Emitting signal from different thread causes QML to crash

pholz

"This bug":https://bugreports.qt-project.org/browse/QTBUG-25647 , which seems to have been solved a long time ago, seems to cause a segfault in the QML signal handler in my program built with qt 5.1.1. I googled the exact error output, which is:

@

Fatal error in ../3rdparty/v8/src/isolate.h, line 446

CHECK(isolate != __null) failed

@

And that led me to the abovementioned bug.

Code:

main.cpp:
@
StateTracker * st = new StateTracker();
QtQuick2ApplicationViewer viewer;
QQmlContext * ctx = viewer.rootContext();
ctx->setContextProperty("stateTracker", st);
viewer.setMainQmlFile(QStringLiteral("qml/quicktest/main.qml"));
viewer.showExpanded();
QThread *sthread = new QThread();
st->moveToThread(sthread);
sthread->start();
@

StateTracker.h:
@
class StateTracker : public QObject
{
Q_OBJECT
// ...
signals:
void positionUpdateTyped(int id, QVector3D pos);
// ...
};
@

main.qml:
@
Item {
Connections {
target: stateTracker
onPositionUpdateTyped: {
// doSomething
}
}
}
@

It worked fine when I was emitting the signal from the main thread, but not now that the StateTracker object is in a different thread. If I manually connect a signal from StateTracker to a QML function (as slot) via connect(), it also works - but the qt docs say that this is bad practice.

Any ideas?

EDIT: I am able to work around it by adding a "relay" class with thread affinity to the main thread that has a signal and a slot for each signal of StateTracker, and acts as an intermediary:

@
signals:
void positionUpdateTypedR(int id, QVector3D pos);

public slots:
void positionUpdateTyped(int id, QVector3D pos)
{
emit positionUpdateTypedR(id, pos);
}
@

and

@
QObject::connect(st, SIGNAL(positionUpdateTyped(int,QVector3D)),
relay, SLOT(positionUpdateTyped(int,QVector3D)),
Qt::QueuedConnection);
@

and then I use the relay class as context property and react to onPositionUpdateTypedR in main.qml. But this adds a lot of unnecessary code for every signal.

Kniebou

Hi,

I answer to that old thread because I have the exact same issue with Qt 5.2.1 and Qt 5.3 RC.
Is it possible to use Connections to connect a C++ signal from a non-main thread to a QML slot ?
The error I get is
@QObject::setParent: Cannot set parent, new parent is in a different thread@