How to handle arrow key event in Cocoa app?

Question

How to handle arrow key event in Cocoa app?

Answer 1

See this code. I assumed the class is subclass of NSView.

#pragma mark    -   NSResponder

- (void)keyDown:(NSEvent *)theEvent
{
    NSString*   const   character   =   [theEvent charactersIgnoringModifiers];
    unichar     const   code        =   [character characterAtIndex:0];
        
    switch (code) 
    {
        case NSUpArrowFunctionKey:
        {
            break;
        }
        case NSDownArrowFunctionKey:
        {
            break;
        }
        case NSLeftArrowFunctionKey:
        {
            [self navigateToPreviousImage];
            break;
        }
        case NSRightArrowFunctionKey:
        {
            [self navigateToNextImage];
            break;
        }
    }
}

A view should be a first responder to receive events. Maybe this code will be required to support that.

#pragma mark    -   NSResponder
- (BOOL)canBecomeKeyView
{
    return  YES;
}
- (BOOL)acceptsFirstResponder
{
    return  YES;
}

To use this method, the class should be subclass of NSResponder. See the other answer handling without subclassing NSResponder.

Answer 2

In my case I wanted a presented NSViewController subclass to be able to listen to arrow key events for navigation with minimal effort. Here's the best solution I've found, a slight variation of Josh Caswell's answer.

Define an event monitor (optional), can be locally in your NSViewController subclass .m

id keyMonitor;

Then start monitoring events, for example in viewDidLoad.

keyMonitor = [NSEvent addLocalMonitorForEventsMatchingMask:NSKeyDownMask handler:^(NSEvent *event) {

    unichar character = [[event characters] characterAtIndex:0];
    switch (character) {
        case NSUpArrowFunctionKey:
            NSLog(@"Up");
            break;
        case NSDownArrowFunctionKey:
            NSLog(@"Down");
            break;
        case NSLeftArrowFunctionKey:
            NSLog(@"Left");
            break;
        case NSRightArrowFunctionKey:
            NSLog(@"Right");
            break;
        default:
            break;
    }
    return event;
}];

To remove the monitor when not required (assuming you defined it)

[NSEvent removeMonitor:keyMonitor];