NSTimer not precise enough

Question

I'm setting up timers to execute code once each one finished. However, it seems that the timing of NSTimer is not completely precise, after a while, the timers seem to finish a little too early.

I've set up the following to test the deviation.

(The start button is done in the Storyboard and linked to.)

import UIKit

class ViewController: UIViewController {

  @IBOutlet weak var startButton: UIButton!

  //var mainTimer: NSTimer!
  var counter: NSTimeInterval = 0.0

  @IBAction func startButtonPressed(sender: AnyObject) {
    startMainTimer()
    startTimers()
  }

  func startMainTimer() {
    let mainTimer = NSTimer.scheduledTimerWithTimeInterval(0.01, target: self, selector: #selector(ViewController.count), userInfo: nil, repeats: true)
    NSRunLoop.mainRunLoop().addTimer(mainTimer, forMode: NSRunLoopCommonModes)
  }

  func count() {
    counter += 0.01
  }

  func startTimers() {
    var lastTimeInterval: NSTimeInterval = 0.0

    for _ in 0..<50 {
      let timeInterval = lastTimeInterval + 1.0
      lastTimeInterval = timeInterval

      // Not setting up a repeating timer as intervals would be different in my original project where the deviation will be even worse.
      let timer = NSTimer.scheduledTimerWithTimeInterval(timeInterval, target: self, selector: #selector(ViewController.printTime), userInfo: nil, repeats: false)
      NSRunLoop.mainRunLoop().addTimer(timer, forMode: NSRunLoopCommonModes)
    }
  }

  func printTime() {
    print(counter)
  }
}

After while, the timers will be early a tenth of a second or even more. Should I be using NSDate for this to have better timing, or would that be overly complicated? How would the above look with NSDate?

Any help / pointers much appreciated!

Have a look at the NSTimer documentation: *"... the effective resolution of the time interval for a timer is limited to on the order of 50-100 milliseconds."* – And see http://stackoverflow.com/questions/31375361/format-realtime-stopwatch-timer-to-the-hundredth-using-swift for an alternative — Martin R, Apr 01 '16 at 13:52

Lou Franco · Answer 1 · 2016-04-01T16:24:27.903

It will never be perfect, but you can stop it from compounding by scheduling the next call at the end of the target selector. Calculate a new interval each time based on the current time and when you want it to trigger.

EDIT: I grabbed some code from a very old project to give the idea (that's why it's in Obj-C) -- You want something like this but not exactly:

- (void)constantIntervalTimer {
   static const NSTimeInterval secondsBetweenMessageSend = 1.0;

   if ( !self.timerShouldStop ) {
      NSDate *startTime = [NSDate date];

      // your code here, it should take less than a
      // second to run or you need to increase the timer interval

      // figure out the right time to preserve the interval
      NSTimeInterval waitTime = secondsBetweenMessageSend -
           [[NSDate date] timeIntervalSinceDate:startTime];

      // in case your code runs in more than the interval,
      // we just call back immediately
      if (waitTime < 0.0) 
         waitTime = 0.0;

      [NSTimer scheduledTimerWithTimeInterval: waitTime
            target:self selector:@selector(constantIntervalTimer) 
            userInfo:nil repeats:NO];
    }
}

Once you call this message, it will keep calling itself every second until you set a property called timerShouldStop to YES. You must declare and define this property and set it to NO in your init for this message to work.

Thank you for the answer, Lou! Would this mean using NSDate to get the current time and set the execution time? I guess I would have to check the current time constantly in a loop. I'd have to look into NSDate then, I've never used it before. Would I still use NSTimer at all? — nontomatic, Apr 01 '16 at 14:02

NSTimer not precise enough

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