Listen and capture Cocoa global shortcut key events, and "eat" them (prevent event trigger)

handleGlobalShortcut.m

CGEventRef myCGEventCallback(CGEventTapProxy proxy, CGEventType type,CGEventRef event, void *refcon){

  // Paranoid sanity check.
  if ((type != kCGEventKeyDown) && (type != kCGEventKeyUp) && (type !=
  NX_SYSDEFINED))
    return event;

  NSEvent *e = [NSEvent eventWithCGEvent:event];

  // We're getting a special event
  if( ([e type] == NSSystemDefined && [e subtype] == 8) ) {
    // do whatever you do with special events
    // return NULL to kill the event
  // we're getting a normal key event
  } else if([e type] == NSKeyDown || [e type] == NSKeyUp) {
    // do whatever you do with regular events
    // return NULL to kill the event
  }
  return event;
}

- (void)listenForKeyEvents 
{
  CFMachPortRef      eventTap, eventTapTest;
  CGEventMask        eventMask;
  CFRunLoopSourceRef runLoopSource;

  eventMask = ((1 << kCGEventKeyDown) | (1 << kCGEventKeyUp));

  // try creating an event tap just for keypresses. if it fails, we need Universal Access.
  eventTapTest = CGEventTapCreate(kCGSessionEventTap,kCGHeadInsertEventTap, 0,eventMask, myCGEventCallback, NULL);
  
  if (!eventTapTest) {
    NSLog(@"no tap");
    NSAlert *alert = [[[NSAlert alloc] init] autorelease];
    [alert addButtonWithTitle:@"Quit"];
    [alert setMessageText:@"Could not create an event tap."];
    [alert setInformativeText:@"Please enable \"access for assistive devices\" in the Universal Access pane of System Preferences."];
    [alert setAlertStyle:NSCriticalAlertStyle];
    [alert runModal];
    [NSApp terminate:self];
    return;
  }
  // disable the test tap
  // causes a crash otherwise (infinite loop with the replacement events, probably)
  CGEventTapEnable(eventTapTest, false);

  // Create an event tap. We are interested in key presses and system defined keys.
  eventTap = CGEventTapCreate(kCGSessionEventTap, kCGHeadInsertEventTap, 0, CGEventMaskBit(NX_SYSDEFINED) | eventMask, myCGEventCallback, NULL);

  // Create a run loop source.
  runLoopSource = CFMachPortCreateRunLoopSource(
      kCFAllocatorDefault, eventTap, 0);

  // Add to the current run loop.
  CFRunLoopAddSource(CFRunLoopGetCurrent(), runLoopSource,
           kCFRunLoopCommonModes);

  // Enable the event tap.
  CGEventTapEnable(eventTap, true);

}