QT Thread issue when display QDialog box - c++

Question

I have developed an app in Qt/C++, it's a file browser like. Currently when deleting some files or copying some files, I have a thread who manage the overall app and copy and another one just created to display a progress bar with the Cancel button. It's very basic but as I'm browsing an Android device file system any access to delete copy freeze the UI and the access is done by the application thread. It's not an issue as I don't want the user to play with the UI when copying or deleting.
My issue is mainly a real time issue with the threading. When I'm using Qt creator as IDE and debugging, I do not have any issue the delete works and the progress bar is also display without any issue. When I'm just using the app out of Qt Creator the app is crashing often when trying to display the dialog box.
I'm pretty sure it's link to the threading. when using the debugger and Qt creator the overall app slowing down to trace, debug...
Here is the code of my TreeView.cpp when asking to delete for example:
DialogProgressIndicator *DeleteProgress = new DialogProgressIndicator;
DeleteProgress->moveToThread(&ProgressThread);
connect(&ProgressThread, &QThread::finished, DeleteProgress, &QObject::deleteLater);
connect(this, &PulsTreeWidget::DisplayProgress, DeleteProgress, &DialogProgressIndicator::ShowDlg);
connect(this, &PulsTreeWidget::UpdateProgress, DeleteProgress, &DialogProgressIndicator::UpdateIndicator);
connect(this, &PulsTreeWidget::CloseProgress, DeleteProgress, &DialogProgressIndicator::CloseDlg);
connect(DeleteProgress,&DialogProgressIndicator::CancelAction, this, &PulsTreeWidget::CatchActionCancel);
ProgressThread.start();
DisplayProgress();
And when the delete is finished, I'm closing everything following the method below:
CloseProgress();
ProgressThread.quit();
disconnect(&ProgressThread, &QThread::finished, FileTransferProgress, &QObject::deleteLater);
disconnect(this, &PulsTreeWidget::DisplayProgress, FileTransferProgress, &DialogProgressIndicator::ShowDlg);
disconnect(this, &PulsTreeWidget::UpdateProgress, FileTransferProgress, &DialogProgressIndicator::UpdateIndicator);
disconnect(this, &PulsTreeWidget::CloseProgress, FileTransferProgress, &DialogProgressIndicator::CloseDlg);
disconnect(FileTransferProgress,&DialogProgressIndicator::CancelAction, this, &PulsTreeWidget::CatchActionCancel);
the class PulsTreeWidget is defined as below:
class PulsTreeWidget : public QTreeWidget
{
Q_OBJECT
QThread ProgressThread;
public:
PulsTreeWidget(PulsDeviceMngr& device, PulsMainUI& parent);
~PulsTreeWidget();
signals:
void DisplayProgress();
void CloseProgress();
void UpdateProgress(int);
The Progress bar is managed by the class
DialogProgressIndicator.cpp
#include <QApplication>
#include "dialogprogressindicator.h"
#include "ui_dialogprogressindicator.h"
DialogProgressIndicator::DialogProgressIndicator(QWidget *parent) :
QDialog(parent),
ui(new Ui::DialogProgressIndicator)
{
ui->setupUi(this);
ui->progressBar->setRange(0,100);
ui->progressBar->setValue(1);
connect(ui->Cancel, SIGNAL(clicked()), this, SLOT(onClickCancel()));
}
void DialogProgressIndicator::ShowDlg() {
ui->progressBar->show();
}
void DialogProgressIndicator::CloseDlg() {
ui->progressBar->close();
}
DialogProgressIndicator::~DialogProgressIndicator()
{
delete ui;
}
void DialogProgressIndicator::UpdateIndicator(int value) {
ui->progressBar->setValue(value);
QApplication::processEvents();
}
void DialogProgressIndicator::onClickCancel() {
emit CancelAction();
disconnect(ui->Cancel, SIGNAL(clicked()), this, SLOT(onClickCancel()));
}
I have followed the Qt guideline but it still crashing when doing the "DisplayProgress"
Any idea ? It's really crashing when in the other thread I'm doing the
void DialogProgressIndicator::ShowDlg() {
ui->progressBar->show();
}
I'm adding the crash log:
Application Specific Information:
abort() called
Thread 0:: Dispatch queue: com.apple.main-thread
0 com.apple.AppKit 0x00007fff89e6561f -[NSView(NSInternal) _allocAuxiliary:] + 833
1 com.apple.AppKit 0x00007fff89e67a59 -[NSView _commonAwake] + 36
2 com.apple.AppKit 0x00007fff89e6c841 -[NSView initWithFrame:] + 457
3 libqcocoa.dylib 0x0000000103c2078f 0x103c0c000 + 83855
4 libqcocoa.dylib 0x0000000103c20a1d 0x103c0c000 + 84509
5 libqcocoa.dylib 0x0000000103c18f36 0x103c0c000 + 53046
6 libqcocoa.dylib 0x0000000103c14b72 0x103c0c000 + 35698
7 QtGui 0x0000000100f77603 QWindow::create() + 51
8 QtWidgets 0x0000000101509e13 QWidgetPrivate::create_sys(unsigned long long, bool, bool) + 1107
9 QtWidgets 0x00000001014df86c QWidget::create(unsigned long long, bool, bool) + 444
10 QtWidgets 0x00000001014eeebd QWidget::setVisible(bool) + 237
11 QtWidgets 0x000000010169679d QDialog::setVisible(bool) + 205
12 com.yourcompany.puls_connect 0x0000000100022305 DialogProgressIndicator::ShowDlg() + 21
13 com.yourcompany.puls_connect 0x000000010001a51e void QtPrivate::FunctionPointer<void (DialogProgressIndicator::*)()>::call<void, void>(void (DialogProgressIndicator::*)(), DialogProgressIndicator*, void**) + 142
14 com.yourcompany.puls_connect 0x000000010001a3fa QtPrivate::QSlotObject<void (DialogProgressIndicator::*)(), void, void>::impl(int, QtPrivate::QSlotObjectBase*, QObject*, void**, bool*) + 202
15 QtCore 0x0000000100bfdda2 QMetaObject::activate(QObject*, int, int, void**) + 1874
16 com.yourcompany.puls_connect 0x00000001000242db PulsTreeWidget::DisplayProgress() + 43

Answer 1

Remember that you can use GUI classes only in main thread. For parallel task use
QFutureWatcher:
WaitDialog waitDlg(this);
connect(this, SIGNAL(progress(int)), &waitDlg, SLOT(setProgress(int)));
QFutureWatcher<void> watcher;
connect(&watcher, SIGNAL(finished()), &waitDlg, SLOT(close()));
connect(&watcher, SIGNAL(canceled()), &waitDlg, SLOT(close()));
QFuture<void> future = QtConcurrent::run([] () {/*do parallel task here*/});
watcher.setFuture(future);
waitDlg.exec();