How to get the result of QDialog::show()? - qt

Question

I want to convert exec usages to show method in my projects. Because, when I use the exec for windows (dialogs) I can not open another window. This is the basic difference between exec() and show() method.
The exec and show work in different ways and I am wondering how can I change the below code with using show() instead of exec().
For example:
int result = exampleWindow->exec();
if ( result == QDialogButtonBox::Ok )
{
exampleWindow->UpdateCalibrationData(&data);
exampleWindow->UpdateFilterData(&filterData);
exampleWindow();
}

Answer 1

show() shows a non-modal window;
exec() shows a modal window.
If you want to get the result of show(), then go with Qt signals/slots:
ExampleWindow::ExampleWindow(QWidget *parent) : QDialog(parent)
{
// Assuming the QDialogButtonBox name is "buttonBox":
connect(buttonBox, SIGNAL(accepted()), this, SLOT(accept()));
connect(buttonBox, SIGNAL(rejected()), this, SLOT(reject()));
}
MainWindow.h:
class MainWindow : public QMainWindow {
Q_OBJECT
private slots:
void updateData();
}
MainWindow.cpp:
MainWindow::MainWindow(QWidget *parent) : QMainWindow(parent)
{
connect(exampleWindow, SIGNAL(accepted()), this, SLOT(updateData()));
}
void MainWindow::updateData()
{
// Your code:
exampleWindow->UpdateCalibrationData(&data);
exampleWindow->UpdateFilterData(&filterData);
exampleWindow();
}

Answer 2

show() simply makes the dialog window visible. It is a QWidget method.
exec(), when provided in a class, always spins an event loop. In case of dialogs specifically, it'll make the dialog visible before spinning the event loop.
You could implement a functionally equivalent exec() yourself, as follows:
void myExec(QDialog * dialog) {
QStateMachine sm;
QState s1(&sm), s2(&sm);
sm.setInitialState(&s1);
QEventTransition transition(dialog, QEvent::Close);
s2.addTransition(&transition);
QEventLoop loop;
QObject::connect(&s2, &QState::entered, &loop, &QEventLoop::quit);
sm.start();
dialog->show();
loop.exec();
}
Generally speaking, you should never use exec() to create a nested event loop, since you're exposing a lot of your own code to possible reentrancy requirements. In case of dialogs, it is always possible to show a modal dialog box without using exec(), so there's really no point to it.

Answer 3

exec() method shows modal dialog as described here
You can use for example closeEvent() of your dialog. In it you can write something like:
void ExmpleWin::closeEvent(QCloseEvent *event)
{
if (/*Your condition*/) {
emit signalUpdateMyData();
event->accept();
}
}
Also you have to connect signal signalUpdateMyData() with apropriate slot where you can make:
exampleWindow->UpdateCalibrationData(&data);
exampleWindow->UpdateFilterData(&filterData);
exampleWindow();