Qt - How to detect closed window in custom event loop without using slots

Question

I'm working on an OpenGL-based game using QGLWidget as a front-end. But due to all of the unnecessary overhead (plus having to distribute the heavy QT libraries!) I plan to migrate to GLUT, and am in the process of replacing 'Qt-stuff' with more standard alternatives before the big leap.

To replace the QTimers that control the framerate and fps timer, I'm trying to wedge those functions into a simple loop that replaces the app.exec() call, as follows:

//main.cpp
#include <QApplication>
#include <windows.h>
#include "mainwindow.h"

int main(int argc, char *argv[]) {
  QApplication app(argc, argv);
  MainWindow mainWin;//only contains a glwidget
  mainWin.show();

  do {
    app.processEvents();
    //call draw functions/fps based on system tick count
    //...
    Sleep(10);
  } while (!app.closingDown()); //<-- this doesn't terminate

  return 0;
}

So far it works fine during runtime, however once you try to close the window (via the system 'X' button), the window goes away but the program hangs in the background. The problem is that I can't find a static flag or query function that indicates that exit() or close() has been called. bool closingDown() always seems to be false, and I've tried toggling a static flag in the ~MainWindow destructor and detecting that, but that doesn't work either since that isn't called until main ends. I know I could probably do it by tying into QApps' aboutToQuit() signal, or maybe making a derived class from Qapp, and intercepting it, but that kind of defeats the purpose of moving away from signals/slots. Is there a simple way to tell if a QApplication has been ordered to shut down, from outside of the class?

Answer 1

You can reimplement

void QWidget::closeEvent ( QCloseEvent * event ) 

method to set your static flag. That method invokes for close events from window.

Answer 2

I recommend adding app.sendPostedEvents(); before that QApplication::processEvents() call. It may actually fix your problem, but if not, you can at least use event handlers as mentioned by @dzhon.

Nitpick: Qt is generally not formatted in all-caps. Saying 'QT' makes it appear as if you are talking about QuickTime!