QFutureWatcher's signal finished does not get emitted.

[VRonin]

Ok, then let's make a slight change, lets move away from QtConcurrent and just use C++:

void ScriptInstance::start()
{
qInfo() << "Executing script " << _scriptFileName << " (" << _scriptInstanceId << ")";
QObject* signalObj = new QObject;
connect(signalObj , &QObject::destroyed, this, &ScriptInstance::slotFinished);
std::async(std::launch::async,[=]()->void{scriptExecutionThread(); delete signalObj;});
}

[0xedd1e]

@VRonin
I've now tested your example and ScriptInstance::slotFinished() is still not called. I also have stepped through the deallocation of the signalObj in start() while having breakpoints set in the MOC-code, but still no luck.

moc_ScriptInstance.cpp (part of it):

void ScriptInstance::qt_static_metacall(QObject *_o, QMetaObject::Call _c, int _id, void **_a)
{
    if (_c == QMetaObject::InvokeMetaMethod) {
        ScriptInstance *_t = static_cast<ScriptInstance *>(_o);
        Q_UNUSED(_t)
        switch (_id) {
        case 0: _t->signalDispose((*reinterpret_cast< quint32(*)>(_a[1]))); break;
        case 1: _t->slotFinished(); break;  // <-- Never called..

Any idea where to look to see where all the invokes/function pointers gets registered, to be able to see if any entry for slotFinished is left out or not, or any other suggestions?

PS. I really appreciate your input!

[SGaist]

Hi,

What about calling signalObj->deleteLater() in place of delete?

[0xedd1e]

@SGaist
I've just tried that and slotFinished() is still not called.

[mrjj]

@0xedd1e
Hi
Are you running ScriptInstance in another thread? (movetothread style)
Have you checked you can emit a signal and have a slot called in the class ?
( or the actual worker thread if that is the case)

I tried Wielands code in main thread and it worked so i wondering
if its something with emitting signals from your actual code.
(clearly it is but i mean in general)

So forget about async/QFutureWatcher for a moment and simply emit a signal to a slot
on same level/class/interclass as the QFutureWatcher is running.

Sorry if you already validated other signals do work. It was just a thought.

[0xedd1e]

@mrjj
I've now done the following;

1. Added a new signal, signalDebug(), to ScriptInstance class.
2. Connected signalDebug() to slot slotFinished() in the constructor for ScriptInstance class.
3. In ScriptInstance::scriptExecutionThread() I emitted the signalDebug() signal.

Result: slotFinished() is called just fine.

FYI: I'm also back to using QtConcurrent mechanisms.

[mrjj]

@0xedd1e
Ok, so signals seems to work.
I was wondering if you needed to specify connection type
but if some signals work, all should.

but just for test
connect(&_scriptWatcher, &QFutureWatcher<void>::finished, this, &ScriptInstance::slotFinished, Qt::QueuedConnection);

Else Im a bit out of idea as all seem ok.

Update
I ran your code and slotFinished was called ?
Can you try it
https://www.dropbox.com/s/rhdk6xxk323ziqh/test_thread.zip?dl=0

[0xedd1e]

@mrjj
Once I discovered the issue I experimented with different type of connections, but no change in behavior.

The project you provided works just fine, so I guess that rules out that there is no issue(s) related to my setup at least.

The ScriptEngine in my case is a singleton, but I don't see why that should matter in this case since all other signals/slots seems to work just fine.

[mrjj]

@0xedd1e
Ok. super. Yes it means nothing wrong inside Qt/the setup.

• ScriptEngine in my case is a singleton

meaning its static ?

But I agree if other signals can get be emitted (from same nesting level) then
it seems unlikely its that.

[0xedd1e]

@mrjj
Correct, it is static.

I'm really out of ideas, so I guess I have to go back to earlier changesets within my project and see if I can pinpoint the exact change that caused this behavior. However, the code connected to the script engine and script instance hasn't changed for quite a while though, so I'll try to back the project to an earlier changeset and see if there are changes elsewhere that causes this. I registered a bug in my own ticket system when I discovered this issue but marked it for check-later-when-qt-upgrades since it worked fine in QT 5.4 and earlier.

Thanks again for all the feedback and I'll report back during the upcoming holiday.

[0xedd1e]

@0xedd1e said in QFutureWatcher's signal finished does not get emitted.:

@mrjj
Correct, it is static.

I'm really out of ideas, so I guess I have to go back to earlier changesets within my project and see if I can pinpoint the exact change that caused this behavior. However, the code connected to the script engine and script instance hasn't changed for quite a while though, so I'll try to back the project to an earlier changeset and see if there are changes elsewhere that causes this. I registered a bug in my own ticket system when I discovered this issue but marked it for check-later-when-qt-upgrades since it worked fine in QT 5.4 and earlier.

Thanks again for all the feedback and I'll report back during the upcoming holiday.

FYI;
I've now gone back and tested previous versions of the project but can't seem to get it working in any of the earlier versions now using QT 5.10. So I guess something has changed in QT, but I can't figure out what though and to my knowledge I follow the recommendations according to the documentation of QT.

Worth trying going back to using earlier versions of QT?

[mrjj]

@0xedd1e
Hi I also looked at the release notes to see
if something with Future/Concurrent was changed but nothing popped up.

• Worth trying going back to using earlier versions of QT?

Well for test. it would be a good test if exact same code just works in say 5.6
and not in 5.10. But being stucked at 5.4 would be not so optimal.

A note about the static.
Just to be sure. the class that expects the Finished signal is a static member of some other class?
As far as i can recall, you cant compile if you try to connect to a static class slot so im inclided to
say that is not the issue.

Since your mini sample works. There must be some difference in the real code. But i cant
think of more to check.

[leochan2009]

same issue i have came across; see my post:
https://forum.qt.io/topic/122640/qvideoprobe-stop-working-when-processing-time-is-long/11