NSTimer not precise enough - ios

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!

Answer 1

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.