Mixed QML/C++ app (solved)

[LaurianeLt]

Hi,
I have created a mixed QML/C++ app, and I would like to be able to create a QML type object and add it to my main.qml as a child, from my main.cpp.

I have created it by using :
QQmlComponent component(&engine, "C:/....", QQmlComponent:PreferSynchronous);
QObject *object = component.create();
But now I would like to be able to specify that its parent should be my main.qml.

Any idea ?

Thanks !

[LaurianeLt]

I forgot to say that I also wrote :
QQmlEngine::setObjectOwnerShip(object, QQmlEngine::CppOwnerShip);
object -> setParent(&engine);

[JKSH]

Hi @LaurianeLt,

I forgot to say that I also wrote :
QQmlEngine::setObjectOwnerShip(object, QQmlEngine::CppOwnerShip);
object -> setParent(&engine);

That code sets the object parent, but you need to set the visual parent. See the differences here: http://doc.qt.io/qt-5/qtquick-visualcanvas-visualparent.html

To set the visual parent, use QQuickItem::setParentItem(). (you need to typecast object to QQuickItem first)

[LaurianeLt]

Hi @JKSH ,
Thank you for your answer, I will try that and let you know how it worked !

[LaurianeLt]

I did as you said but I still have some problems, here is what I wrote :

QQmlApplicationEngine engine;
...
QQmlComponent component (&engine, "C:/Qt/.../Rectangle.qml", QQmlComponent:PreferSynchronous);
QQuickItem * object = qobject_cast<QQuickItem*>(component.create());

QQmlComponent component1 (&engine, "C:/Qt/.../main.qml", QQmlComponent:PreferSynchronous);
QQuickItem * parent = qobject_cast<QQuickItem*>(component1.create());

object->setParent(&engine);
object->setParentItem(parent);
engine.load(QUrl(QStringLiteral("qrc:/main.qml)));

First I know that the way I create the "parent", when I run it, it opens two windows, the "main.qml" is launched twice. I would like to be able to define my "engine" as the parent but it's not a QQuickItem..

And even like this, my qml object "rectangle.qml" does not appear in the window when I run it.

[LaurianeLt]

So I tried to define the visual parent this way :

QQuickItem root = qobject_cast<QQuickItem>(engine.rootObjects().first());
object->setParentItem(root);

It's running fine but once again, the rectangle.qml is not displayed...

Any idea ?

[JKSH]

Hi,

Please add three backticks (```) before and after your code to make it render nicely.

@LaurianeLt said:

it opens two windows, the "main.qml" is launched twice.

That's because you created the "main.qml" component twice:

QQmlComponent component (&engine, "C:/Qt/.../Rectangle.qml", QQmlComponent:PreferSynchronous);
QQuickItem * object = qobject_cast<QQuickItem*>(component.create()); // <-- First creation
...
engine.load(QUrl(QStringLiteral("qrc:/main.qml))); // <-- Second creation

I would like to be able to define my "engine" as the parent but it's not a QQuickItem.

Why do you want to set "engine" as the parent?

An item is painted "inside" its visual parent. An engine cannot be painted seen, so it cannot be a visual parent.

And even like this, my qml object "rectangle.qml" does not appear in the window when I run it.

I just realized that you are using QQmlEngine, which means that the root type inside main.qml is probably a Window, is that correct? If so, note that Window does not inherit Item, so it is not a QQuickItem either. Look for the QQuickItems inside the Window by calling engine.rootObjects().first()->children().

I suggest you check the application output on the console. They provide many hints to help you solve your problems. You can print your own messages using qDebug():

qDebug() << engine.rootObjects();

[LaurianeLt]

Hi @JKSH,

Thank you so much for your answer and the tip.

I realised I was creating it twice but I wanted to see if I was able this way to make my "main.qml" be the parent of "rectangle.qml".

I realize that what I said was not clear at all. I don't want to set "engine" as the parent, but my "main.qml" which is the qml file I load with the engine.

I am using QQmlEngine indeed so my main.cpp in its simplest form is this :

int main( int argc, char *argv[])
{
QApplication app(argc, argv);
QQmlApplicationEngine engine;
engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
return app.exec();
}

So what I want to be able to do is to create and declare an object "rectangle.qml" a child of "main.qml" from my "main.cpp".

I hope I am making more sense now, I will try to figure more out by looking at the children of engine.rootObjects().first().

Thanks again!

[LaurianeLt]

@JKSH I forgot : the root type inside main.qml is an ApplicationWindow.

[LaurianeLt]

I tried this :

qDebug << engine.rootObjects().first()->children();

And I get something like this, with the list of the children :
(QQuickLoader(0x26c5ff98, QQuickItem(0x26c5fcf8), ... )

So I tried to get the second one, which is a QQuickItem like that :

QQuickItem parent = qobject_cast<QQuickItem*>(engine.rootObjects().first()->children().at(2));

But I get a error message saying that "QQuickItem::QQuickItem(const QQuickItem&)' is private" .

[JKSH]

@LaurianeLt said:

@JKSH I forgot : the root type inside main.qml is an ApplicationWindow.

Yep, that inherits Window. It's not a QQuickItem.

QQuickItem parent = qobject_cast<QQuickItem*>(engine.rootObjects().first()->children().at(2));

But I get a error message saying that "QQuickItem::QQuickItem(const QQuickItem&)' is private" .

You got that error message because the type of parent needs to be QQuickItem*, not QQuickItem.

Also note that counting starts from 0, so if you want the second item you should call .at(1).

[LaurianeLt]

@JKSH Thank you so much, this is finally working !

[JKSH]

@LaurianeLt You're welcome :) Happy coding!