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Add sleep(for:) to Clock

Available from Swift 5.9

Paul Hudson      @twostraws

SE-0374 adds a new extension method to Swift’s Clock protocol that allows us to suspend execution for a set number of seconds, but also extends duration-based Task sleeping to support a specific tolerance.

The Clock change is a small but important one, particularly if you’re mocking a concrete Clock instance to remove delays in tests that would otherwise exist in production.

For example, this class can be created with any kind of Clock, and will sleep using that clock before triggering a save operation:

class DataController: ObservableObject {
    var clock: any Clock<Duration>

    init(clock: any Clock<Duration>) {
        self.clock = clock

    func delayedSave() async throws {
        try await clock.sleep(for: .seconds(1))

Because that uses any Clock<Duration>, it’s now possible to use something like ContinuousClock in production but your own DummyClock in testing, where you ignore all sleep() commands to keep your tests running quickly.

In older versions of Swift the equivalent code would in theory have been try await clock.sleep(until: .seconds(1))), but that wouldn’t work in this example because isn’t available as Swift doesn’t know exactly what kind of clock has been used.

As for the change to Task sleeping, it means we can go from code like this:

try await Task.sleep(until: .now + .seconds(1), tolerance: .seconds(0.5))

To just this:

try await Task.sleep(for: .seconds(1), tolerance: .seconds(0.5))
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