How to use QPaint object in QML?

[abanksdev]

I'm referencing this doc: http://doc.qt.io/qt-5/qtquick-customitems-painteditem-example.html but am still missing a crucial step I believe

Basically, I'm making a custom loader using QPaint. I make a class for the custom loader, put all methods in the header and include the paint event in the corresponding cpp file.

CustomLoader::CustomLoader(QQuickItem *parent)
    : QQuickPaintedItem(parent)
{
Q_ASSERT(m_maxValue>=m_minValue); // make sure max is the same or more than the min
    QWidget::paintEvent(event); // call the base class implementation
    QPainter painter(this); // set a painter to paint on the widget
}

void CustomLoader::paint(QPainter *painter){
}

void CustomLoader::setProgress(int progress) //this sets the percentage of completed the loader is 
{

}

I then use the application engine to setContextProperty

CustomLoader customLoader(NULL);

    engine.rootContext()->setContextProperty("customLoader", customLoader);
    customLoader.setProgress(50);

And get the following error:

main.cpp:15: error: member access into incomplete type 'QQmlContext'
    engine.rootContext()->setContextProperty("customLoader", customLoader);
                        

my CustomLoader.qml is simply this:

import QtQuick 2.0

Item {
    width: 300
    height: 300

}

I am very new to this and thus have a few questions.
1.) Any ideas what is causing the error I am receiving about accessing an incomplete type?
2.) How do I pass the CustomLoader paint object to the CustomLoader QML so that it displays in the QML element?

Thank you!

[hsolter]

maybe include <QQmlContext> in your main.cpp? . It seems it does not find the definition of the Context class.

Look up, definition vs declaration, in a c++ book or online.

[AndyS]

Hi @abanksdev

It is not safe to have a widget inside QQuickPaintedItem since it is not supported to create a QWidget on anything but the GUI thread, and your QQuickPaintedItem might end up painting via the render thread. Therefore you should change it so that you don't need to create a QWidget just for the purpose of painting it in QQuickPaintedItem.

In order to at least get the rendering to work, assuming you have changed the QWidget to be a different paint device, such as QImage for example. Then you would reimplement paint() and then use the QPainter passed in to render with. Although I would strongly advise against using the following, but in case it helps you could do:

void CustomLoader::paint(QPainter *painter)
{
    MyWidget w;
    w.render(painter);
}

But that is purely for demostration purposes only.

Andy

[jw23578]

did you qmlregistertype your new qml type?
https://doc.qt.io/qt-5/qtqml-cppintegration-definetypes.html

jens