21

How do I use the new Swift 5.5 await keyword to wait for a duration of time?

Normally, with completion handlers, you would have something like this by using DispatchQueue's asyncAfter(deadline:execute:):

func someLongTask(completion: @escaping (Int) -> Void) {
    DispatchQueue.global().asyncAfter(deadline: .now() + 1) {
        completion(Int.random(in: 1 ... 6))
    }
}

someLongTask { diceRoll in
    print(diceRoll)
}

How can this be converted to using async & await in Swift 5.5?

George
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2 Answers2

48

iOS 16+ / macOS 13+

There's a newer API, sleep(until:tolerance:clock:), used like so:

// 3 seconds
try await Task.sleep(until: .now + .seconds(3), clock: .continuous)

iOS <16 / macOS <13

You can use Task.sleep(nanoseconds:) to wait for a specific duration. This is measured in nanoseconds, not seconds.

Here's an example:

func someLongTask() async -> Int {
    try? await Task.sleep(nanoseconds: 1 * 1_000_000_000) // 1 second
    return Int.random(in: 1 ... 6)
}

Task {
    let diceRoll = await someLongTask()
    print(diceRoll)
}

It may be easier to use an extension for sleep so you can just pass in seconds:

extension Task where Success == Never, Failure == Never {
    static func sleep(seconds: Double) async throws {
        let duration = UInt64(seconds * 1_000_000_000)
        try await Task.sleep(nanoseconds: duration)
    }
}

Which would now be called like so:

try await Task.sleep(seconds: 1)

Note that sleep is called with try. An error is thrown if the sleep is cancelled. If you don’t care if it’s cancelled, just try? is fine.

George
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  • Is the bug fixed where this wasn't actually working in the early betas? – matt Aug 09 '21 at 16:49
  • @matt It's running fine for me now, what was the issue during early betas? – George Aug 09 '21 at 16:50
  • Anything longer than a few nanoseconds we wouldn't actually sleep. :) See the comments after my https://stackoverflow.com/a/67929769/341994 Anyway, glad to hear it's working now! – matt Aug 09 '21 at 16:52
  • @matt Ah it is working now. I also just timed the example with a regular `DispatchQueue` and they both take the correct amount of time (roughly). – George Aug 09 '21 at 16:55
  • What's the reason for the `where Success == Never, Failure == Never`? Isn't that rather limiting? – matt Aug 09 '21 at 18:57
  • @matt The linked documentation (in the answer) to the sleep method says: "`Available when Success is Never and Failure is Never.`". I found this after I couldn't get it to compile without this `where` clause. If there is a less limiting version, that would be better. – George Aug 09 '21 at 19:14
  • Yeah, this is basically the case for _all_ the Thread static funcs so perhaps we just shouldn't worry about it. – matt Aug 09 '21 at 19:46
9

From iOS 16 you can use .seconds directly like this:

try await Task.sleep(for: .seconds(10))
Alejandro L.Rocha
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