Unexpected second print value in a repeating schedule timer of DispatchSource, what is it?

Using Swift
1

I'm testing a timer that is returning two fired messages when I only expect one at each time or trigger:

let queue = DispatchQueue(label: "com.domain.app.timer", qos: .userInteractive)
let timer = DispatchSource.makeTimerSource(flags: .strict, queue: queue)

timer.schedule(deadline: .now(), repeating: .seconds(1), leeway: .nanoseconds(0))

var elapsedTime = DispatchTime.now().uptimeNanoseconds

timer.setEventHandler {
  elapsedTime = DispatchTime.now().uptimeNanoseconds - elapsedTime
  print("Timer Fired: elapsedTime: \(elapsedTime/1000000000)")
}

timer.resume()

The output is:

**Timer Fired: elapsedTime: 0**
**Timer Fired: elapsedTime: 131188**

**Timer Fired: elapsedTime: 1**
**Timer Fired: elapsedTime: 131189**

**Timer Fired: elapsedTime: 2**
**Timer Fired: elapsedTime: 131190**

**Timer Fired: elapsedTime: 3**
**Timer Fired: elapsedTime: 131191**

**Timer Fired: elapsedTime: 4**
**Timer Fired: elapsedTime: 131192**

**Timer Fired: elapsedTime: 5**
**Timer Fired: elapsedTime: 131193**

What's the second value per call?

2

I tried your code, it fires once per second, as expected. But your elapsedTime computation is a little fishy. Try this:

var last = DispatchTime.now().uptimeNanoseconds

timer.setEventHandler {
  let current = DispatchTime.now().uptimeNanoseconds
  let elapsed = current - last
  last = current
  print("Timer Fired: elapsedTime: \(Double(elapsed)/1000000000)")
}

PS

I don't know how you run it, but there's no runloop in your sample code, so I use dispatchMain for that.

Another thing. If you don't plan to relax the firing time, you don't need to specify leeway value (which is already 0 by default). Even if you specify leeway of 0, it's not strictly guaranteed that it will fire at/before the deadline.

3

Yours works fine! Thanks once again @Lantua
Yeh, I was using the PlayGround, sorry. So, I thought this part was fine:

elapsedTime = DispatchTime.now().uptimeNanoseconds - elapsedTime

where elapsedTime is a mutable variable outside the setEventHandler, was that the problem? Do I have to keep all values in separate variables in Swift? I thought it'd compute and update elapsedTime for each subsequent call...

4

Well, umm, I think your understanding of the variable works fine, and every call does use the shared elapsedTime. Just that the way you compute it doesn't make much sense. Try to hand-calculate it on the paper for the first few values and you might see. With now returning something like 0s, 1s, 2s, etc.

5

You're right, haven't noticed and that was the problem; Thank you : )