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Qt6: prevent QML object from being garbage collected

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It appears that the QML object that is created when assigning the source
to the StatsView in the ui can be garbage collected and therefore
destroyed and re-instantiated without our knowledge. So instead of
trying to keep a pointer around, we end up looking up the object address
when needed.
We still ask the JS code to not garbage collect the object - but that
clearly isn't enough to prevent it from being destroyed when the parent
widget changes.

Without this fix opening the statistics will crash with Qt6.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
This commit is contained in:
Dirk Hohndel 2022-02-16 15:42:21 -08:00
parent 2ede1a4563
commit 2368a183e1
2 changed files with 26 additions and 9 deletions
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33
desktop-widgets/statswidget.cpp
View file

@ -5,6 +5,7 @@
#include <QCheckBox>
#include <QPainter>
#include <QStyledItemDelegate>
#include <QQmlEngine>
class ChartItemDelegate : public QStyledItemDelegate {
private:
@ -90,12 +91,24 @@ StatsWidget::StatsWidget(QWidget *parent) : QWidget(parent)
ui.stats->setSource(urlStatsView);
ui.stats->setResizeMode(QQuickWidget::SizeRootObjectToView);
QQuickItem *root = ui.stats->rootObject();
view = qobject_cast<StatsView *>(root);
(void)getView();
}
// hack around the Qt6 bug where the QML object gets destroyed and recreated
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)
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.
}
return view;
}
// Initialize QComboBox with list of variables
@ -143,13 +156,14 @@ void StatsWidget::updateUi()
connect(check, &QCheckBox::stateChanged, [this,id] (int state) { featureChanged(id, state); });
ui.features->addWidget(check);
}
StatsView *view = getView();
if (view)
view->plot(state);
}
void StatsWidget::updateRestrictionLabel()
{
StatsView *view = getView();
if (!view)
return;
int num = view->restrictionCount();
@ -211,6 +225,7 @@ void StatsWidget::featureChanged(int idx, bool status)
{
state.featureChanged(idx, status);
// No need for a full chart replot - just show/hide the features
StatsView *view = getView();
if (view)
view->updateFeatures(state);
}
@ -221,20 +236,21 @@ void StatsWidget::showEvent(QShowEvent *e)
updateUi();
QWidget::showEvent(e);
// Apparently, we have to manage the visibility of the view ourselves. That's mad.
if (view)
view->setVisible(true);
// this is implicitly done in getView() - so we can ignore the return value
(void)getView();
}
void StatsWidget::hideEvent(QHideEvent *e)
{
QWidget::hideEvent(e);
// Apparently, we have to manage the visibility of the view ourselves. That's mad.
if (view)
view->setVisible(false);
// this is implicitly done in getView() - so we can ignore the return value
(void)getView();
}
void StatsWidget::restrict()
{
StatsView *view = getView();
if (view)
view->restrictToSelection();
updateRestrictionLabel();
@ -242,6 +258,7 @@ void StatsWidget::restrict()
void StatsWidget::unrestrict()
{
StatsView *view = getView();
if (view)
view->unrestrict();
updateRestrictionLabel();

2
desktop-widgets/statswidget.h
View file

@ -31,7 +31,7 @@ slots:
private:
Ui::StatsWidget ui;
StatsState state;
StatsView *view;
StatsView *getView();
void updateUi();
void updateRestrictionLabel();
std::vector<std::unique_ptr<QCheckBox>> features;