Question

My application is a windowing application that performs certain complex mathematical algorithms. Because I started with the application a long time ago, most of it is still single-threaded. To be more precise, the main thread executes all the complex calculation logic. It’s important to mention that during the calculations, I show some progress on the screen.

In most cases, the mathematical algorithms only take several seconds, so after the user has started the action, an hourglass (or the running circle in Windows 7) is shown, and a few seconds later the results are shown.

In some cases, the algorithm can take several minutes. During this time, I show the hourglass, and while the algorithm is busy, I show the progress in my window. But, if the user clicks in the application after it has been busy for a while, the Window becomes ‘more white’ (as if a non-completely-transparent piece of plastic is laid over the window), the Window is not updated anymore, and Windows reports ‘the application is not responding’.

I use Qt and I use the Qt function QWidget::repaint to force a repaint while my algorithm is busy. The repaint works for some time, but as said above, Windows seems to block this after a while.

What is the correct way to tell Windows that your application is still busy so that the window keeps on updating? If I enter an explicit message loop, the user might trigger other actions in the application which I don’t want.

• Is it enough to call PeekMessage?
• It is enough to call GetMessage?
• Or should I call DispatchMessage? And how do I prevent the user from starting another action (actually, prevent all user input)
• Should I call one of these messages every time I update my window, or can I limit myself to call it every few seconds (10 seconds?, 30 seconds? …)

Notice that moving the calculation logic to a separate thread is currently not an option.

I’m using Visual Studio 2010 on Windows 7, in combination with Qt 4.7.

Answer 1

The DisableProcessWindowGhosting function (see http://msdn.microsoft.com/en-us/library/ms648415(v=vs.85).aspx) tells Windows that it must not show the ‘ghost window’ if an application is not responsive.

My colleague did some experiments with it and noticed the following:

• the animation showing the progress continues nicely (this is actually what I wanted to achieve)
• the user can still minimize, move, … the window (great)
• on the downside: if the application is really hanging, the user must use Task Manager to kill it

So, this solves my problem.