When a custom attaching object also inherits QQmlParserStatus and implements the required "classBegin" and "classComplete" methods, I find that these methods do not get called when the attached object is created because of a usage like this:

Rectangle {
   MyAttachingType.myAttachedProperty: "myAttachedPropertyValue"
}

Is this because the QML engine is not responsible for the creation of the attaching object because it is done in the qmlAttachedProperties function? e.g.

MyAttachingObject *MyAttachingType::qmlAttachedProperties(QObject *object)
{
    return newMyAttachingObject(object);
}