Modal QProgressDialog updated from worker thread - c++

Question

I want to update a modal QProgressDialog from my worker thread. However, if I set the dialog to be modal, my application crashes (and the dialog did not show any progress). If I do not, everything goes fine (but the user can tinker around with the rest of the program, which may cause issues).
What am I doing wrong?
Minimum code sample follows:
filereader qfr;
QProgressDialog progress("Importing file.", "Cancel", 0, file_size);
connect(&qfr, &filereader::signalProgress, &progress, &QProgressDialog::setValue, Qt::QueuedConnection);
QThread worker_thread;
std::atomic<bool> success = false;
connect(&worker_thread, &QThread::started,
[&]() {
success = qfr.read_file(/* parameters */);
worker_thread.quit();});
worker_thread.start();
//progress.setWindowModality(Qt::WindowModal); // Works only fine when this line is commented !!
while (worker_thread.isRunning()) {
QApplication::processEvents();
QThread::sleep(0);
}
progress.close();

Answer 1

Your thread is pretty much pointless. It serves no real purpose. You could have as well just called QApplication::processEvents in your read_file method. But you shouldn't, calling processEvents is bad practice.
What you should do is remove that while loop, and make your progress dialog a member of your class. I don't really like how that lambda looks either. I would personally just use filereader::read_file as a slot.
Note that Qt::windowModal blocks input to the parent window. Your progress dialog has no parent. So you would either have to call progress->setModal(true), or progress.setWindowModality(Qt::ApplicationModal);. Or set a parent to it.
Here is a small example (it is not tailor made for your application, but it should point you in the right direction):
#include <QtWidgets>
class Worker : public QObject
{
Q_OBJECT
public:
Worker(QObject *parent = nullptr) : QObject(parent){}
public slots:
void simulateLongProcess()
{
for(int i = 0; i < 101; i++)
{
emit progressChanged(i);
QThread::msleep(100);
}
emit finishedWorking(true);
}
signals:
void progressChanged(int progress);
void finishedWorking(bool result);
};
class Widget : public QWidget
{
Q_OBJECT
public:
Widget(QWidget *parent = nullptr) : QWidget(parent)
{
setLayout(new QHBoxLayout);
progress_dialog.setModal(true);
progress_dialog.setAutoReset(false);
progress_dialog.setCancelButton(nullptr);
QThread *thread = new QThread(this);
connect(thread, &QThread::started, &worker, &Worker::simulateLongProcess);
connect(&worker, &Worker::finishedWorking, thread, &QThread::quit);
connect(&worker, &Worker::progressChanged, &progress_dialog, &QProgressDialog::setValue);
connect(&worker, &Worker::finishedWorking, &progress_dialog, &QProgressDialog::close);
connect(&worker, &Worker::finishedWorking, this, &Widget::handleResult);
QPushButton * start_button = new QPushButton("START");
connect(start_button, &QPushButton::clicked, this, [=]
{
progress_dialog.show();
thread->start();
});
layout()->addWidget(start_button);
resize(400, 300);
}
public slots:
void handleResult(bool result)
{
// do something with the result
}
private:
QProgressDialog progress_dialog;
Worker worker;
};
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
Widget w;
w.show();
return a.exec();
}
#include "main.moc"