Cannot change cursor in Qt main window from menu action - qt

Question

In my MainWindow, I have a push button and a menu bar item whose signals are both connected to the same slot. In the slot function, I have written:
mainWindow->setCursor(QCursor(Qt::WaitCursor));
This works as expected when the slot function is invoked via the button; however, when the same function is invoked from the menu, the wait cursor doesn't appear. Any idea why?
I also considered using QApplication::setOverrideCursor; however, that causes other problems.
Any recommendations? Thanks!
(I am using Qt 4.7 and doing my development on Windows 7 using Qt Creator with the default MinGW compiler.)
Here's more detail.
in MainWindow constructor: this->setCursor(Qt::CrossCursor);
signal/slot connections:
QObject::connect(button, SIGNAL(clicked()), MainWindow, SLOT(showMessageBox()));
QObject::connect(action, SIGNAL(triggered()), MainWindow, SLOT(showMessageBox()));
showMessageBox function:
void MainWindow::showMessageBox()
{
this->setCursor(Qt::WaitCursor);
// display wait cursor briefly before showing message box
for (int i = 0; i < 1<<30; ) {++i;}
QMessageBox msgBox;
msgBox.setText("Hello!");
msgBox.setStandardButtons(QMessageBox::Ok);
msgBox.setCursor(Qt::PointingHandCursor);
msgBox.exec();
this->setCursor(Qt::CrossCursor);
}
When showMessageBox is invoked with 'button', the wait cursor is displayed as expected.
When showMessageBox is invoked through 'action', the wait cursor does not appear; instead the cursor changes from Qt::CrossCursor to a Qt::ArrowCursor as soon as the user selects the 'action' menu item, and then changes to Qt::PointingHandCursor once the message box opens. The wait cursor never appears.

Answer 1

Your code is synchronous and uses a delay loop. When you're in the delay loop, there's no way for any Qt code to execute. A cursor change requires the event loop to be spinning -- so it appears from the symptoms you give.
Here's how to do it correctly -- remember, if you use delays/sleeps and other blocking calls in your GUI code, your users will hate you, and rightly so. Using exec() in message/dialog boxes is also bad style. Your application is asynchronous, code it so. Make sure your slots are declared as such (in the protected slots: section of MainWindow declaration).
void MainWindow::showMessageBox()
{
this->setCursor(Qt::WaitCursor);
QTimer::singleSlot(200, this, SLOT(slot1()); // fire slot1 after 200ms
}
void MainWindow::slot1()
{
QMessageBox * msgBox = new QMessageBox(this);
msgBox->setText("Hello!");
msgBox->setStandardButtons(QMessageBox::Ok);
msgBox->setCursor(Qt::PointingHandCursor);
msgBox->show();
connect(msgBox, buttonClicked(QAbstractButton*), SLOT(slot2(QAbstractButton*)));
}
void MainWindow::slot2(QAbstractButton* button)
{
// a button was clicked on the message box
this->setCursor(Qt::CrossCursor);
}