Add a running countup display timer to an iOS app, like the Clock stopwatch?

I'm working with an app that processes device motion events and updates interface in 5 second increments. I would like to add an indicator to the app that would display the total time the app has been running. It seems that a stopwatch-like counter, like the native iOS Clock app is a reasonable way to count time that the app has been running and display it to the user.

What I'm not sure of is the technical implementation of such a stopwatch. Here's what I'm thinking:

  • if I know how long between interface updates, I can add up seconds between events and keep a count of seconds as a local variable. Alternatively, a 0.5 second interval scheduled timer can provide the count.

  • If I know the start date of the app, I can convert the local variable to date for each interface update using [[NSDate dateWithTimeInterval:(NSTimeInterval) sinceDate:(NSDate *)]

  • I can use a NSDateFormatter with a short time style to convert the updated date to a string using stringFromDate method

  • The resulting string can be assigned to a label in the interface.

  • The result is that the stopwatch is updated for each "tick" of the app.

It appears to me that this implementation is a bit too heavy and is not quite as fluid as the stopwatch app. Is there a better, more interactive way to count up time that the app has been running? Maybe there's something already provided by iOS for this purpose?


Almost what @terry lewis suggested but with an algorithm tweak:

1) schedule a timer

NSTimer *timer = [NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:@selector(timerTick:) userInfo:nil repeats:YES];

2) when the timer fires, get the current time (that's the tweak, don't count ticks because if there is wobble in the timer, tick counting will accumulate the error), then update the UI. Also, NSDateFormatter is a simpler and more versatile way to format time for display.

- (void)timerTick:(NSTimer *)timer {
    NSDate *now = [NSDate date];

    static NSDateFormatter *dateFormatter;
    if (!dateFormatter) {
        dateFormatter = [[NSDateFormatter alloc] init];
        dateFormatter.dateFormat = @"h:mm:ss a";  // very simple format  "8:47:22 AM"
    self.myTimerLabel.text = [dateFormatter stringFromDate:now];

If you look in the iAd sample code from Apple in the basic banner project they have a simple timer:

NSTimer *_timer; 
_timer = [NSTimer scheduledTimerWithTimeInterval:0.1 target:self selector:@selector(timerTick:) userInfo:nil repeats:YES];

and the the method they have

- (void)timerTick:(NSTimer *)timer
    // Timers are not guaranteed to tick at the nominal rate specified, so this isn't technically accurate.
    // However, this is just an example to demonstrate how to stop some ongoing activity, so we can live with that inaccuracy.
    _ticks += 0.1;
    double seconds = fmod(_ticks, 60.0);
    double minutes = fmod(trunc(_ticks / 60.0), 60.0);
    double hours = trunc(_ticks / 3600.0);
    self.timerLabel.text = [NSString stringWithFormat:@"%02.0f:%02.0f:%04.1f", hours, minutes, seconds];

It just runs from start up, pretty basic.

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