QML chartview crash when changing data - qt

Question

I'm new to qml and I didn't really get its logic yet.
Here is my issue: I have a chartview with barseries, and I expect that when the user clicks on a bar, the bar chart changes.
So I clear the barSeries and append new data to it. But it causes a segmentation fault.
If I do the same processing from a button click, it works well.
Do you have any idea what is the problem, and how to fix it?
Below is a code sample.
main.qml
import QtQuick 2.7
import QtQuick.Controls 2.0
import QtQuick.Layouts 1.0
import QtCharts 2.2
ApplicationWindow {
visible: true
width: 640
height: 480
title: qsTr("Chartview test")
Item {
id: histoView
property int globalForJs: 0
anchors.fill: parent
visible: true
Component.onCompleted: {
redraw();
}
signal redrawSignal()
ColumnLayout {
anchors.fill: parent
Button {
id:myBtn
anchors { top: parent.top; left: parent.left; leftMargin: 20 }
height: 30
width: 80
text: "new chart"
onClicked: {
console.debug("myBtn onClicked")
histoView.redrawSignal()
}
}
ChartView {
id: chartView
anchors { bottom: parent.bottom; left: parent.left; right: parent.right }
height: parent.height - 40
title: "chartview"
BarSeries {
id: mySeries
axisX: BarCategoryAxis { categories: [" "] }
axisY: ValueAxis {
id: ordinate
min:0
max:100
}
onClicked: {
console.debug("mySeries onClicked")
histoView.redrawSignal();
//myBtn.clicked()
}
}
}
}
function redraw() {
// clear data
mySeries.clear();
// add 5 new data
for (var i=0;i<5;i++) {
var tab=[];
globalForJs++;
tab.push(globalForJs);
mySeries.append("tutu"+i,tab);
}
// update Y axis maximum value
if (ordinate.max < globalForJs) {
ordinate.max = globalForJs + 10;
}
}
onRedrawSignal:redraw()
}
}
main.cpp
#include <QApplication>
#include <QQmlApplicationEngine>
int main(int argc, char *argv[])
{
QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
QApplication app(argc, argv);
QQmlApplicationEngine engine;
engine.load(QUrl(QLatin1String("qrc:/main.qml")));
return app.exec();
}

Answer 1

Short answer:
Looks like a bug in QGraphicsScene or Qt Charts' usage of it. Please report a bug at bugreports.qt.io.
Long answer (debugging crashes in Qt):
When you run into a crash where there's only QML, and nothing suspicious-looking going on in your C++ code, it's probably a bug in Qt. In order to increase the changes of the bug being fixed quickly, it's a good idea to get a stack trace, because the developer may spot a simple fix for it by looking at where it crashes. To get a stack trace, you'll have to build a debug build of Qt.
When debugging your application with a debug build of Qt, I can see that QGraphicsItem::mouseReleaseEvent() is calling a member function of an object that has been deleted. An easy way to verify this is to add some debug code to the destructor of QGraphicsItem that prints out the memory address of each object that is destroyed. In addition, add a line to the beginning of QGraphicsItem::mouseReleaseEvent() that uses qDebug()'s printf syntax (to avoid QGraphicsItem's << operator being called, which would cause a crash before it could print anything):
diff --git a/src/widgets/graphicsview/qgraphicsitem.cpp b/src/widgets/graphicsview/qgraphicsitem.cpp
index f2b8b66..9f2302d 100644
--- a/src/widgets/graphicsview/qgraphicsitem.cpp
+++ b/src/widgets/graphicsview/qgraphicsitem.cpp
## -1501,6 +1501,7 ## QGraphicsItem::QGraphicsItem(QGraphicsItemPrivate &dd, QGraphicsItem *parent)
*/
QGraphicsItem::~QGraphicsItem()
{
+ qDebug() << Q_FUNC_INFO << this;
if (d_ptr->isObject) {
QGraphicsObject *o = static_cast<QGraphicsObject *>(this);
QObjectPrivate *p = QObjectPrivate::get(o);
## -7328,6 +7329,7 ## void QGraphicsItem::mouseMoveEvent(QGraphicsSceneMouseEvent *event)
*/
void QGraphicsItem::mouseReleaseEvent(QGraphicsSceneMouseEvent *event)
{
+ qDebug("About to access %p\n", this);
if (event->button() == Qt::LeftButton && (flags() & ItemIsSelectable)) {
bool multiSelect = (event->modifiers() & Qt::ControlModifier) != 0;
if (event->scenePos() == event->buttonDownScenePos(Qt::LeftButton)) {
Build qtbase and then debug your application again. The stack trace will now look something like this (copied from Creator):
1 QGraphicsItem::flags qgraphicsitem.cpp 1854 0x6dcf8cf9
2 QGraphicsItem::mouseReleaseEvent qgraphicsitem.cpp 7333 0x6dd05003
3 QtCharts::Bar::mouseReleaseEvent bar.cpp 90 0x7ffd83fd4a9b
4 QGraphicsItem::sceneEvent qgraphicsitem.cpp 6845 0x6dd03ac6
5 QGraphicsScenePrivate::sendEvent qgraphicsscene.cpp 1251 0x6dd58732
6 QGraphicsScenePrivate::sendMouseEvent qgraphicsscene.cpp 1325 0x6dd594fd
7 QGraphicsScene::mouseReleaseEvent qgraphicsscene.cpp 4099 0x6dd52c26
8 QGraphicsScene::event qgraphicsscene.cpp 3412 0x6dd51423
9 QApplicationPrivate::notify_helper qapplication.cpp 3713 0x6d7772de
10 QApplication::notify qapplication.cpp 3085 0x6d771d83
11 QCoreApplication::notifyInternal2 qcoreapplication.cpp 1013 0x6cf72c16
12 QCoreApplication::sendEvent qcoreapplication.h 231 0x6cf7d5f2
13 QtCharts::DeclarativeChart::mouseReleaseEvent declarativechart.cpp 850 0x7ffd876dd16a
14 QQuickItem::event qquickitem.cpp 7756 0x7ffd7e206583
15 QApplicationPrivate::notify_helper qapplication.cpp 3713 0x6d7772de
16 QApplication::notify qapplication.cpp 3085 0x6d771d83
17 QCoreApplication::notifyInternal2 qcoreapplication.cpp 1013 0x6cf72c16
18 QCoreApplication::sendEvent qcoreapplication.h 231 0x6cf7d5f2
19 QQuickWindow::sendEvent qquickwindow.cpp 2807 0x7ffd7e2305f9
20 QQuickWindowPrivate::deliverMouseEvent qquickwindow.cpp 1671 0x7ffd7e235df1
... <More>
Console output (shortened):
Debugging starts
QML debugging is enabled. Only use this in a safe environment.
QML Debugger: Waiting for connection on port 52234...
[...]
__cdecl QGraphicsItem::~QGraphicsItem(void) QGraphicsItem(0x2da77915480, parent=0x2da77b0fc00, pos=0,0, flags=(ItemIsSelectable))
[...]
About to access 0x2da77915480
You can see that the QGraphicsItem whose memory address is 0x2da77915480 is destroyed, and then its mouseReleaseEvent() function is called.