QMessageBox::warning replacement that wont process the event loop

[stefanwoe]

I agree that in a perfect world this shouldn't happen. And i rather see this problem on user reports than on debugging. The situations are typically more complex as described in my example. In most cases its not a problem. In some - very rare but unexpected ones - it is.

Unfortunately a nullptr parent does not help. See the stack below, which starts with ```
QMessageBox::warning(0x0000000000000000 and this still sends a activate event when switching to the application etc.:

Qt6Cored.dll!doActivate<0>(0x0000021a8dad3f90, 7, 0x000000af0bf92a58) Line 3919	C++
Qt6Cored.dll!QMetaObject::activate(0x0000021a8dad3f90, 0x00007ffa8df943d0, 0, 0x000000af0bf92a58) Line 3965	C++
Qt6Widgetsd.dll!QMdiArea::subWindowActivated(0x0000021a928c29b0) Line 309	C++
Qt6Widgetsd.dll!QMdiAreaPrivate::emitWindowActivated(0x0000021a928c29b0) Line 1083	C++
Qt6Widgetsd.dll!QMdiAreaPrivate::_q_processWindowStateChanged({...}, {...}) Line 758	C++
Qt6Widgetsd.dll!QMdiArea::qt_static_metacall(0x0000021a8dad3f90, InvokeMetaMethod, 10, 0x000000af0bf92fc8) Line 181	C++
Qt6Cored.dll!doActivate<0>(0x0000021a928c29b0, 7, 0x000000af0bf92fc8) Line 3919	C++
Qt6Cored.dll!QMetaObject::activate(0x0000021a928c29b0, 0x00007ffa8df9f5f0, 0, 0x000000af0bf92fc8) Line 3965	C++
Qt6Widgetsd.dll!QMdiSubWindow::windowStateChanged({...}, {...}) Line 229	C++
Qt6Widgetsd.dll!QMdiSubWindowPrivate::setActive(true, false) Line 1470	C++
Qt6Widgetsd.dll!QMdiAreaPrivate::activateCurrentWindow() Line 1016	C++
Qt6Widgetsd.dll!QMdiArea::eventFilter(0x000000af0bf9f7b8, 0x000000af0bf94388) Line 2594	C++
Qt6Cored.dll!QCoreApplicationPrivate::sendThroughApplicationEventFilters(0x000000af0bf9f7b8, 0x000000af0bf94388) Line 1171	C++
Qt6Widgetsd.dll!QApplicationPrivate::notify_helper(0x000000af0bf9f7b8, 0x000000af0bf94388) Line 3372	C++
Qt6Widgetsd.dll!QApplication::notify(0x000000af0bf9f7b8, 0x000000af0bf94388) Line 2760	C++
Qt6Cored.dll!QCoreApplication::notifyInternal2(0x000000af0bf9f7b8, 0x000000af0bf94388) Line 1063	C++
Qt6Cored.dll!QCoreApplication::sendSpontaneousEvent(0x000000af0bf9f7b8, 0x000000af0bf94388) Line 1484	C++
Qt6Guid.dll!QGuiApplicationPrivate::setApplicationState(ApplicationActive, false) Line 3688	C++
Qt6Guid.dll!QGuiApplicationPrivate::processActivatedEvent(0x0000021a93d338b0) Line 2482	C++
Qt6Guid.dll!QGuiApplicationPrivate::processWindowSystemEvent(0x0000021a93d338b0) Line 2033	C++
Qt6Guid.dll!QWindowSystemInterface::sendWindowSystemEvents({...}) Line 1172	C++
Qt6Guid.dll!QWindowsGuiEventDispatcher::sendPostedEvents() Line 81	C++
Qt6Cored.dll!QEventDispatcherWin32::processEvents({...}) Line 474	C++
Qt6Guid.dll!QWindowsGuiEventDispatcher::processEvents({...}) Line 72	C++
Qt6Cored.dll!QEventLoop::processEvents({...}) Line 140	C++
Qt6Cored.dll!QEventLoop::exec({...}) Line 232	C++
Qt6Widgetsd.dll!QDialog::exec() Line 600	C++
Qt6Widgetsd.dll!showNewMessageBox(0x0000000000000000, Warning, {...}, {...}, {...}, NoButton) Line 1646	C++
Qt6Widgetsd.dll!QMessageBox::warning(0x0000000000000000, {...}, {...}, {...}, NoButton) Line 1747	C++

[mpergand]

@stefanwoe said in QMessageBox::warning replacement that wont process the event loop:

Id like to replace QMessageBox::warning with a own implementation that will not process events.

Hi,
You can create your own eventloop like that:

/** BUG Qt 5.12
    *  Le sheet saute au milieu de l'écran
    */  
   
   fileDialog.open();
   
   QEventLoop eventLoop;
   connect(&fileDialog, &QFileDialog::finished, &eventLoop,&QEventLoop::quit);
   eventLoop.exec();
   
   if(fileDialog.result() == QDialog::Accepted)
       {
       QStringList list=fileDialog.selectedFiles();
       
       if(!list.isEmpty())
   		filename=list.at(0);
       }
   
   return filename;

This silly code is the result of a silly bug in Qt5.12

But, It should be unecessary because QDialog::exec or QMessage::exec already do that and block the main event loop.

[J.Hilk]

@stefanwoe the solution is to use complete QMessageBox instance and show it by calling show() and not exec().

exec() will spann the event loop, show will rely on your main event loop

the static call off QMessageBox::warning will also spawn an additional event loop.

but, this will result in your having to refactor stuff, as the show() call does not "wait" inplace on the user input!

[stefanwoe]

@mpergand said in QMessageBox::warning replacement that wont process the event loop:

But, It should be unecessary because QDialog::exec or QMessage::exec already do that and block the main event loop.

Well they process events at least - see the stack above.

Inspired by your code i tried:

QMessageBox msgBox(this);
msgBox.setText("My message");
msgBox.setStandardButtons(QMessageBox::Yes | QMessageBox::No);
msgBox.setDefaultButton(QMessageBox::Yes);
QEventLoop eventLoop;
connect(&msgBox, &QMessageBox::finished, &eventLoop,&QEventLoop::quit);
int ret = eventLoop.exec();  

The Dialog wont show, but eventLoop.exec(); blocks. probably i missed something?

[jsulm]

@stefanwoe said in QMessageBox::warning replacement that wont process the event loop:

probably i missed something?

Yes, you do not show your dialog

[stefanwoe]

@jsulm Youre right. I added msgBox.show();

QMessageBox msgBox(this);
msgBox.setText("My message");
msgBox.setStandardButtons(QMessageBox::Yes | QMessageBox::No);
msgBox.setDefaultButton(QMessageBox::Yes);

QEventLoop eventLoop;
connect(&msgBox, &QMessageBox::finished, &eventLoop,&QEventLoop::quit);
msgBox.show();
int ret =    eventLoop.exec();

This works, but still it processes events - as far as i understand it this connect(...) just adds a additional event loop, but does not replace the default one:

Qt6Cored.dll!doActivate<0>(0x00000268d32e5e70, 7, 0x0000006f304f29f8) Line 3919	C++
Qt6Cored.dll!QMetaObject::activate(0x00000268d32e5e70, 0x00007ffa80a243d0, 0, 0x0000006f304f29f8) Line 3965	C++
Qt6Widgetsd.dll!QMdiArea::subWindowActivated(0x00000268d378ae20) Line 309	C++
Qt6Widgetsd.dll!QMdiAreaPrivate::emitWindowActivated(0x00000268d378ae20) Line 1083	C++
Qt6Widgetsd.dll!QMdiAreaPrivate::_q_processWindowStateChanged({...}, {...}) Line 758	C++
Qt6Widgetsd.dll!QMdiArea::qt_static_metacall(0x00000268d32e5e70, InvokeMetaMethod, 10, 0x0000006f304f2f68) Line 181	C++
Qt6Cored.dll!doActivate<0>(0x00000268d378ae20, 7, 0x0000006f304f2f68) Line 3919	C++
Qt6Cored.dll!QMetaObject::activate(0x00000268d378ae20, 0x00007ffa80a2f5f0, 0, 0x0000006f304f2f68) Line 3965	C++
Qt6Widgetsd.dll!QMdiSubWindow::windowStateChanged({...}, {...}) Line 229	C++
Qt6Widgetsd.dll!QMdiSubWindowPrivate::setActive(true, false) Line 1470	C++
Qt6Widgetsd.dll!QMdiAreaPrivate::activateCurrentWindow() Line 1016	C++
Qt6Widgetsd.dll!QMdiArea::eventFilter(0x0000006f304ff4e8, 0x0000006f304f4328) Line 2594	C++
Qt6Cored.dll!QCoreApplicationPrivate::sendThroughApplicationEventFilters(0x0000006f304ff4e8, 0x0000006f304f4328) Line 1171	C++
Qt6Widgetsd.dll!QApplicationPrivate::notify_helper(0x0000006f304ff4e8, 0x0000006f304f4328) Line 3372	C++
Qt6Widgetsd.dll!QApplication::notify(0x0000006f304ff4e8, 0x0000006f304f4328) Line 2760	C++
Qt6Cored.dll!QCoreApplication::notifyInternal2(0x0000006f304ff4e8, 0x0000006f304f4328) Line 1063	C++
Qt6Cored.dll!QCoreApplication::sendSpontaneousEvent(0x0000006f304ff4e8, 0x0000006f304f4328) Line 1484	C++
Qt6Guid.dll!QGuiApplicationPrivate::setApplicationState(ApplicationActive, false) Line 3688	C++
Qt6Guid.dll!QGuiApplicationPrivate::processActivatedEvent(0x00000268d9547bb0) Line 2482	C++
Qt6Guid.dll!QGuiApplicationPrivate::processWindowSystemEvent(0x00000268d9547bb0) Line 2033	C++
Qt6Guid.dll!QWindowSystemInterface::sendWindowSystemEvents({...}) Line 1172	C++
Qt6Guid.dll!QWindowsGuiEventDispatcher::sendPostedEvents() Line 81	C++
Qt6Cored.dll!QEventDispatcherWin32::processEvents({...}) Line 474	C++
Qt6Guid.dll!QWindowsGuiEventDispatcher::processEvents({...}) Line 72	C++
Qt6Cored.dll!QEventLoop::processEvents({...}) Line 140	C++
Qt6Cored.dll!QEventLoop::exec({...}) Line 232	C++

[stefanwoe]

@J-Hilk This seems like a good starting point, but i dont get this to work ad hoc. How can this be used as a modal dialog? Any references?

[J.Hilk]

@stefanwoe said in QMessageBox::warning replacement that wont process the event loop:

w can this be used as a modal dialog? Any references?

:D

setWindowModality(Qt::ApplicationModal);

https://doc.qt.io/qt-5/qmessagebox.html#setWindowModality

[stefanwoe]

@J-Hilk
i tried this here (of course a incomplete test):

QMessageBox msgBox(this);
msgBox.setText("My message");
msgBox.setStandardButtons(QMessageBox::Yes | QMessageBox::No);
msgBox.setDefaultButton(QMessageBox::Yes);
msgBox.setWindowModality(Qt::ApplicationModal);
msgBox.show();

show() now of course will not wait until the dialog is closed etc. How can i accomplish this? It seems like the correct way, but so far i dont get it.

[J.Hilk]

@stefanwoe
make msgBox persistent, for example as a class member, then it should at least show.

[stefanwoe]

@J-Hilk It shows - but i how can do i add a loop or the like to process the dialog until a button is pressed?

[J.Hilk]

@stefanwoe you can't that's why I wrote:

but, this will result in your having to refactor stuff, as the show() call does not "wait" inplace on the user input!

your code/ function block will have to end on the show() call of your message box, and than you "continue" in a slot/function connected to one of its signals, for example
https://doc.qt.io/qt-5/qdialog.html#finished

[stefanwoe]

@J-Hilk That i understand - but this will continue code execution after the msgBox.show() statement - as it wont block control flow. So the code after msgBox.show() will be executed while the dialog is shown. But what i want, is a replacement for QMessageBox::warning which blocks further execution while the dialog is shown.
How can that be done?

[J.Hilk]

@stefanwoe said in QMessageBox::warning replacement that wont process the event loop:

How can that be done?

it can't not without calls to processEvents() or additional QEventLoops

[mchinand]

This post is deleted!

[JonB]

@stefanwoe
As @J-Hilk says.
And since your earlier traceback showed

...
Qt6Cored.dll!QEventDispatcherWin32::processEvents({...}) Line 474	C++
Qt6Guid.dll!QWindowsGuiEventDispatcher::processEvents({...}) Line 72	C++
Qt6Cored.dll!QEventLoop::processEvents({...}) Line 140	C++
Qt6Cored.dll!QEventLoop::exec({...}) Line 232	C++

it seems to me likely/possible that you will be in the same state as you were originally.....

[stefanwoe]

I now tried:

QMessageBox msgBox(this);
msgBox.setText("My message");
msgBox.setStandardButtons(QMessageBox::Yes | QMessageBox::No);
msgBox.setDefaultButton(QMessageBox::Yes);
msgBox.setWindowModality(Qt::ApplicationModal);
msgBox.show();
QEventLoop eventLoop(&msgBox);
eventLoop.exec(QEventLoop::DialogExec);

Still the same problem.

Inspecting the call stack in the debugger i think the root problem lies in

Qt6Guid.dll!QGuiApplicationPrivate::processWindowSystemEvent(0x000001577107c8c0) 
Qt6Guid.dll!QWindowSystemInterface::sendWindowSystemEvents({...}) Line 1172	C++
Qt6Guid.dll!QWindowsGuiEventDispatcher::sendPostedEvents() Line 81	C++
Qt6Cored.dll!QEventDispatcherWin32::processEvents({...}) Line 474	C++
Qt6Guid.dll!QWindowsGuiEventDispatcher::processEvents({...}) Line 72	C++
Qt6Cored.dll!QEventLoop::processEvents({...}) Line 140	C++
Qt6Cored.dll!QEventLoop::exec({...}) Line 232	C++

I can not find any configurable way to - temporarily - skip this call chain, when using QEventLoop.

[JonB]

@stefanwoe
I don't think you can, that is my point. You need an event loop to deal with any dialog, and some event being processed is upsetting something in the system....
Really ISTM the original issue, whatever it is/causes it, ought be sorted out. Like a (reproducible) bug report. But that may take time for a resolution.....

[stefanwoe]

I found this SO discussion which states the same problem:
https://stackoverflow.com/questions/26502673/will-qmessagebox-block-the-running-of-the-whole-main-thread-in-qt

The solution here is:

This is why we have a policy in our company that forbids to use QMessageBox::question() (and similar) and to call exec() on dialogs in our applications. We are creating modal dialogs on the heap and use their signals instead.

Well - can anybody tell what that means? Would this work without a event loop?

[SGaist]

Hi,

It just means that they impose a single event loop throughout the whole application lifetime.

Basically, you should refactor your logic so that you do not depend on the fact that the event loop is processing events.

Can you explain how you implement your logic ?