Make SHIFT+3 produce `#` not `£` on OSX by code

Question

On my UK MacBook Air, # can be typed using OPT+3. SHIFT+3 currently produces £. How can I rewire so that SHIFT+3 produces #?

This question is motivated by https://apple.stackexchange.com/questions/156154/use-option-as-meta-key-with-ability-to-type-the-symbol

I think it's a failing that the default setting on OSX's Terminal app doesn't register OPT (which is needed e.g. for keyboard shortcuts in the nano editor), and that if you switch the setting to enable it, you are now unable to type #, which is generally done by OPT+3.

I frequently need # while editing configuration files on my remote server in a Terminal session using nano.

I believe it may be possible to use Ukelele but I would much prefer a code-level solution.

If I can accomplish this by code, I will have learned something!

I've tried asking a more generic question but OSX has a complex machinery for processing keystrokes. This question belongs first! If I can understand this, it will illuminate a component of the more general case.

Answer 1

This answer come straight from Modify keyDown output with very minor adaptation:

// compile and run from the commandline with:
//    clang -fobjc-arc -framework Cocoa  ./foo.m  -o foo
//    sudo ./foo 

#import <Foundation/Foundation.h>
#import <AppKit/NSEvent.h>

typedef CFMachPortRef EventTap;

// - - - - - - - - - - - - - - - - - - - - -

@interface KeyChanger : NSObject
{
@private
    EventTap            _eventTap;
    CFRunLoopSourceRef  _runLoopSource;
    CGEventRef          _lastEvent;
}
@end

// - - - - - - - - - - - - - - - - - - - - -

CGEventRef _tapCallback(
                        CGEventTapProxy proxy,
                        CGEventType     type,
                        CGEventRef      event,
                        KeyChanger*     listener
                        );

// - - - - - - - - - - - - - - - - - - - - -

@implementation KeyChanger

- (BOOL)tapEvents
{
    if (!_eventTap) {
        NSLog(@"Initializing an event tap.");

        _eventTap = CGEventTapCreate(kCGSessionEventTap,
                                     kCGTailAppendEventTap,
                                     kCGEventTapOptionDefault,
                                     CGEventMaskBit( kCGEventKeyDown ),
                                     (CGEventTapCallBack)_tapCallback,
                                     (__bridge void *)(self));
        if (!_eventTap) {
            NSLog(@"unable to create event tap. must run as root or "
                    "add privlidges for assistive devices to this app.");
            return NO;
        }
    }
    CGEventTapEnable(_eventTap, TRUE);

    return [self isTapActive];
}

- (BOOL)isTapActive
{
    return CGEventTapIsEnabled(_eventTap);
}

- (void)listen
{
    if( ! _runLoopSource ) {
        if( _eventTap ) { //dont use [self tapActive]
            _runLoopSource = CFMachPortCreateRunLoopSource( kCFAllocatorDefault,
                                                            _eventTap, 0);
            // Add to the current run loop.
            CFRunLoopAddSource( CFRunLoopGetCurrent(), _runLoopSource,
                                kCFRunLoopCommonModes);

            NSLog(@"Registering event tap as run loop source.");
            CFRunLoopRun();
        }else{
            NSLog(@"No Event tap in place! You will need to call "
                    "listen after tapEvents to get events.");
        }
    }
}

- (CGEventRef)processEvent:(CGEventRef)cgEvent
{
    NSEvent* event = [NSEvent eventWithCGEvent:cgEvent];

    NSUInteger modifiers = [event modifierFlags] &
        (NSCommandKeyMask | NSAlternateKeyMask | NSShiftKeyMask | NSControlKeyMask);

    enum {
       kVK_ANSI_3 = 0x14,
    };

    // TODO: add other cases and do proper handling of case
    if (
        //[event.characters caseInsensitiveCompare:@"3"] == NSOrderedSame
        event.keyCode == kVK_ANSI_3
        && modifiers == NSShiftKeyMask
        ) 
    {
        NSLog(@"Got SHIFT+3");

        event = [NSEvent keyEventWithType: event.type
                                 location: NSZeroPoint
                            modifierFlags: event.modifierFlags & ! NSShiftKeyMask
                                timestamp: event.timestamp
                             windowNumber: event.windowNumber
                                  context: event.context
                               characters: @"#"
              charactersIgnoringModifiers: @"#"
                                isARepeat: event.isARepeat
                                  keyCode: event.keyCode];
    }
    _lastEvent = [event CGEvent];
    CFRetain(_lastEvent); // must retain the event. will be released by the system
    return _lastEvent;
}

- (void)dealloc
{
    if( _runLoopSource ) {
        CFRunLoopRemoveSource( CFRunLoopGetCurrent(), _runLoopSource, kCFRunLoopCommonModes );
        CFRelease( _runLoopSource );
    }
    if( _eventTap ) {
        //kill the event tap
        CGEventTapEnable( _eventTap, FALSE );
        CFRelease( _eventTap );
    }
}

@end

// - - - - - - - - - - - - - - - - - - - - -

CGEventRef _tapCallback(
                        CGEventTapProxy proxy,
                        CGEventType     type,
                        CGEventRef      event,
                        KeyChanger*     listener
                        )
{
    //Do not make the NSEvent here.
    //NSEvent will throw an exception if we try to make an event from the tap timout type
    @autoreleasepool {
        if( type == kCGEventTapDisabledByTimeout ) {
            NSLog(@"event tap has timed out, re-enabling tap");
            [listener tapEvents];
            return nil;
        }
        if( type != kCGEventTapDisabledByUserInput ) {
            return [listener processEvent:event];
        }
    }
    return event;
}

// - - - - - - - - - - - - - - - - - - - - -

int main(int argc, const char * argv[])
{
    @autoreleasepool {
        KeyChanger* keyChanger = [KeyChanger new];
        [keyChanger tapEvents];
        [keyChanger listen];//blocking call.
    }
    return 0;
}