listening for changes to active application

FROM : Robert Nikander
DATE : Tue Oct 14 17:58:05 2008

Hi,

How can I be notified when the foreground application changes?  I 
found "activeApplication" in NSWorkspace.  Is there a way to observe 
changes to that property?  I tried the following, which doesn't seem 
to work...

[[NSWorkspace sharedWorkspace] addObserver:obs 
forKeyPath:@"activeApplication" options: 0 context:NULL];
// where obs implements...
- (void)observeValueForKeyPath:ofObject:change:context:

thanks,
Rob