Losing key-up event on OS X - objective-c

Question

I am writing a game-like app for OS X and need to know if the left and right arrows are pressed during the game loop. I also check if the space bar is down. For this I implement
-(void)keyDown:(NSEvent *)event;
-(void)keyUp:(NSEvent *)event
in my view and store the state of each key in some flags. I also check if the command button is pressed using:
-(void)flagsChanged:(NSEvent *)event;
If the window isn't main (another window gets focus) or resigns key (spotlight opens) I reset the flags. I do this by implementing:
-(void)windowDidBecomeKey:(NSNotification *)notification;
-(void)windowDidResignKey:(NSNotification *)notification;
-(void)windowDidResignMain:(NSNotification *)notification;
-(void)windowDidBecomeMain:(NSNotification *)notification;
This works almost all the time. If I press command and then space, spotlight opens and my app resigns key. However, if I hold for example the left arrow key, and first presses and holds space, then command, spotlight won't show up, but I sometimes lose the key-up event for the arrow key when I release them. It doesn't happen every time but (could depend on the release order) but it is easily reproduced in a few tries. So the key gets stuck down until the next press.
Is there another state my app enters that I'm not aware of? If not, this approach seems a bit fragile.
Is there a more robust way of checking that a key is down (that doesn't require installation of a logger tool or enabling accessibility for the app in preferences)?

Answer 1

For this purpose I use NSEvent's addLocalMonitorForEventsMatchingMask:handler:-static method. It works while the app is active. Use addGlobalMonitor... for handling global events but be aware your app may be rejected from the AppStore. Here some code sample.
id monitor=[NSEvent addLocalMonitorForEventsMatchingMask:NSKeyDownMask handler:(NSEvent *)^(NSEvent *theEvent){
if (theEvent.keyCode==/*your key code*/) // you should check the key modifiers too
{
// your code here
}
return theEvent; // you may return the event to pass the key to the receiver or nil if no need
}];
// remove monitor
[NSEvent removeMonitor:monitor];