Countdown with several decimal slots, using NSTime

问题:

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

回答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)
  }
}

回答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.