[Solved] QFileDialog::getOpenFileName breaks message boxes etc.



  • Hi!

    The following code sequence (a slot being called) causes big problems:
    @
    void CMainWidget::doMain() {
    //QWidget *const pParent = nullptr;
    QWidget *const pParent = this;

    // check that the message box is working as expected!
    QMessageBox::information(pParent, "Test Msgbox", tr("Qt Version: %1").arg(QT_VERSION_STR));

    // THIS QFILEDIALOG BREAKS SOMETHING!
    // comment the next line out to see everything working normal!
    QFileDialog::getOpenFileName(pParent, "Open some file", QString(), tr("All files (.)"), nullptr, QFileDialog::ReadOnly);

    // using this "non-static" version doesn't work, too
    //QFileDialog FileDialog;
    //FileDialog.getOpenFileName(pParent, "Open some file", QString(), tr("All files (.)"), nullptr, QFileDialog::ReadOnly);

    QInputDialog::getText(pParent, "Enter a text", "Blablabla:");

    // you'll hear the message box sound, but don't see anything
    QMessageBox::warning(pParent, "Test Msgbox 2", "Hi, I'm another msg box");

    // the error msg will show up, but it will stay "unusable" in the background
    // also the program will execute the following file dialog at the same time without waiting
    QErrorMessage ErrMsg;
    ErrMsg.setModal(true);
    ErrMsg.showMessage(tr("This is some sort of error message"));
    ErrMsg.exec();

    // this pops up too early
    QFileDialog::getOpenFileName(pParent, "Open another file", QString(), tr("All files (.)"), nullptr, QFileDialog::ReadOnly);

    cout << "doMain() done. Time to quit()." << endl;
    qApp->quit();
    }//end method
    @

    The problem is, if "getOpenFileName()" is called (no matter whether from an QFileDialog object or as static method), then the following message boxes don't show up -- you do hear the message box sound though. The QInputDialog (in this case, getText, but tested for other methods, too) doesn't show either. The QErrorMessage shows, but stays in the background and is unusable. The 2nd QFileDialog does surprisingly work.

    I tried many things. The slot "doMain()" belongs to a QWidget, but I also tried it with a QObject (I'd prefer the latter). I tried to pass "this" (i.e. the QWidget) as parent-pointer, but I also tried passing NULL. It all doesn't make a difference.

    I upgraded to Qt 4.7.3. I'm using VS2008 32bit. The problem occurs on both Windows 7 and Windows XP SP3 (32bit each).

    I posted something similar before, but this is the the most "common" version of the problem I could derive. Old thread was http://developer.qt.nokia.com/forums/viewthread/6896 .

    PLEASE help me, or try to reproduce the problem on your machine. Is my event loop messed up? I simply cannot understand what's happening.

    Here's a zip-file with a qmake Pro-File and a Visual Studio solution: https://gigamove.rz.rwth-aachen.de/download/id/CbLuQWMNeobPuB (direct download link should be https://gigamove.rz.rwth-aachen.de/d/id/CbLuQWMNeobPuB/dd/100 )

    Many thanks for any comment!
    Linus



  • -Works perfectly on Ubuntu.-
    EDIT: Just downloaded your sources, same problem as you have. I'll investigate.



  • The problem doesn't occur when QTimer isn't used. Weird!!



  • Thanks for testing!

    As a work-around, could you tell me how I could invoke "doMain()" without the QTimer? Basically I want the event-loop to be running so I can display static input dialogs and execute some sequential procedure (like, in this case, doMain). I.e., what did you do avoid using QTimer?



  • @ // call doMain of the main QObject or QWidget
    myMain->doMain();
    @
    Why do you need the event loop to run?

    EDIT: Another result: It doesn't matter where the filedialog is!
    EDIT2: This is a workaround:
    @
    QFileDialog FileDialog = new QFileDialog(pParent,"Open some file", QString(), tr("All files (.)"));
    FileDialog->exec();
    delete FileDialog;
    @
    EDIT3: This workd, too BTW:
    @ QFileDialog d(pParent,"Open some file", QString(), tr("All files (
    .*)"));
    d.exec();
    @
    EDIT4: I guess that means the problem lies in the code that executes platform-specific dialog. Looking through the sources now...



  • [quote author="loladiro" date="1308761399"]
    Why do you need the event loop to run?
    [/quote]
    At least I think I need it to run. Basically to have dialogs I occasionally pop up working. I don't really remember right now, but in my huge project I've always been using this main() routine and CMain class as boiler-plate code. IIRC then in the beginning I only needed QErrorMessage to work, but now I sometimes have a QGLWidget which needs to react on key-presses.

    Maybe you're right, and I don't need the event-loop, I'm a bit confused. But after all, isn't that what you do with an QApplication? I always thought that without the app.exec(), you couldn't use dialogs/GUI elements at all...

    Anyway, thanks very much for the workarounds, I'll try and gladly use them!



  • You need the event loop for non nmodal windows / dialogs. Modal dialogs spin an own event loop.



  • Ok, the problem is definitely in the platform specific file dialog if you use:
    @
    QFileDialog::getOpenFileName(pParent, "Open some file", QString(), tr("All files (.)"), nullptr, QFileDialog::ReadOnly | QFileDialog::DontUseNativeDialog);
    @
    There's no problem either. The only thing that's weird about that is that it occurs on both Linux and Windows.

    EDIT: This is getting more and more confusing!!!! If the main windows is visible there's no problem!



  • The problem is, that you give a modal window a parent window, which was never shown. If you call mainWnd.show() before, everything works fine.



  • [deleted original post myself because of stupid question ;]
    It does work. SO yeah problem solved.



  • [quote author="loladiro" date="1308761399"]
    EDIT3: This workd, too BTW:
    @ QFileDialog d(pParent,"Open some file", QString(), tr("All files (.)"));
    d.exec();
    @
    [/quote]
    While this does in fact work, it shows a different looking file dialog -- it feels "non-native". I'd like to show the platform-specific default thing, in this case the default Windows version.

    [quote author="loladiro" date="1308761399"]
    EDIT4: I guess that means the problem lies in the code that executes platform-specific dialog. Looking through the sources now...
    [/quote]
    Yes, that's what I suspect now, too.

    Especially, if you change the workaround to this,
    @ QFileDialog d(pParent,"Open some file", QString(), tr("All files (.)"));
    //d.exec();
    d.getOpenFileName();
    @
    then it's broken again right away.

    I get the feeling getOpenFileName() breaks something, or indeed I do something unallowed with my event loop, which I don't understand...



  • [quote author="Gerolf" date="1308764209"]The problem is, that you give a modal window a parent window, which was never shown. If you call mainWnd.show() before, everything works fine.[/quote]

    Well no, I don't give a parent window. Ideally, I'd like to pass nothing (NULL). Just during debugging, I introduced pParent and tried setting it to "this", as well as "nullptr". It doesn't make a difference.

    My favourite case would be just to use an QObject, which I can't pass as parent anyway. I only tried deriving QMainWidget from QWidget, so that I could try passing "this" as parent for a test.

    If you could explain what exactly I should do without a main window, than I'll gladly try this.

    [quote author="loladiro" date="1308764395"]
    It does work. SO yeah problem solved.[/quote]
    Could you elaborate? Did you find something new? In my eyes, the problem is still not solved I'm afraid :-(. I'd like to somehow use "getOpenFileName()" while my event loop is running, and I still don't get how to show message boxes afterwards.

    So I could either refrain from starting my QApplication's event loop (which I don't want), or I could just not use getOpenFileName(). That can't be it, right? I hope there's just something wrong I'm doing with my "concept"...?



  • You're right it is a little odd that it only happens with native file dialogs.
    The problem (at least the trivial one is that) the following code in QEventLoop is executed (q->threadData->quitNow is set) after using native file dialogs. I 'm trying to find out why (just out of interest):
    qeventloop.cpp:178,179
    @
    if (d->threadData->quitNow)
    return -1;
    @



  • For what do you need an event loop in your example?
    You want to open fife modal dialogs, one after each other, no event loop is needed for that.
    You need an event loop for mode less dialogs, mode less windows or if you have a system of objects, that react on signal/slot in a not synchronous way (no queued connection, no threads)



  • [quote]For what do you need an event loop in your example?
    You want to open fife modal dialogs, one after each other, no event loop is needed for that.
    You need an event loop for mode less dialogs, mode less windows or if you have a system of objects, that react on signal/slot in a not synchronous way (no queued connection, no threads)[/quote]
    I agree with Gerolf, however, I'm still interested in why it's happening and will report here if I find anything. In the meantime, just use a workaround.

    EDIT: I found out why the problem doesn't occur with a visible window. Consider the following code from QApplication:
    @
    void QApplicationPrivate::emitLastWindowClosed()
    {
    if (qApp && qApp->d_func()->in_exec) {
    if (QApplicationPrivate::quitOnLastWindowClosed) {
    // get ready to quit, this event might be removed if the
    // event loop is re-entered, however
    QApplication::postEvent(qApp, new QEvent(QEvent::Quit));
    }
    emit qApp->lastWindowClosed();
    }
    }@
    EDIT2: This also doesn't happen if native dialog aren't used (The event loop gets reentered immidiately, but native widgets don't use the Qt event loop, so the application has enough time to process the quit event - it is actually done explicitly in the function that calls the native widgets - very interesting).



  • Hi,

    I tried with no message loop, and it worked as expected:

    @
    void mydoMain() {
    // check that the message box is working as expected!
    QMessageBox::information(0, "Test Msgbox", QObject::tr("Qt Version: %1").arg(QT_VERSION_STR));

        QFileDialog::getOpenFileName(0, "Open some file", QString(), QObject::tr("All files (*.*)"), nullptr, QFileDialog::ReadOnly);
    
        QInputDialog::getText(0, "Enter a text", "Blablabla:");
    
        // you'll hear the message box sound, but don't see anything
        QMessageBox::warning(0, "Test Msgbox 2", "Hi, I'm another msg box");
    
        // the error msg will show up, but it will stay "unusable" in the background
        // also the program will execute the following file dialog at the same time without waiting
        QErrorMessage ErrMsg;
        ErrMsg.setModal(true);
        ErrMsg.showMessage(QObject::tr("This is some sort of error message"));
        ErrMsg.exec&#40;&#41;;
    
        // this pops up too early
        QFileDialog::getOpenFileName(0, "Open another file", QString(), QObject::tr("All files (*.*)"), nullptr, QFileDialog::ReadOnly);
    
        cout << "doMain() done. Time to quit()." << endl;
    

    }//end method

    int main(int argc, char *argv[]) {
    QApplication app(argc, argv);

        mydoMain();
        return 0;
    

    }//end main
    @



  • Yes you didn't start the event loop. Use QTimer. And consider replacing the FileDialog with QApplication::processEvents();.
    I would say this qualifies as a bug.



  • [quote author="Gerolf" date="1308939653"]For what do you need an event loop in your example?
    [/quote]
    The example is a minimal stripped down version of my real project, just to demonstrate this problem (and to avoid the dependencies of including Boost and OpenCV).

    In my project (a framework actually), I want to later have the option to compose everything into a big GUI. Certain parts of the system might get their parameters programmatically for batch processing, but if they don't, they show these dialogs to ask the user "on the fly". So I definitely want to have the option to use my classes in a Qt project where a global event loop is running!

    Right now I have a modeless QGLWidget which reacts on key-presses, which probably needs an event-loop anyway. And I sometimes have multiple OpenCV Windows of their HighGUI subsystem with Qt elements, which also needs an event loop I think.

    [quote author="Gerolf" date="1308939653"]
    You want to open fife modal dialogs, one after each other, no event loop is needed for that.
    You need an event loop for mode less dialogs, mode less windows or if you have a system of objects, that react on signal/slot in a not synchronous way (no queued connection, no threads)
    [/quote]
    Just to make absolute sure there is no misunderstanding: I want the full capabilities of a Qt GUI application, including a global event loop to have the freedom of later adding a main window or any other GUI element.

    The big question to me is: Did I break thinks by using modal input/file dialogs in a strange unusual way, or might this really be some Qt bug or quirk? It just doesn't make any sense to me why this example code doesn't work the way it should...

    Thanks for the comments.



  • [quote author="loladiro" date="1308942428"]Yes you didn't start the event loop. Use QTimer. And consider replacing the FileDialog with QApplication::processEvents();.
    I would say this qualifies as a bug.[/quote]
    During my tests, I tried to put "qApp->processEvents();" and "qApp->sendPostedEvents();" between the dialogs -- no luck.



  • I have an idea what could happen:

    I was playing around a bit more now. Without event loop, everything is fine. With a visible main window, everything is fine. with event loop and without main window, it does not work.

    why? QApplication has a signal lastWindowClosed, which is emitted, if the last top level widget is closed:

    bq. By default,

    • this attribute is set for all widgets except transient windows such as splash screens, tool windows, and popup menus
    • QApplication implicitly quits when this signal is emitted.

    so, to work around it, use a property: "QApplication::quitOnLastWindowClosed":http://doc.qt.nokia.com/4.7/qapplication.html#quitOnLastWindowClosed-prop and set it to false, and voila, it works :-)

    @
    int main(int argc, char *argv[]) {
    QApplication app(argc, argv);

        CMainWidget w;
        // w.show();
    
        app.setQuitOnLastWindowClosed(false);
        QTimer::singleShot(0, &w, SLOT(doMain()));
    
        return app.exec();
    

    }//end main
    @

    This means, it's not a bug, it's a feature :-)



  • Yes, I meant QApplication::processEvent() is the problem here. Sorry if I was unclear. I'll reupload your example in two files, so that other people can see the problem.

    So: Problem with comment why (Please use this code for tests):
    http://dl.dropbox.com/u/32965023/bugTest/bugTest.pro
    http://dl.dropbox.com/u/32965023/bugTest/main.cpp
    http://dl.dropbox.com/u/32965023/bugTest/test.h

    EDIT: @Gerolf, yes see my previous comment as to why it happens
    EDIT2: Well yes, it might be a feature, but the problem is that QWidget removes that event again, as soon as another one gets created (If there's no processEvents in between. I think the same code should be added to native widgets



  • Hi loladrio,

    this solves the issue:

    @
    app.setQuitOnLastWindowClosed(false);
    @

    No processEvents needed



  • Yes I know. My problem with the issue is more conceptual not practical. IMO, Qt should behave the same way independent of whether QFileDialog uses a native window or not. But in this point it doesn't.



  • [quote author="Gerolf" date="1308943261"]
    This means, it's not a bug, it's a feature :-)[/quote]
    Wow, thank you so much!!! Best one-line-fix ever ;-)

    However, there is one strange thought left: Why do several standard dialogs show different behaviours? How come that message and input boxes will silently "go away", a QErrorMessage will "hang", and QFileDialogs won't be impressed at all by this QApplication::quitOnLastWindowClosed property (try it out, file dialogs are still working in that state).

    And then the differences in behaviour depending on which constructor you use and whether it's a native look or not -- that's just odd.

    But anyway, thanks again, huge relief for me. Solved!



  • This is part of the QWidget constructor: IMO, it should also be executed when constructing native windows.

    EDIT: Oops, forgot code
    @
    if (isWindow() || parentWidget()->isVisible()) {
    // remove posted quit events when showing a new window
    QCoreApplication::removePostedEvents(qApp, QEvent::Quit);
    @



  • But QFileDialog with use native dialog opens a nativ windiows file dialog. This does not look at the Qt event loop.

    [quote author="loladiro" date="1308943850"]@
    if (isWindow() || parentWidget()->isVisible()) {
    // remove posted quit events when showing a new window
    QCoreApplication::removePostedEvents(qApp, QEvent::Quit);
    @[/quote]

    This is the close event. The signal quit is different and as it is in the same thread, it's executed directly.



  • No it's not and that on purpose so that if there is a brief period with no active widget, the event is remove again:
    @
    void QApplicationPrivate::emitLastWindowClosed()
    {
    if (qApp && qApp->d_func()->in_exec) {
    if (QApplicationPrivate::quitOnLastWindowClosed) {
    // get ready to quit, this event might be removed if the
    // event loop is re-entered, however
    QApplication::postEvent(qApp, new QEvent(QEvent::Quit));
    }
    emit qApp->lastWindowClosed();
    }
    }
    @

    EDIT: To explain (I'm sure @Gerolf, you know what this means, but now everybody reading this thread will), the close event on QApplication is one executed directly (which would be sendEvent()), but postponed until the eventLoop is run next, in order to give QWidget a chance to remove it again.


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