Override keydownevent in NSTextfield

You can do it without subclassing by using NSTextFieldDelegate methods:

As @Darren Inksetter said, you can use control:textView:doCommandBySelector:

First, declare NSTextFieldDelegate in your interface tag. Then implement the method:

- (BOOL)control:(NSControl *)control textView:(NSTextView *)textView doCommandBySelector:(SEL)commandSelector
{
    if( commandSelector == @selector(moveUp:) ){
        // Do yourthing here, like selectKeyViewFollowingView
        return YES;    // Return YES means don't pass it along responders chain. Return NO if you still want system's action on this key.
    }
    if( commandSelector == @selector(moveDown:) ){
        // Do the same with the keys you want to track
        return YES;
    }
    return NO;
}

The keydown event can't be overriden in a NSTextField, if you want , you could override the keydown event of the super view or you could use a NSTextView or just override the keyup event in the NSTextField

You will want to look at

control:textView:doCommandBySelector:

http://developer.apple.com/library/mac/#documentation/cocoa/reference/NSControlTextEditingDelegate_Protocol/Reference/Reference.html