如何使NSTimer更准确？

Question

In my app, I put a label on the view controller and I want that label to display a number which is incremented by 1 every millisecond. (I know I can just update it every second by 1000, but I want it to look smooth)

@IBOutlet var label: UILabel!
var number = 0
var timer: NSTimer!

override func viewDidLoad() {
    super.viewDidLoad()
    timer = NSTimer.scheduledTimerWithTimeInterval(0.001, target: self, selector: Selector("addNumber"), userInfo: nil, repeats: true)
}

func addNumber () {
    number++
    label.text = number.description
}

override func viewDidDisappear(animated: Bool) {
    super.viewDidDisappear(animated)
    timer?.invalidate()
}

The code all looks very sensible to me, but when I run the app, I see that the number is increasing by about 50 every second, not 1000 every second. That error is far from acceptable!

Surprisingly, when I lock the screen for a few seconds and unlock it again, the number suddenly increases by a few thousands!

I think this is because I am updating the UI when the timer fires so it requires more time. But how can I fix this?

Answer 1

Your question is asking how to make NSTimer more accurate and I have some bad news: you can't. A reasonable expectation of NSTimer is that it can be called ~30 times per second and, if you're lucky, up to ~60 times per second. And that's okay.

Thinking of this from a practical point of view: the display can only be updated up to 60 times per second, so there is no need to give user feedback more often than that.

If you're trying to max out the hardware and rely on it firing as fast as possible, then you're going to have issues on older and slower hardware where it might only fire ~20 times per second. Relying on the time interval of NSTimer will get you in trouble quickly.

Since you're looking for milliseconds, you can set a start time and every time your function is called look at the number of milliseconds since your start time and that's the number of transpired milliseconds.

Your current method does everything Apple's documentation says it should: number is just the count of times that method has been called (not the number of transpired milliseconds).

Answer 2

Building on @Fennelouski's answer, what I suggest is you swap to a CADisplayLink which is pretty much made for this kind of task. It will call your method once every screen update (or once every 2nd or 3rd, or whatever you make the frameInterval property). it also has a timestamp variable that you can use to calculate how much time has passed and update your label accordingly