Countdown with several decimal slots, using NSTimer in Swift

Question

I want to make an app that has a timer starting at 10.0000000 for example, and I want it to countdown perfectly Here's my code so far:

import UIKit

class ViewController: UIViewController {

    @IBOutlet weak var labelTime: UILabel!

    var counter = 10.0000000

    var labelValue: Double {
        get {
            return NSNumberFormatter().numberFromString(labelTime.text!)!.doubleValue
        }
        set {
            labelTime.text = "\(newValue)"
        }
    }


    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
        labelValue = counter
        var timer = NSTimer.scheduledTimerWithTimeInterval(0.0000001, target: self, selector: ("update"), userInfo: nil, repeats: true)
    }

    func update(){
        labelValue -= 0.0000001
    }


}

What happens is that my countdown is really slow, it's just not working and it would take like 1 hour to get to 0 seconds, instead of just 10 seconds. Any ideas? What changes should I make to my code? Thanks

Juback

Could you please edit and split your code in several lines. Hart to read (and understand) when it's on a single line. Or is that common practice with swift ? – Collapse

Answer 1

Timers are not super-accurate, and the resolution of NSTimer is about 1/50th of a second.

Plus, the refresh rate of the iPhone screen is 60 frames/second, so it's totally pointless to run your timer any faster than that.

Rather than trying to use a timer to decrement something every time it fires, create a timer that fires like 50 times a second, and have it use clock math to update the display based on the remaining time:

var futureTime: NSTimeInterval 

override func viewDidLoad() {
    super.viewDidLoad()
    labelValue = counter

    //FutureTime is a value 10 seconds in the future.
    futureTime = NSDate.timeIntervalSinceReferenceDate() + 10.0 

    var timer = NSTimer.scheduledTimerWithTimeInterval(
      0.02, 
      target: self, 
      selector: ("update:"), 
      userInfo: nil, 
      repeats: true)
}

func update(timer: NSTimer)
{
  let timeRemaining = futureTime - NSDate.timeIntervalSinceReferenceDate()
  if timeRemaining > 0.0
  {
    label.text = String(format: "%.07f", timeRemaining)
  }
  else
  {
    timer.invalidate()
    //Force the label to 0.0000000 at the end
    label.text = String(format: "%.07f", 0.0)
  }
}

Disclaim

I edited my sample code slightly. See the updated version. – Disclaim

this works great! Thank you! In swift, how can I make my float only have 7 decimal cases? Seems as if it doesn't work with the @"%.07f" – Rackley

What does it give you? "It doesn't work" is incredibly, maddeningly UN-helpful. – Disclaim

Ahaha sorry. It is working now, had to remove the '@' – Rackley

Oh, sorry. That's an Objective-C habit that is hard to break. (Fixed) – Disclaim

Hey sorry to bother you, but I was wondering how could I raise the precision of the timer countdown? I'm performing a println and it's not going through all the slots when I have 3 decimal slots, and I want it to go through EVERY number... How? PLEASE help I'm desperate – Rackley

It isn't clear what you want to accomplish. Desperate to do what, exactly? The debug console (where println output goes) is actually quite slow. It can't display a message every 1/1000th of a second. If you want a timer that counts every 0.001 seconds, that's thousandths of a second. You simply can't do that. – Disclaim

You could create a high priority task in GCD that runs on a separate thread and takes some action every thousandth of a second, but anything that interacts with the screen simply can't keep up. The screen only refreshes once every 1/60th of a second. You can't update the screen faster than that under any circumstances. – Disclaim

I have a game where I have a timer that starts at 5.000 and it starts counting down infinitely. The objective of the game is for the user to stop the timer at 0.000. I don't necessarily need for the screen to update EVERY mili-mili-mili second, all I want is for when I press the button to stop the timer, for it to show me THE EXACT result! And right now it isn't doing that... Is this possible? Thank you – Rackley

Post your button IBAction code. That's where you need to calculate your actual time remaining. You should model your code after the code I posted above. – Disclaim

BTW, you should use Double, not Float, to do math on time intervals. – Disclaim

Answer 2

Are you trying to make it display every combination between 0.0000001 and .99999999 in a period of one second? The screen would literally have to update a hundred million times in order to display every number. There's no feasible way to do this in a single second on any present technology or likely any future technology. The screen itself cannot update any faster than 60 times a second, so that's the fastest this will work for you.

You can try using NSTimer for that rate (1/60 = 0.0167). NSTimer itself isn't guaranteed to be very precise. In order to update the screen at every frame, you'll have to use CADisplayLink (https://developer.apple.com/library/ios/documentation/QuartzCore/Reference/CADisplayLink_ClassRef/).

This gives you the chance to run a selector on every frame update, which is as fast as the system can change the frame by definition.

Negrillo