How to put delay for each iteration of loop

Question

Say I had this loop:

count = 0

for i in 0...9 {

    count += 1

}

and I want to delay it.

Delay function:

// Delay function
func delay(_ delay:Double, closure:@escaping ()->()) {

DispatchQueue.main.asyncAfter( deadline: DispatchTime.now() + Double(Int64(delay * Double(NSEC_PER_SEC))) / Double(NSEC_PER_SEC), execute: closure)

}

.

This means if I want to increase count by 1 every second, I would do:

count = 0

for i in 0...9 {

    delay(1) {

        count += 1

    }

}

but this doesn't work as it only delays code in brackets. How do I delay the actual loop? I would like the delay to stop from iterating until the time has passed, and then the loop/code can repeat again.

Possible duplicate of [How to create a delay in Swift?](https://stackoverflow.com/questions/27517632/how-to-create-a-delay-in-swift) — GIJOW, Mar 19 '18 at 18:57
Your delay function has an asynchronous dispatch, so it doesn't block the current thread (which would be bad anyway). It is better to use a `Timer` — Paulw11, Mar 19 '18 at 18:58

Sweeper · Accepted Answer · 2018-03-19T21:57:39.347

15

Your current code doesn't work because you are doing the increment asynchronously. This means that the for loop will still run at its normal speed.

To achieve what you want, you can use a timer like this:

var count = 0
let timer = Timer.scheduledTimer(withTimeInterval: 1, repeats: true){ _ in
    count += 1
    print(count)
}

If you want it to stop after 5 times, do this:

var count = 0
var timer = Timer.scheduledTimer(withTimeInterval: 1, repeats: true){ t in
    count += 1
    print(count)
    if count >= 5 {
        t.invalidate()
    }
}

edited Mar 19 '18 at 21:57

answered Mar 19 '18 at 19:04

Sweeper

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Checking with a print statement, and it doesn't seem to work – George_E -old Mar 19 '18 at 19:18
@George_E Oops! My bad. Edited. – Sweeper Mar 19 '18 at 19:22
How do I break out the loop? `break` doesn't work here – George_E -old Mar 19 '18 at 19:30
@Sweeper I'm compiling this as a standalone `.swift` file. The async call never finishes because the program terminates as soon as the end of the file is reached. Could you show how to keep the program alive while the async call is ticking? – Alexander Mar 19 '18 at 19:31
1

@George_E Look at the [documentation](https://developer.apple.com/documentation/foundation/timer). In particular, [`Timer.invalidate()`](https://developer.apple.com/documentation/foundation/timer/1415405-invalidate). – Alexander Mar 19 '18 at 19:32
@Alexander Can't figure out why `invalidate()` won't work?! – George_E -old Mar 19 '18 at 19:47
@Alexander `Instance member 'invalidate' cannot be used on type 'Timer'; did you mean to use a value of this type instead?` is the error – George_E -old Mar 19 '18 at 19:48
@George_E See the variable `timer` that I declared? Use that to call `invalidate`: `timer.invalidate()` – Sweeper Mar 19 '18 at 20:23
@Sweeper `variable used within its own initial value` error now ): – George_E -old Mar 19 '18 at 20:31
1

@George_E Oh so you want to stop it inside the closure. See the edit. – Sweeper Mar 19 '18 at 20:44
@Sweeper You don't need the IUO. The parameter passed into the closure of the timer is the timer itself. You can call invalidate on that. – Alexander Mar 19 '18 at 21:48

score 4 · Answer 2 · answered Mar 19 '18 at 19:13

As @Paulw11 and @Sweeper have suggested, you can use a Timer to do this. However, if the code does need to be asynchronous for some reason, you can reimplement the loop asynchronously by making it recursive:

func loop(times: Int) {
    var i = 0

    func nextIteration() {
        if i < times {
            print("i is \(i)")

            i += 1

            DispatchQueue.main.asyncAfter(deadline: .now() + .seconds(1)) {
                nextIteration()
            }
        }
    }

    nextIteration()
}

How to put delay for each iteration of loop

2 Answers2

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