Timer.md

Timer

• Author(s): Philippe Hausler

[ Source | Tests ]

Introduction

Producing elements at regular intervals can be useful for composing with other algorithms. These can range from invoking code at specific times to using those regular intervals as a delimiter of events. There are other cases this exists in APIs however those do not currently interact with Swift concurrency. These existing APIs are ones like Timer or DispatchTimer but are bound to internal clocks that are not extensible.

Proposed Solution

We propose to add a new type; AsyncTimerSequence which utilizes the new Clock, Instant and Duration types. This allows the interaction of the timer to custom implementations of types adopting Clock.

This asynchronous sequence will produce elements of the clock's Instant type after the interval has elapsed. That instant will be the now at the time that the sleep has resumed. For each invocation to next() the AsyncTimerSequence.Iterator will calculate the next deadline to resume and pass that and the tolerance to the clock. If at any point in time the task executing that iteration is cancelled the iteration will return nil from the call to next().

public struct AsyncTimerSequence<C: Clock>: AsyncSequence {
  public typealias Element = C.Instant
  
  public struct Iterator: AsyncIteratorProtocol {
    public mutating func next() async -> C.Instant?
  }
  
  public init(
    interval: C.Instant.Duration, 
    tolerance: C.Instant.Duration? = nil, 
    clock: C
  )
  
  public func makeAsyncIterator() -> Iterator
}

extension AsyncTimerSequence where C == SuspendingClock {
  public static func repeating(every interval: Duration, tolerance: Duration? = nil) -> AsyncTimerSequence<SuspendingClock>
}

extension AsyncTimerSequence: Sendable { }
extension AsyncTimerSequence.Iterator: Sendable { }

Since all the types comprising AsyncTimerSequence are Sendable these types are also Sendable.

Credits/Inspiration

https://developer.apple.com/documentation/foundation/timer

https://developer.apple.com/documentation/foundation/timer/timerpublisher