wxWidgets - multitasking with single thread

Question

I have a GUI app that I am creating with wxWidgets. As part of the functionality, I have to run "tasks" simultaneously with manipulation of the GUI window. For example, I may run the code:

long currentTime = wxGetLocalTime();
long stopTime = wxGetLocalTime() + 3;
while (wxGetLocalTime() != stopTime) {}
wxMessageBox("DONE IN APP");

For the duration of those 3 seconds, my application would essentially be frozen until the wxMessageBox is shown. Is there a way to have this run in the background without the use of multiple threads? It creates problems for the application that I've developing.

I was wondering if there are some types of event handling that could be used. Any sort of help is greatly appreciated.

Answer 1

There are 3 ways to run time-consuming tasks in GUI wx applications:

1. By far the most preferred is to use a different thread. The explanation of the application being "very GUI intensive" really doesn't make any sense to me, I think you should seriously reconsider your program design if its GUI intensity (whatever it is) prevents you from using background worker threads. If you do use this approach, it's pretty simple but pay special attention to the thread/program termination issues. In particular, you will need to either wait for the thread to finish (acceptable if it doesn't take a long time to run) or cancel it explicitly before exiting the program.

2. Use EVT_IDLE event to perform your task whenever there are no other events to process. This is not too bad for small tasks which can be broken in small enough pieces as you need to be able to resume processing in your handler. Don't forget to call event.RequestMore() to continue getting idle events even when nothing is happening otherwise.

3. The worst and most dangerous was is to call wxYield() as suggested by another answer. This can seem simple initially but you will regret doing it later because this can create extremely difficult to debug reentrancy problems in your code. If you do use it, you need to guard against reentrancy everywhere yourself and you should really understand what exactly this function does.

Answer 2

Try this:

long currentTime = wxGetLocalTime();
long stopTime = wxGetLocalTime() + 3;
while (wxGetLocalTime() != stopTime) {
   wxYield();
}
wxMessageBox("DONE IN APP");

Answer 3

I know this is late to the game, but...

I've successfully used the EVT_IDLE method for YEARS (back in the 90's with Motif originally). The main idea is to break your task up into small pieces, where each piece calls the next piece (think linked-list). The mechanism to do this is using the CallAfter() method (using C++, of course). You just "CallAfter()" as the last step in the piece and that will allow the GUI main loop to run another iteration and possibly update GUI elements and such before calling your next piece. Just remember to keep the pieces small.

Using a background thread is really nice, but can be trickier than you imagine... eventually. As long as you know the data you're working on in the background won't be touched/viewed by anything else, you're OK. If you know this is the case, then that is the way to go. This method allows the GUI to remain fully responsive during background calculations (resizing/moving the window, etc.)

In either case, just don't forget to desensitize appropriate GUI elements as the first step so you won't accidentally launch the same background task multiple times (for example, accidentally clicking a push button multiple times in succession that launches the background thread).