Measures elapsed time in Cocoa app

KDJStopwatch.h

#import <Foundation/Foundation.h>

// Calculates the time elapsed since it was created,
// or since the last time it was reset.

@interface KDJStopwatch : NSObject

// Reset the start time to the current time
- (void)reset;

// Return number of seconds elapsed between initialization (or the last call to -reset) and now
- (NSTimeInterval)elapsedTimeInterval;

// Return user-readable string for elapsed time
- (NSString *)elapsedTimeString;

@end

KDJStopwatch.m

#import "KDJStopwatch.h"
#import <QuartzCore/QuartzCore.h>

@implementation KDJStopwatch {
    NSTimeInterval _startTime;
}

- (id)init {
    self = [super init];
    if (self) {
        [self reset];
    }
    return self;
}

- (void)reset {
    _startTime = CACurrentMediaTime();
}

- (NSTimeInterval)elapsedTimeInterval {
    return CACurrentMediaTime() - _startTime;
}

- (NSString *)elapsedTimeString {
    double interval = [self elapsedTimeInterval];
    if (interval < 1) {
        return [NSString stringWithFormat:NSLocalizedString(@"%.1f ms", @"KDJStopwatch elapsedTimeFormat ms"), interval * 1000];
    }
    else {
        return [NSString stringWithFormat:NSLocalizedString(@"%.2f s", @"KDJStopwatch elapsedTimeFormat sec"), interval];
    }
}

- (NSString *)description {
    return [NSString stringWithFormat:@"<KDJStopwatch: %p; elapsed: %g>",
            self, (double)self.elapsedTimeInterval];
}

@end

Example usage:

KDJStopwatch *stopwatch = [[KDJStopwatch alloc] init];

// Do some time-consuming operation
// ...

NSLog("elapsed time: %@", [stopwatch elapsedTimeString]);

See also

• Swift: https://gist.github.com/kristopherjohnson/7528fbbed80cd74edc69
• Android: https://gist.github.com/kristopherjohnson/6009597