Convenience Async[Throwing]Stream.makeStream methods

Paul Hudson      @twostraws

SE-0388 adds a new makeStream() method to both AsyncStream and AsyncThrowingStream that sends back both the stream itself alongside its continuation.

So, rather than writing code like this:

var _continuation: AsyncStream<String>.Continuation!
let stream = AsyncStream<String> { _continuation = $0 }
let continuation = _continuation!

We can now get both at the same time:

let (newStream, newContinuation) = AsyncStream.makeStream(of: String.self)

This is going to be particularly welcome in places where you need to access the continuation outside of the current context, such as in a different method. For example, previously we might have written a simple number generator like this one, which needs to store the continuation as its own property in order to be able to call it from the queueWork() method:

struct OldNumberGenerator {
    private var continuation: AsyncStream<Int>.Continuation!
    var stream: AsyncStream<Int>!

    init() {
        stream = AsyncStream(Int.self) { continuation in
            self.continuation = continuation
        }
    }

    func queueWork() {
        Task {
            for i in 1...10 {
                try await Task.sleep(for: .seconds(1))
                continuation.yield(i)
            }

            continuation.finish()
        }
    }
}

With the new makeStream(of:) method this code becomes much simpler:

struct NewNumberGenerator {
    let (stream, continuation) = AsyncStream.makeStream(of: Int.self)

    func queueWork() {
        Task {
            for i in 1...10 {
                try await Task.sleep(for: .seconds(1))
                continuation.yield(i)
            }

            continuation.finish()
        }
    }
}

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Other changes in Swift 5.9…

• if and switch expressions
• Value and Type Parameter Packs
• Macros
• Noncopyable structs and enums
• consume operator to end the lifetime of a variable binding
• Add sleep(for:) to Clock
• Discarding task groups

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