QT QML Items both in C++ and QML

Question

My goal is to describe a qml item in a qml file and then use a corresponding C++ class to add to the visual representation of this item.

I know it is possible to describe a qml item and use it in qml file and the communicate with it from c++ through signal/slots.

I also know it is possible to describe a qml item in C++ using QQuickPaintedItem and use it in a qml file (making it available through the register mechanism). Are there other ways?

So, the actual question is it possible to combine both in one item? So, I want to have a qml item and a c++ class painting the same area, e.g. I have specific OpenGL code in c++ and a usual Rectangle Frame with MouseArea defined in qml.

Create your own item in C++. C++ is compiling language, qml is interpreted (in runtime), so it is not possible to use qml definitions in compile-time. — Dmitry Sazonov
With QQuickPaintedItem I am able to use C++ objects in qml. With object->findChild<QObject*>("myobject") I can use qml objects in C++. The question is, whether there is an object which is like QQuickPaintedItem but also can have a qml visual representation. The qt.labs.templates seem interesting. — PsiX
You can combine C++ and QML in a single element by putting your QQuickPaintedItem-derived element above your QML element. You can do that by adjusting the z property or just by ordering the elements accordingly in a single QML file. Since QQuickPaintedItem has transparent background, you will see your drawing output from both C++ and QML. — Andrej Repiský
Just add stuff to it in QML: MyPaintedItem { Rectangle{} } and save it into MyPaintedItemWithRectangle.qml. Then use the MyPaintedItemWithRectangle type. — Velkan

PsiX PsiX · Accepted Answer · 2016-07-13T17:08:08

After the hint from Andrej I decided to realize my goal of having both the qml representation and a C++ rendering class by creating a wrapper class which derives from QQuickPaintedItem and by this overriding the paint method. With this I can render into the item in code. The item is used in my main qml file.

It is a wrapper class because it loads the qml file I want to show through a QQmlComponent which creates the QuickItem I want to show, too. I do this by setting the parent of the loaded/created item to my wrapper class. So in my QuickPaintedItem class (best done in classbegin, when engine is already initialized):

QQmlComponent component(engine,QUrl("qrc:/myqml.qml")); QObject* object = component.create(); QQuickItem* quickItem = qobject_cast<QQuickItem*>(object); quickItem->setParentItem(this);

Then myqml.qml is rendered and my paint method, too. And I have a central place where I can use both.

After the comments from Velkan, another way is to put the loading of the component in a Loader qml item:

Wrapper { Loader{ onQmlChanged: source = newQml } }

where onQmlChanged would be a slot which consumes a signal:

signal onQmlChanged(string newQml);

I cannot say which way is better performance wise. Defining the structure in qml seems easier and cleaner. A difference to the code version, is that it loads the item at creation time of the Wrapper, so during the creation of the main.qml and before it is shown.

QT QML Items both in C++ and QML

1 Answers