Quick focus ring

FocusView.h

#import <Cocoa/Cocoa.h>

@interface FocusView : NSTextView

@end

FocusView.m

#import "FocusView.h"

@interface FocusView ()
@property BOOL focusRing;
@end

@implementation FocusView

- (BOOL)becomeFirstResponder {
    BOOL res = [super becomeFirstResponder];
    self.focusRing = res;
    [self setNeedsDisplay:YES];
    return res;
}

- (BOOL)resignFirstResponder {
    BOOL res = [super resignFirstResponder];
    self.focusRing = !res;
    [self setNeedsDisplay:YES];
    return res;
}

- (void)drawRect:(NSRect)dirtyRect {
    [super drawRect:dirtyRect];

    if (self.focusRing) {
        CGContextRef ctx = NSGraphicsContext.currentContext.CGContext;
        CGRect r = (CGRect){CGPointZero, self.frame.size};
        CGContextSetStrokeColorWithColor(ctx, NSColor.keyboardFocusIndicatorColor.CGColor);

        CGContextSetShadowWithColor(ctx, CGSizeMake(0, 0), 4, NSColor.keyboardFocusIndicatorColor.CGColor);
        CGContextSetLineWidth(ctx, 2);
        CGContextStrokeRect(ctx, r);
    }
}

@end

You might also check if the Window is in the background, to not draw the focus ring when the window is not the front most.