Multiple AsyncSequences from one?

kelin · November 17, 2025, 11:41am

For my use case I made this restricted clone of Publisher's share() based on escaping continuations. Since documentation explicitely mentions, that the Continuation supports escaping, I believe it's valid. So far it works. Feel free to use and contribute.

extension AsyncStream where Element: Sendable {
    func double() -> (AsyncStream<Element>, AsyncStream<Element>) {
        var continuation1: AsyncStream<Element>.Continuation!
        var continuation2: AsyncStream<Element>.Continuation!
        let stream1 = AsyncStream { continuation in
            continuation1 = continuation
        }
        let stream2 = AsyncStream { continuation in
            continuation2 = continuation
        }
        Task {
            for await element in self {
                continuation1.yield(element)
                continuation2.yield(element)
            }
            continuation1.finish()
            continuation2.finish()
        }
        return (stream1, stream2)
    }
}


let integers = ... // as above

let (integers1, integers2) = integers.double()

let evens = integers1.compactMap { i in
    i % 2 == 0 ? i : nil
}
let odds = integers2.compactMap { i in
    i % 2 == 0 ? nil : i
}
for await e in evens {
    print("even:", e)
}
for await o in odds {
    print("odd:", o)
}
print("Print over.")

Output

even: 10
even: 8
even: 6
even: 4
even: 2
odd: 9
odd: 7
odd: 5
odd: 3
odd: 1
Print over.