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

    {

        NSUInteger flags = [event modifierFlags];

        if (flags & NSAlphaShiftKeyMask)

            NSLog(@"capsLock on");

        else

            NSLog(@"capsLock off");

    }

}

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