When to use what classes in Qt Quick 2 and how?

Question

I am trying to write a game with Qt 5.1 and Qt Quick 2 and it's new (faster) graphic engine. I have been reading documentation for hours but still can't figure out what classes to use if I want to move items on screen whose positions are determined by C++ code.

In QGraphics it was easy: I create an instance of QGraphicsScene add any inheritor of QGraphicsItem to it and then create an instance of QGraphicsView that is a Widget and displays all items and their changes. I connect QGraphicsItem to signals to make changes.

In Qt Quick 2 I first read the

• official examples that only used QML and Javascript,
• then I thought I found the equivalents to QGraphics, namely QDeclarativeEngine, QDeclarativeComponent and QDelcarativeView and was ready to create custom QML elements with Q_PROPERTY
  • UPDATE: Just now I found out that QDeclarative* is Qt Quick 1 and QQml* is the equivalent Qt Quick 2 prefix.
• But then I also found QQuickItem, QQuickWindow, QSGNode etc..

After reading many tutorials documentations I still don't know what the "default" solution is. Every tutorial shows something different. I am overwhelmed with all the examples and classes.

• Can someone please give me a basic example that is based on Qt Quick 2 and where images on a canvas are moved around by signals that are being sent from regular QObjects written in C++?
• Can you please help me categorize all the classes I mentioned. Which do I use when?

Answer 1

I think you're making confusion about things on different levels, and that acually makes this question a "compound" question which should be split in smaller questions...

Anyhow:

• when you need a QWindow which is able to host QtQuick 2 content, then you need a QQuickView or a QQuickWindow (usually the former, has provides more convenience; see their docs).
• QtQuick2 is not QGraphicsView-based. It's not even in QtWidgets -- you can't use any widgets-related class or API there.
• The easiest way to bind a QML element property to the one of a C++ object is just exposing that object to the QML engine, then perform an ordinary binding.

For instance:

class MyObject : public QObject {
    Q_OBJECT
    Q_PROPERTY(int horizontalPos READ horizontalPos NOTIFY horizontalPosChanged)
public:
    int horizontalPos() const { return m_horizontalPos; }
signals:
    void horizontalPosChanged();
    // etc.
}

Then you can expose an instance of MyObject to the QML engine:

MyObject obj;
QQuickView view;
// exposes the object under the "_myObject" name
view.engine()->context()->setContextProperty("_myObject", &obj); 

The underscore is a nice touch to underline the fact that this name comes from the C++ world.

Finally, in QML, you can just bound to the property:

Rectangle {
    x: _myObject.horizontalPos // voilà, they're bound together
}

Answer 2

You could dynamically instantiate custom qml components from the c++ side and modify the properties, still from the C++ side.

You could also just work on some C++ list/model/multiple qobjects containing the positions of your sprites/characters/whatever you display on the screen, and then bind this list with your qml side.

You could also write some QQuickItem from C++ that layouts its children (like proposed here: http://qt-project.org/forums/viewthread/29407/)

-

And here are some links completing peppe's answer / detailing how to mix QtQuick2 & C++ :

The doc pages for Qt Quick 2 / C++ integration (the Qt5 doc isn't well indexed by Google):

• http://qt-project.org/doc/qt-5.0/qtqml/qtqml-cppintegration-topic.html
• http://qt-project.org/doc/qt-5.0/qtqml/qtqml-cppintegration-interactqmlfromcpp.html

The blog post explaining QWidget::createWindowContainer (it's only if you want to embed QtQuick2 in a QtWidget application... otherwise, you can just use the skeleton for a QtQuick2 application from QtCreator):

• http://blog.qt.digia.com/blog/2013/02/19/introducing-qwidgetcreatewindowcontainer/?utm_source=rss&utm_medium=rss&utm_campaign=introducing-qwidgetcreatewindowcontainer