Technical Q&A QA1519

Detecting the Caps Lock Key

Q: How do I detect when the caps lock key is turned on or off in my Cocoa application?

A: You must implement flagsChanged: in order to detect this key.

NSResponder keyDown: and keyUp: methods do not fire when just a modifier key is pressed. So if you want to catch this event, you will need to override:

- (void)flagsChanged:(NSEvent*) and examine the NSEvent from there.

The flagsChanged: method comes from NSResponder, so any responder in the responder chain can react to it.

Listing 1: Detecting the caps lock key.

- (void)flagsChanged:(NSEvent*)event
{
    if ([event keyCode] == 0x39)  // 57 = key code for caps lock
    {
        unsigned flags = [event modifierFlags];
        if (flags & NSAlphaShiftKeyMask)
            NSLog(@"capsLock on");
        else
            NSLog(@"capsLock off");
    }
}

Related Documentation

• Cocoa - Handling Key Events

• Keyboard Layout and Key Codes - for a list of available key codes to use in responding to modifier and key events.

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Document Revision History

Posted: 2007-05-11