newData)@

, you can access it in QML with
@
import NamespaceForImport 1.0

QMLItemName {
onDataChanged: { ... "newData" is available as parameter ... }
}
@

[yohannvf]

Hello again, I think I'm doing it wrong

I made a call to my C++ object when QML completed the launched of MainMenu.qml like this:

in main.cpp:

@QDeclarativeView view;
view.rootContext()->setContextProperty("MyClass",myClass);@

in MainMenu.qml

@Component.onCompleted: {
MyClass.mainMenuCreated();
}@

in MyClass c++

@void MyClass::mainMenuCreated()
{
qDebug() << "Hello I'm created";

}@

This is working but now, How can I have access to QDeclarativeView / QDeclarativeContext from MyClass to connect my freshly created qml file with c++ signal ? I only have access in main.cpp and tried to pass it in MyClass constructor, but it seems private. I think I'm doing something wrong :D

[feldifux]

You should be able to pass it with the constructor - what do you mean that it is private? You have control over MyClass implementation, so you can just do something like the following:
@
//in main.cpp:
QMLApplicationViewer viewer;

MyClass myClass = new MyClass(viewer);

//and in MyClass constructor you can access the QDeclarativeEngine from the QMLApplicationViewer with:

MyClass::MyClass(QMLApplicationViewer &viewer) {
QDeclarativeEngine *engine = viewer.engine();
QDeclarativeContext *context = engine->rootContext;
}
@

From the QDeclarativeView you can call the method rootContext() to get the context in your MyClass.

Alternatively, to receive the context from a QDeclarativeItem which you instantiate from QML, you can use the following:
@QDeclarativeContext *currentContext = QDeclarativeEngine::contextForObject(this);
QDeclarativeEngine *declarativeEngine = currentContext->engine();
QDeclarativeContext *rootContext = declarativeEngine->rootContext();@

[yohannvf]

I surely made a mistake somewhere :(

In main.cpp

@QDeclarativeView view;
MyClass *myClass = new MyClass(view);
@

MyClass.h

@class MyClass : public QObject
{
Q_OBJECT
public:
explicit MyClass();
explicit MyClass(QDeclarativeView &view);

signals:

public slots:

private:
QDeclarativeView mQDeclarativeView;

};@

MyClass.cpp

@MyClass::MyClass() {
}

MyClass::MyClass(QDeclarativeView &view) {
mQDeclarativeView = view;
}@

Got this error:

@ error : 'QDeclarativeView& QDeclarativeView::operator=(const QDeclarativeView&)' is private
@

[sierdzio]

Use a pointer. You are trying to use an assignment operator while all QObject children cannot be copied.

[yohannvf]

It was what I did first but you gave me an example with references :P

It's working now :)

The final code for visitors:

@void MyClass::mainMenuCreated()
{
qDebug() << "Hello I'm created";

QObject *menu = (mQDeclarativeView->rootObject())->findChild<QObject*>("mainMenu");
QObject::connect(this, SIGNAL(data(QVariant)), menu, SLOT(updateData(QVariant)));

}@

And now my function from MainMenu.qml is called :)

@updateData(text) { console.log("I'm here"); }
@

If you see any improvment that could be made or other better technique, don't hesitate :)

Thank you so much for this lesson :P

[feldifux]

No worries :)

As a tiny improvement, it's good practice to name context properties with a small first letter to be able to distinguish them from classes/QML components. So instead of "MyClass", name it "myClass" - you could access enums for instance from QML with "MyClass.enumName" then, and so don't mix it up with a context property access.

Also, the data-signal could be renamed to dataChanged, as it is probably reflects more what it does. But that's syntactic sugar, just a suggestion..