Color indicator of an input source in MacOS

lang-dot.m

// Color indicator of an input source in MacOS

// Shows a dot at the top right corner that changes color depending on the current input source.
// By default, it is set to show a green dot for the US keyboard layout and a red one for anything else.
// You can show or hide it by pressing Cmd+Opt+Ctrl+Shift.

// Why would one need such an indicator?

// In order to have more screen space, I keep my menu bar hidden and only show up when I move mouse to the top of the screen.
// Thus, I cannot see which input source is currently selected and often I find myself typing in non-US symbols 
// when in fact I want to use US layout, and vice versa. 
// This indicator removes this guesswork by providing me with a quick and non-intrusive way to determine the current layout.

// Usage:

// Dependencies:
// - If using `clang` to compile: `xcode-select --install`
// - If using `gcc` to compile: `brew install gcc`

// Compile:
// clang -framework Cocoa -o lang-dot lang-dot.m
// or
// gcc -framework Cocoa -o lang-dot lang-dot.m

// Run: ./lang-dot
// Automatically start upon reboot:
//   crontab -e
//   @reboot /path/to/lang-dot

#import <Cocoa/Cocoa.h>

NSString *targetLayout = @"com.apple.keylayout.US";
NSString *domain = @"com.apple.HIToolbox";
NSString *key = @"AppleCurrentKeyboardLayoutInputSourceID";
static BOOL isWindowVisible = YES;


void checkAndUpdate(NSWindow *window) {
    NSUserDefaults *userDefaults = [[NSUserDefaults alloc] initWithSuiteName:domain];
    id keyboardLayout = [userDefaults objectForKey:key];

    NSColor *backgroundColor = [keyboardLayout isEqualToString:targetLayout] ? [NSColor greenColor] : [NSColor redColor];

    // Must run asynchronously as otherwise the color doesn't update
    dispatch_async(dispatch_get_main_queue(), ^{
        [window setBackgroundColor:backgroundColor];
    });
}

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        NSApplication *application = [NSApplication sharedApplication];

        NSRect mainScreenFrame = [NSScreen mainScreen].frame;
        // Place it in the top right corner
        NSRect frame = NSMakeRect(mainScreenFrame.size.width - 3,
                                  mainScreenFrame.size.height - 3,
                                  3,
                                  3);

        NSWindow *window = [[NSWindow alloc] initWithContentRect:frame
                                                       styleMask:NSWindowStyleMaskBorderless
                                                         backing:NSBackingStoreBuffered
                                                           defer:NO];

        // Place the window above most other windows
        [window setLevel:NSFloatingWindowLevel + 1];
        [window setCollectionBehavior:NSWindowCollectionBehaviorCanJoinAllSpaces];
        [window makeKeyAndOrderFront:nil];  // Bring the window to the front

        // Detect Cmd+Opt+Ctrl+Shift even when the app is not focused
        [NSEvent addGlobalMonitorForEventsMatchingMask:NSEventMaskFlagsChanged handler:^(NSEvent *event) {
            unsigned int flags = [event modifierFlags];
            bool cmdPressed = (flags & NSEventModifierFlagCommand) != 0;
            bool optPressed = (flags & NSEventModifierFlagOption) != 0;
            bool ctrlPressed = (flags & NSEventModifierFlagControl) != 0;
            bool shiftPressed = (flags & NSEventModifierFlagShift) != 0;

            if (cmdPressed && optPressed && ctrlPressed && shiftPressed) {
                isWindowVisible = !isWindowVisible;

                if (isWindowVisible) {
                    [window orderFront:nil];
                } else {
                    [window orderOut:nil];
                }
            }
        }];

        // Use NSRunLoop to check and update every second
        NSTimer *timer = [NSTimer timerWithTimeInterval:1.0
                                                 target:[NSBlockOperation blockOperationWithBlock:^{
                                                     checkAndUpdate(window);
                                                 }]
                                               selector:@selector(main)
                                               userInfo:nil
                                                repeats:YES];
        [[NSRunLoop mainRunLoop] addTimer:timer forMode:NSRunLoopCommonModes];

        [application run];
    }
    return 0;
}