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  1. Qt
  2. QTBUG-121361

Document that QML_EXTENDED is *not* meant for extending types you don't control

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      I want to add a new property to the QtMultimedia VideoOutput QML object. It appears I should use QML_EXTENDED to do that.
      https://doc.qt.io/qt-6/qtqml-cppintegration-definetypes.html#registering-extension-objects

      It's not clear from the documentation how to do this. I see two approaches, include the private header so I can use QQuickVideoOutput - the undocumented private implementation of the VideoOutput QML object:

      #include <QtMultimediaQuick/private/qquickvideooutput_p.h>
struct VideoOutputForeign
{
    Q_GADGET
    QML_FOREIGN(QQuickVideoOutput)
    QML_NAMED_ELEMENT(VideoOutput)
    QML_EXTENDED(VideoOutputExtension)
};

      Or add my property to the public base class of QQuickVideoOutput - QQuickItem:

      #include <QQuickItem>
struct VideoOutputForeign
{
    Q_GADGET
    QML_FOREIGN(QQuickItem)
    QML_NAMED_ELEMENT(Item)
    QML_EXTENDED(VideoOutputExtension)
};

      and then deal with users attempting to use my property on classes I don't support (i.e. anything other than VideoOutput)

      I implemented both approaches in the attached project and both seem to work.
      cmake -DPRIVATE=1 to use the QQuickVideoOutput approach, and don't define PRIVATE to use the QQuickItem approach.

      One oddity is I log the object I'm extending in my extension constructor:

      VideoOutputExtension::VideoOutputExtension(QObject *object)
    : QObject(object)
    , m_videoOutput(object)
{
    QVideoSink* videoSink = nullptr;
    auto* mo = m_videoOutput->metaObject();
    mo->invokeMethod(m_videoOutput, "videoSink", Q_RETURN_ARG(QVideoSink*, videoSink));
    qDebug() << m_videoOutput << "class" << mo->className() << "videoSink" << videoSink;
}

      In both cases it logs itself as VideoOutputExtension instead of QQuickItem or QQuickVideoOutput - but when it is attached to a VideoOutput, it has a videoSink property so it must be the correct object.

      QQuickItem implementation logs:

      QMetaObject::invokeMethod: No such method VideoOutputExtension::videoSink()
VideoOutputExtension(0x600001a4c0e0) class VideoOutputExtension videoSink QObject(0x0)
setFoobar 42
VideoOutputExtension(0x154a18a80) class VideoOutputExtension videoSink QVideoSink(0x600002305540)
setFoobar 23
qml: loaded video-item.qml

      QQuickVideoOutput implementation logs:

      VideoOutputExtension(0x142106ad0) class VideoOutputExtension videoSink QVideoSink(0x600000948a00)
setFoobar 23
qml: loaded video.qml

      Could the QML_EXTENDED documentation be enhanced to recommend the preferred way to extend QML objects that have private Qt implementations?

        1. videooutput.zip
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            qtqmlteam Qt Qml Team User
            rectalogic Andrew Wason
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              Created:
              Updated:

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