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desktop: update comment to reflect reason for QQuickItem destruction

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Explain the ominous behavior that leads to the StatsView being
destroyed under our feet by Qt6.

Also, don't generate the view in the destructor. No point in
doing that, when we have to regenerate it anyway for each
access.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>
This commit is contained in:
Berthold Stoeger 2023-04-28 20:22:12 +02:00
parent 515f76e9a9
commit 2f044039dd
1 changed files with 10 additions and 5 deletions
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15
desktop-widgets/statswidget.cpp
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@ -91,19 +91,24 @@ StatsWidget::StatsWidget(QWidget *parent) : QWidget(parent)
ui.stats->setSource(urlStatsView);
ui.stats->setResizeMode(QQuickWidget::SizeRootObjectToView);
(void)getView();
}
// hack around the Qt6 bug where the QML object gets destroyed and recreated
// The Qt-developers do not have their ownership management under control.
// This is not surprising to anyone who has looked at their source code.
// In contradiction to the documentation, starting with Qt6, QQuickItems
// _will_ be deleted when the parent widgets are reparented, even when
// CppOwnership is set. This can be prevented for OpenGL backends, but not
// for others. Therefore, we have to _always_ refetch the view from the QML page.
// For details and precise conditions, see comments and code in
// QQuickWidgetPrivate::handleWindowChange().
// Ultimately, we will have to change the way widgets are hidden/shown in
// the MainWindow.
StatsView *StatsWidget::getView()
{
StatsView *view = qobject_cast<StatsView *>(ui.stats->rootObject());
if (!view)
qWarning("Oops. The root of the StatsView is not a StatsView.");
if (view) {
// try to prevent the JS garbage collection from freeing the object
// this appears to fail with Qt6 which is why we still look up the
// object from the ui.stats rootObject
ui.stats->engine()->setObjectOwnership(view, QQmlEngine::CppOwnership);
view->setParent(this);
view->setVisible(isVisible()); // Synchronize visibility of widget and QtQuick-view.