Forcing the Qt GUI to update before entering a separate function

Question

This seems like it should be automatic, but apparently it's not. I have the following code:

    ui.my_label->setText("Test 1...");
    ui.my_label->adjustSize();

    processThatTakesAbout30SecondsToFinish(files[0].toStdString());

    ui.my_label->setText("Finished.");
    ui.my_label->adjustSize();

What is happening is that I never see "Test1...", as the GUI seems to hang until the following function completes, and I eventaully only see "Finished.".

How can I make sure the GUI is updating before it enters that function?
Thanks.

Answer 1

You shuld be able to process the event queue before entering your code if you;

#include <QApplication>

and, when you want to refresh your GUI, call;

qApp->processEvents();

Note that it may be a good idea to let your long running process call that function now and then, to make your GUI feel more responsive.

Answer 2

If you don't care about your GUI being responsive during this time, then a call to my_label->repaint() would do the trick. Qt can't do anything automatically for you unless you yield to the event loop.

For maximimum responsiveness you might consider running your process in a separate thread and use signal/slot connections (which are thread-safe by default) to signal to your main GUI thread when your processing is complete.

Answer 3

I just wanted to add that for me it took a combo of the two answers I saw here. So what worked for me was:

ui.my_label->setText("Test 1...");
ui.my_label->adjustSize();

//! Both of these lines needed
ui.my_label->repaint();
qApp->processEvents();

processThatTakesAbout30SecondsToFinish(files[0].toStdString());

ui.my_label->setText("Finished.");
ui.my_label->adjustSize();

Hope this helps someone.