Details
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Bug
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Resolution: Unresolved
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P2: Important
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None
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5.15.2
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None
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Windows 10 20H2 x64, MSVC 142 2019
Description
In a Qt Quick application, a QProcess (running a CLI program) started in a C++ class object, "freezes" (or at least the slot connected to its finished signal never executes) during user interaction with Qt Quick application window.
The C++ code of the class:
Backend::Backend(QObject *parent) : QObject(parent)
{
_someCounter = 0;
_whoAmI.setProgram("whoami");
connect(
&_whoAmI, QOverload<int, QProcess::ExitStatus>::of(&QProcess::finished),
[=](int exitCode, QProcess::ExitStatus exitStatus) { doneSomething(exitCode, exitStatus); }
);
connect(&_timer, &QTimer::timeout, this, &Backend::doSomething);
_timer.setSingleShot(false);
_timer.start(800);
}
void Backend::doSomething()
{
if (_whoAmI.state() == 0) { _whoAmI.start(); }
else { qDebug() << "waiting till previously started process ends..."; }
}
void Backend::doneSomething(int exitCode, QProcess::ExitStatus exitStatus)
{
qDebug() << "Exit code:" << exitCode << " | status:" << exitStatus;
_someCounter++;
QString rez = _whoAmI.readAllStandardOutput().trimmed();
emit gotResults(
QString("%1 - %2 - %3").arg(
QDateTime::currentDateTime().toString(),
QString::number(_someCounter),
rez
)
);
}
This class is then registered in main.cpp:
qmlRegisterType<Backend>("io.qt.Backend", 1, 0, "Backend");
and used in main.qml:
Backend {
id: backend
onGotResults: {
applicationLog.append(msg);
}
}
The full project source code is attached in qml-cpp-blocking.zip
Here's a video of the problem occurring: blocking.mp4
So in case of either of these interactions:
1. Moving the application window
2. Resizing the application window
3. Navigating the application menu
the QProcess running in Backend object never finishes (or its finished signal never triggers the connected slot?).
I've encountered this problem with Qt 5.15.2 on Windows. The same application works fine on Linux and Mac OS, I couldn't reproduce this problem there.
Also I couldn't reproduce it with Qt 6.2.0 on Windows, which is good news.