How to update the QT Mainwindow at each loop in mainwindow.cpp - qt

Question

My main.cpp look like this:
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
MainWindow w;
w.show();
return a.exec();
}
An in my mainwindow.cpp I want to show a different image at each loop in "while", so it would look like this:
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
image = load_an_image
int i=0;
while (i<15)
{
show image in the MainWindow
waitkey (wait until I press a key or wait some time)
do something to this image for the next loop
i++
}
}
However the Mainwindow does not show up until the "while" is finished and I cannot find how to show the MainWindow at each loop.
Can anyone give me any advice ?

Answer 1

GUI will not update itself until gui thread is free of other tasks. However, you can force it using
qApp->processEvents();
Following is example of very bad coding style, but that might be what you want.
#include "mainwindow.h"
#include <QApplication>
#include <thread>
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
MainWindow w;
w.show();
qint8 k = 15;
using namespace Qt;
QPalette pal;
QColor col = red;
while (k--)
{
std::this_thread::sleep_for(std::chrono::milliseconds(250));
pal.setBrush(w.backgroundRole(), QBrush(col));
w.setPalette(pal);
col = col == red ? blue : red;
qApp->processEvents();
}
return a.exec();
}
To run this, you will have to add QMAKE_CXXFLAGS += -std=c++11 to your '.pro' file.
And if you want to understand things better, i recommend to read about qt events.

Answer 2

You could delay handling the image by using a Qtimer. Something like this: -
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
QTimer::singleShot(5, this, SLOT(timeout());
}
// create as a slot in the MainWindow derived class
void MainWindow::timeout()
{
image = load_an_image();
int i=0;
while (i<15)
{
// show image in the MainWindow
// waitkey (wait until I press a key or wait some time)
// do something to this image for the next loop
i++
}
}
However, it would be better handled by loading the first image and then reacting to key events, rather than waiting directly in the main thread...
void MainWindow::keyReleaseEvent(QKeyEvent* keyEvent)
{
if(keyEvent->key() == Qt::Key_Space) // use the space bar, for example
{
if(m_imageFrame < 15)
{
// update the image
}
}
else
{
QMainWindow::keyReleaseEvent(keyEvent);
}
}