QML working in QQuickView, but not in custom QQuickWindow

[kloffy]

My aim is to use QML for GUI on top of some custom rendering that I am doing in OpenGL. Before going into that, I thought it would be good to get a better understanding of QML. Therefore, I thought I might start by implementing my own version of QQuickView on top of QQuickWindow (slowly working my way deeper into the workings of QML). However, while doing so, I quickly ran into a problem with the following QML:

import QtQuick 2.5
import QtQuick.Controls 1.4

Item {
	anchors.fill: parent

	ListModel {
		id: fruitModel

		ListElement {
			title: "Apple"
			page: "Banana"
		}
		ListElement {
			title: "Orange"
			page: "Banana"
		}
		ListElement {
			title: "Banana"
			page: "Banana"
		}
	}

	ListView {
		anchors.fill: parent
		model: fruitModel
		delegate: Rectangle{
			width: parent.width; height: 80
			color: "red"
			Text {
				anchors.fill: parent
				color: "black"
				text: title
			}
		}
	}
}

This is a pretty simple scene that works perfectly well if I display it in a QQuickView. However, if I use my custom window, only the first list item is displayed. Can anybody point me towards what is missing in my QQuickWindow subclass to make this work?

#! /usr/bin/python3

import os
import sys

from PyQt5 import QtCore, QtGui, QtNetwork, QtQml, QtQuick, QtWidgets

def application_path(*paths):
    return os.path.join(os.path.dirname(os.path.realpath(__file__)), *paths)

class MainWindow(QtQuick.QQuickWindow):
    def __init__(self):
        super(MainWindow, self).__init__()

        self._engine = QtQml.QQmlEngine(self)
        if not self._engine.incubationController():
            self._engine.setIncubationController(self.incubationController())
        
        self._engine.quit.connect(QtWidgets.QApplication.instance().quit)
    
    def setSource(self, url):
        context = QtQml.QQmlContext(self._engine)

        component = QtQml.QQmlComponent(self._engine)
        component.loadUrl(url)

        if component.isError():
            logger.error(component.errorString())
            return

        obj = component.create(context)

        if component.isError():
            logger.error(component.errorString())
            return

        if isinstance(obj, QtQuick.QQuickItem):
            obj.setParentItem(self.contentItem())
            self._root = obj

if __name__ == '__main__':
    app = QtWidgets.QApplication(sys.argv)
    
    #view = QtQuick.QQuickView()
    view = MainWindow() 
    view.setSource(QtCore.QUrl.fromLocalFile(application_path("main.qml")))
    view.resize(800, 600)
    view.show()
    
    sys.exit(app.exec_())

[p3c0]

Hi @kloffy and Welcome,

I'm not aware of PyQt so I'm not sure what is causing that problem in that case. But I replicated the same code using C++ and it worked as expected. All 3 items are shown. Following is what I tried:
MainWindow.cpp

MainWindow::MainWindow()
{
    QQmlEngine *eng = new QQmlEngine(this);
    eng->setIncubationController(incubationController());

    QQmlContext *con = new QQmlContext(eng);

    QQmlComponent *comp = new QQmlComponent(eng);
    comp->loadUrl(QUrl("qrc:/main.qml"));

    QQuickItem *obj = qobject_cast<QQuickItem*>(comp->create(con));
    obj->setParentItem(contentItem());
}

main.cpp

QGuiApplication app(argc, argv);

MainWindow win;
win.resize(800,600);
win.show();

return app.exec();

[kloffy]

Thank you for testing the code in C++, I should have probably tried that myself. Your response has led me towards a solution. It appears that the problem is indeed specific to PyQt5, and in particular I have a feeling it might be related to object lifetimes. Something might be getting destroyed before it should be, which has me a bit worried about the robustness of PyQt5 in general. I can make the example work by keeping a reference to the context in a member of MainWindow, i.e.:

self._context = QtQml.QQmlContext(self._engine)

I was under the impression that in Qt's ownership model, the QQmlContext would be kept alive by its parent the QQmlEngine. I am really puzzled by what is going on, but I suppose in order to get clarity I will have to consult the PyQt5 guys.

[kloffy]

Oh, mea culpa to the PyQt5 guys. I just saw that the parent is passed as the second parameter to QQmlContext. If I pass QQmlEngine twice, it also works:

context = QtQml.QQmlContext(self._engine, self._engine)

On the other hand, in that case I am a bit confused as to why this is not an issue in C++.

[p3c0]

@kloffy I too have no idea about the implementation :) You can try asking it on the mailing lists.
http://lists.qt-project.org/mailman/listinfo/interest
http://lists.qt-project.org/mailman/listinfo
The Qt developers might know the exact reason behind these internals.

[kloffy]

Thanks again, the mailing list will come in handy, no doubt. As far as the original problem goes, I think it can be considered resolved! :)