How to await x seconds with async await Swift 5.5

Question

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?

Sternson

Answer 1

iOS 16+ / macOS 13+

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

// 3 seconds
try await Task.sleep(for: .seconds(3))

---

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.

Sternson

Is the bug fixed where this wasn't actually working in the early betas? – Coffelt

@Coffelt It's running fine for me now, what was the issue during early betas? – Sternson

Anything longer than a few nanoseconds we wouldn't actually sleep. :) See the comments after my https://mcmap.net/q/246832/-swift-5-5-async-let-error-expression-is-39-async-39-but-is-not-marked-with-39-await-39 Anyway, glad to hear it's working now! – Coffelt

@Coffelt 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). – Sternson

What's the reason for the where Success == Never, Failure == Never? Isn't that rather limiting? – Coffelt

@Coffelt 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. – Sternson

Yeah, this is basically the case for all the Thread static funcs so perhaps we just shouldn't worry about it. – Coffelt

Answer 2

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

try await Task.sleep(for: .seconds(10))

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