Modify NSEvent to send a different key than the one that was pressed

I'm trying to create an OS X keyboard hook for assistive technology purposes (i.e. don't worry, not a keylogger).

When a user presses a key, I want to prevent the real keypress and send a fake keypress (character of my choosing) instead.

I have the following code:

- (void) hookTheKeyboard {
    CGEventMask keyboardMask = CGEventMaskBit(kCGEventKeyDown);
    id eventHandler = [NSEvent addGlobalMonitorForEventsMatchingMask:keyboardMask handler:^(NSEvent *keyboardEvent) {
        NSLog(@"keyDown: %c", [[keyboardEvent characters] characterAtIndex:0]);
        //Want to: Stop the keyboard input
        //Want to: Send another key input instead
    }];
}

Any help accomplishing either of those goals? Basically modifying the NSEvent "keyboardEvent" to send a different character. Thanks.

You can't do this with the NSEvent API, but you can do this with a CGEventTap. You can create an active event tap and register a callback that receives a CGEventRef and can modify it (if necessary) and return it to modify the actual event stream.

EDIT

Here's a simple program that, while running, replaces every "b" keystroke with a "v":

#import <Cocoa/Cocoa.h>

CGEventRef myCGEventCallback(CGEventTapProxy proxy, CGEventType type, CGEventRef event, void *refcon) {
  //0x0b is the virtual keycode for "b"
  //0x09 is the virtual keycode for "v"
  if (CGEventGetIntegerValueField(event, kCGKeyboardEventKeycode) == 0x0B) {
    CGEventSetIntegerValueField(event, kCGKeyboardEventKeycode, 0x09);
  }

  return event;
}

int main(int argc, char *argv[]) {
  NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
  CFRunLoopSourceRef runLoopSource;

  CFMachPortRef eventTap = CGEventTapCreate(kCGHIDEventTap, kCGHeadInsertEventTap, kCGEventTapOptionDefault, kCGEventMaskForAllEvents, myCGEventCallback, NULL);

  if (!eventTap) {
    NSLog(@"Couldn't create event tap!");
    exit(1);
  }

  runLoopSource = CFMachPortCreateRunLoopSource(kCFAllocatorDefault, eventTap, 0);

  CFRunLoopAddSource(CFRunLoopGetCurrent(), runLoopSource, kCFRunLoopCommonModes);

  CGEventTapEnable(eventTap, true);

  CFRunLoopRun();

  CFRelease(eventTap);
  CFRelease(runLoopSource);
  [pool release];

  exit(0);
}

(Funny story: as I was editing this post, I kept on trying to write "replaces every 'b' keystroke", but it kept on coming out as "replaces every 'v' keystroke". I was confused. Then I remembered that I hadn't stopped the app yet.)