Capture all key press events in the system.

gistfile1.m

- (void)applicationDidFinishLaunching:(NSNotification *)aNotification
{
    CGEventMask eventMask = CGEventMaskBit(kCGEventKeyDown);
    CFMachPortRef eventTap = CGEventTapCreate(kCGHIDEventTap,
                                              kCGHeadInsertEventTap,
                                              kCGEventTapOptionDefault,
                                              eventMask,
                                              eventHandler,
                                              NULL);
    CFRunLoopSourceRef runLoopSource = CFMachPortCreateRunLoopSource(kCFAllocatorDefault, eventTap, 0);
    CFRunLoopAddSource(CFRunLoopGetCurrent(), runLoopSource, kCFRunLoopCommonModes);
    CGEventTapEnable(eventTap, true);
    CFRelease(eventTap);
    CFRelease(runLoopSource);
}

CGEventRef eventHandler(CGEventTapProxy proxy, CGEventType type, CGEventRef event, void *refcon)
{
    NSEvent *sysEvent = [NSEvent eventWithCGEvent:event];

    // get the application that trigger the event
    NSRunningApplication *activedApp = nil;
    NSArray *appList = [[NSWorkspace sharedWorkspace] runningApplications];
    for (NSRunningApplication *app in appList)
    {
        if (app.active)
        {
            activedApp = app;
            break;
        }
    }

    NSLog(@"[%@] %@", [activedApp localizedName], [sysEvent characters]);
    return event; // return NULL;
}