OS X: Detect system-wide keyDown events?

If you are okay with a minimum requirement of OS X 10.6+, and can suffice with "read-only" access to the stream of events, you can install a global event monitor in Cocoa: Cocoa Event-Handling Guide: Monitoring Events.

If you need to support OS X 10.5 and earlier, and read-only access is okay, and don't mind working with the Carbon Event Manager, you can basically do the Carbon-equivalent using GetEventMonitorTarget(). (You will be hard-pressed to find any (official) documentation on that method though). That API was first available in OS X 10.3, I believe.

If you need read-write access to the event stream, then you will need to look at a slightly lower-level API that is part of ApplicationServices > CoreGraphics:CGEventTapCreate() and friends. This was first available in 10.4.

Note that all 3 methods will require that the user have "Enable access for assistive devices" enabled in the System Preferences > Universal Access preference pane (at least for key events).

I'm posting the code that worked for my case.

I'm adding the global event handler after the app launches. My shortcut makes ctrl+alt+cmd+T open my app.

- (void) applicationWillFinishLaunching:(NSNotification *)aNotification
{
    // Register global key handler, passing a block as a callback function
    [NSEvent addGlobalMonitorForEventsMatchingMask:NSKeyDownMask
                                           handler:^(NSEvent *event){

        // Activate app when pressing cmd+ctrl+alt+T
        if([event modifierFlags] == 1835305 && [[event charactersIgnoringModifiers] compare:@"t"] == 0) {

              [NSApp activateIgnoringOtherApps:YES];
        }
    }];

}