Format realtime stopwatch timer to the hundredth using Swift

Question

I have an app using an NSTimer at centisecond (0.01 second) update intervals to display a running stopwatch in String Format as 00:00.00 (mm:ss.SS). (Basically cloning the iOS built-in stopwatch to integrate into realtime sports timing math problems, possibly needing millisecond accuracy in the future)

I use (misuse?) the NSTimer to force-update the UILabel. If the user presses Start, this is the NSTimer code used to start repeating the function:

displayOnlyTimer = NSTimer.scheduledTimerWithTimeInterval(0.01, target: self, selector: Selector("display"), userInfo: nil, repeats: true)

And here is the function that is executed by the above NSTimer:

func display() {
    let currentTime = CACurrentMediaTime() - timerStarted + elapsedTime

    if currentTime < 60 {
        timeDisplay.text = String(format: "%.2f", currentTime)
    }else if currentTime < 3600 {
        var minutes = String(format: "%00d", Int(currentTime/60))
        var seconds = String(format: "%05.2f", currentTime % 60)
        timeDisplay.text =  minutes + ":" + seconds
    }else {
        var hours = String(format: "%00d", Int(currentTime/3600))
        var minutes = String(format: "%02d", (Int(currentTime/60)-(Int(currentTime/3600)*60)))
        var seconds = String(format: "%05.2f", currentTime % 60)
        timeDisplay.text =  hours + ":" + minutes + ":" + seconds
    }
}

There will be at least 2 display links running at the same time. Will this method be too inefficient once all other elements are in play?

The display is then updated without using NSTimer when the user presses stop/pause/reset. I didn't find anything that directly translated into Swift. I'm fairly certain I'm using an inefficient method to force update the text UILabel quickly in the UIView.

More Details: I'm working on less messy code for the running timer format (mm:ss.SS). I will update this once more when I've finished that.

UPDATE: Thanks to Rob and jtbandes for answering both of my questions (formatting method and display update method). It was easy to replace the NSTimer (see above) with CADisplayLink():

displayLink = CADisplayLink(target: self, selector: Selector("display"))
        displayLink.addToRunLoop(NSRunLoop.currentRunLoop(), forMode: NSRunLoopCommonModes)

And then replace all instances in code of

displayOnlyTimer.invalidate() 

with

displayLink.paused = true 

(this will pause the display link from updating)

Answer 1

For rapid UI updates you should use a CADisplayLink. Anything faster than the display refresh rate is a waste of processing power since it physically cannot be displayed. It also provides a timestamp of the previous frame so you can try to predict when the next frame will be.

You're calculating CACurrentMediaTime() - timerStarted + elapsedTime multiple times. I would recommend doing it only once and saving it in a local variable.

Consider using NSDateComponentsFormatter. Try to reuse one instance of the formatter rather than creating a new one each time (which is usually the most expensive part). Overall, the less string manipulation you can do, the better.

You can check CACurrentMediaTime at the beginning and end of your display method to see how long it takes. Ideally it should be much less than 16.6ms. Keep an eye on the CPU usage (and general power consumption) in the Xcode debug navigator.