How to get properly manage QMessageBox’s close signal

[LucaPolese]

Hi everyone,
As I wrote in the title, I cannot manage the call of the close button ("X") of a QMessageBox.
To give you a better understanding of what I mean, here is a picture of the window and the code I have written:

void MainWindow::closeEvent (QCloseEvent *event) {
    QMessageBox* messaggio;
    if(controller->getModificato()){
        messaggio = new QMessageBox(QMessageBox::Question, tr("QtDrive"), tr("Vuoi salvare le modifiche prima di uscire dall'applicazione"), QMessageBox::Yes | QMessageBox::No, this);
        messaggio->setButtonText(QMessageBox::Yes, tr("Sì"));
        messaggio->setButtonText(QMessageBox::No, tr("No"));
        messaggio->setWindowFlags(messaggio->windowFlags() & ~Qt::WindowCloseButtonHint);
        int ret = messaggio->exec();
        if(ret == QMessageBox::Yes){
            salvaIlFile();
            //Se ho salvato effettivamente il file -> modificato sarà false.
            //Altrimenti modificato continua ad avere valore true
            //Se chiudessi perderei tutta l'informazione
            if(controller->getModificato()) event->ignore();
            else event->accept();
        }else if(ret == QMessageBox::No){
            event->accept();
        }else event->ignore();
    }else{
        ...
    }

}

As the name of the function suggests, what is written in this code should only be executed when the programme is closed.

Only if there has been a change in the data (during the life of the program) the QMessageBox is presented to the user to ask if he wants to save these changes, otherwise the program is closed.

In practice, what I want to understand is how to manage the button "X" (at the top right of the photo), because when I click it, the check stops at the second one (else if(ret == QMessageBox::No)), whereas it should simply make the event->ignore() call in else.

I would also like to point out that I tried to remove the close button from the QMessageBox with the following codes, but the results were more than in vain:

• messaggio->setWindowFlags((messaggio->windowFlags() | Qt::CustomizeWindowHint) & ~Qt::WindowCloseButtonHint);
• messaggio->setWindowFlags(messaggio->windowFlags() & ~Qt::WindowCloseButtonHint);
• messaggio->setWindowFlags(Qt::CustomizeWindowHint | Qt::WindowTitleHint); as suggested here

But nothing has changed

(Maybe) useful information:
Qt version: 5.15.2
Qt Creator version: 4.15.2
Architecture: x86_64

[Cobra91151]

@LucaPolese

Hello!

First of all, you can remove the QMessageBox close button by using this code: setWindowFlags(Qt::Dialog | Qt::WindowTitleHint | Qt::CustomizeWindowHint);

Screenshot 1:

Secondly, you can handle the QMessageBox close event without QMessageBox::Cancel button by adding some bool class member variable to keep tracking the data change.
In close event, you can check if your data has been changed or not. For example: I have created bool isDataChanged in dialog.h file. In constructor (dialog.cpp) file I initialized the boolean variable to isDataChanged = false;. When pressing on Update data button isDataChanged sets to true. Finally, you can check if isDataChanged to display the QMessageBox in the closeEvent method.

void Dialog::closeEvent(QCloseEvent *event)
{
    qDebug() << "closeEvent";

    if (isDataChanged) {
        QMessageBox *messaggio = new QMessageBox(
                    QMessageBox::Question,
                    tr("QtDrive"),
                    tr("Vuoi salvare le modifiche prima di uscire dall'applicazione"),
                    QMessageBox::Yes | QMessageBox::No,
                    this);
        messaggio->setButtonText(QMessageBox::Yes, tr("Sì"));
        messaggio->setButtonText(QMessageBox::No, tr("No"));
        messaggio->setWindowFlags(Qt::Dialog | Qt::WindowTitleHint | Qt::CustomizeWindowHint);
        int ret = messaggio->exec();

        if (ret == QMessageBox::Yes) {
            event->accept();
        } else {
            event->ignore();
        }

        messaggio->deleteLater();
    }
}

Screenshot 2:

bool isDataChanged is just a variable for a test case, in your case you can use the controller->getModificato() method instead. Happy coding!

[SamurayH]

You can try adding a Cancel button, since the Close button is interpreted as a No:

QMessageBox::Yes | QMessageBox::No | QMessageBox::Cancel

[LucaPolese]

@SamurayH I could indeed, but the problem remains: if I press the "X" button, this is interpreted as a "No", when it should be interpreted according to the else case, and should therefore make the event->ignore() call

[SamurayH]

@LucaPolese Here's what I've tested and it works (Qt 5.14.2):

void Widget::closeEvent(QCloseEvent *event)
{
    if(! controller->getModificato())
        return;

    QMessageBox messaggio(
                QMessageBox::Question,
                tr("QtDrive"),
                tr("Vuoi salvare le modifiche prima di uscire dall'applicazione"),
                QMessageBox::Yes|QMessageBox::No|QMessageBox::Cancel,
                this);


    messaggio.setButtonText(QMessageBox::Yes, tr("Sì"));
    messaggio.setButtonText(QMessageBox::No, tr("No"));
    messaggio.setWindowFlags(messaggio.windowFlags() & ~Qt::WindowCloseButtonHint);

    int ret = messaggio.exec();

    if(ret == QMessageBox::Yes) {
        // Save settings
        event->accept();
    }
    else if(ret == QMessageBox::No) {
        // Close app
        event->accept();
    }
    else {
        // Do nothing (Cancel or Close)
        event->ignore();
    }
}

[LucaPolese]

@SamurayH It works fine for me too!
I believe that at this point I will adopt this version too.

The only thing that bothers me is that I can't handle the closing event of the QMessageBox ("X"), without adding the QMessageBox::Cancel button.
Would you happen to know if there is an additional solution for this?

[Cobra91151]

@LucaPolese

Hello!

First of all, you can remove the QMessageBox close button by using this code: setWindowFlags(Qt::Dialog | Qt::WindowTitleHint | Qt::CustomizeWindowHint);

Screenshot 1:

Secondly, you can handle the QMessageBox close event without QMessageBox::Cancel button by adding some bool class member variable to keep tracking the data change.
In close event, you can check if your data has been changed or not. For example: I have created bool isDataChanged in dialog.h file. In constructor (dialog.cpp) file I initialized the boolean variable to isDataChanged = false;. When pressing on Update data button isDataChanged sets to true. Finally, you can check if isDataChanged to display the QMessageBox in the closeEvent method.

void Dialog::closeEvent(QCloseEvent *event)
{
    qDebug() << "closeEvent";

    if (isDataChanged) {
        QMessageBox *messaggio = new QMessageBox(
                    QMessageBox::Question,
                    tr("QtDrive"),
                    tr("Vuoi salvare le modifiche prima di uscire dall'applicazione"),
                    QMessageBox::Yes | QMessageBox::No,
                    this);
        messaggio->setButtonText(QMessageBox::Yes, tr("Sì"));
        messaggio->setButtonText(QMessageBox::No, tr("No"));
        messaggio->setWindowFlags(Qt::Dialog | Qt::WindowTitleHint | Qt::CustomizeWindowHint);
        int ret = messaggio->exec();

        if (ret == QMessageBox::Yes) {
            event->accept();
        } else {
            event->ignore();
        }

        messaggio->deleteLater();
    }
}

Screenshot 2:

bool isDataChanged is just a variable for a test case, in your case you can use the controller->getModificato() method instead. Happy coding!

[SGaist]

@Cobra91151 one small thing to your suggestion: there's no need to allocated your message box on the heap. Just make it a local instance and it will be automatically destroyed at the end of the method.

[LucaPolese]

@Cobra91151 That's exactly what I was looking for
Thanks a lot!

[Cobra91151]

@SGaist

Hello!

I know that. I just copied @LucaPolese code and modify it a bit. Besides, it calls messaggio->deleteLater(); at the end of the method, so no memory leak occurs. I forgot to mention it in my post above. For a personal use, I have created MsgDialog class which inherits from QMessageBox class and call it as a local instance. Using MsgDialog class I can make it stay always on top, add custom icon etc. The example below displays a simple usage of my class.

Code:

MsgDialog testDlg(this, "Title", "This is a test?", QMessageBox::Question, QMessageBox::Yes | QMessageBox::No);
int userRes = testDlg.exec();

if (userRes == QMessageBox::Yes) {
    //confirm
} else {
   //decline
}

[SGaist]

@Cobra91151 No worries, I saw that you deleted the dialog at the end of the method. Just pointing out that it's an easy to forget step and that in this case it was not necessary to use the heap for that. It's also one small thing that allows for reduced heap fragmentation.