Updating Qt Images does not work until exec is called - c++

Question

I need to create a simple GUI which displays images, the images in this example can change and the GUI will need to update it's contents.
I wrote the following update function in my widget class:
void myClass::updatePic() {
QPixmap pix("./pic.png");
int width = ui->picLabel->width();
int height = ui->picLabel->height();
ui->picLabel->setPixmap(pix.scaled(width,height,Qt::KeepAspectRatio));}
I try to use it in the following manner:
int main(int argc, char *argv[]) {
QApplication a(argc, argv);
myClass w;
w.show();
sleep(3);
w.updatePic();
sleep(3);
w.updatePic();
sleep(3);
return a.exec();}
But the window just opens and does not display the images until we get to the a.exec() line, and then it opens the last image. What am I doing wrong?
EDIT:
Clarification, the trigger for changing the images comes from an external program (specifically, the gui will be a node in ros, and will be triggered by another node). Is there a way to push a button not from the gui via an external program? the timer will work but I dislike this "busy wait" style solutions.
Thanks for the suggestions so far

Answer 1

exec runs the QT event loop, which includes rendering widgets.
So move your updatePic call into your widget and activate it by for example a button or in the show event

Answer 2

At first learn more about event loop. In particular, you must know that all events like paintEvent or resizeEvent are usually called on corresponding events handle. The events handle is usually called by the event loop, i.e. inside of exec function.
Let's unite answers of #MohaBou and #RvdK. You need to handle timer shots after the exec call. Use QObject::timerEvent for this.
myClass::myClass()
{
<...>
// This two variables are members of myClass.
_timerId = startTimer(3000);
_updatesCount = 0;
}
myClass::~myClass()
{
<...>
// For any case. As far as I remember, otherwise the late event
// may be handled after the destructor. Maybe it is false, do
// not remember...
if (_timerId >= 0) {
killTimer(_timerId);
_timerId = - 1;
}
}
myClass::timerEvent(QTimerEvent *event)
{
if (event->timerId() == _timerId) {
if (_updatesCount < 2) {
updatePic();
++_updatesCount;
} else {
killTimer(_timerId);
_timerId = - 1;
}
}
}
The startTimer method here adds especial timer event to the event query every 3 seconds. As all events, it may be handled only when the event loop will take control and all earlier events are handled. Because of it you can have a duration if many "heavy" events are handled.
EDIT: sorry, I didn't understand #MohaBou at first read. His answer with explicit QTimer is also good enough (but I still don't understand a part about modality).

Answer 3

The function exec also renders the child widgets. exec() blocks the application flow while show() doesn't. So, exec is mainly used for modal dialogs.
I recommend to update it in your custom witget by using a refresh timer. Use a QTimer to update the image every 3 secs:
QTimer* timer = new QTimer(this);
timer->setInterval(3000);
connect(timer, SINGAL(timeout()), this, SLOT(updatPicture()));
Update your picture in your custom slot:
MainWindow::updatePicture() {
updatePic()
}
If you want, you could use a lambda function:
connect(timer, &QTimer::timeout, this, [&w]() {
updatePic()
});