init(unfolding:)

Declaration

@preconcurrency init(unfolding produce: @escaping  @Sendable () async throws -> Element?) where Failure == any Error

Parameters

• produce:

  A closure that asynchronously produces elements for the stream.

Discussion

Use this convenience initializer when you have an asynchronous function that can produce elements for the stream, and don’t want to invoke a continuation manually. This initializer “unfolds” your closure into a full-blown asynchronous stream. The created stream handles adherence to the AsyncSequence protocol automatically. To terminate the stream with an error, throw the error from your closure.

The following example shows an AsyncThrowingStream created with this initializer that produces random numbers on a one-second interval. If the random number is divisible by 5 with no remainder, the stream throws a MyRandomNumberError.

let stream = AsyncThrowingStream<Int, Error> {
    await Task.sleep(1 * 1_000_000_000)
    let random = Int.random(in: 1...10)
    if random % 5 == 0 {
        throw MyRandomNumberError()
    }
    return random
}

// Call point:
do {
    for try await random in stream {
        print(random)
    }
} catch {
    print(error)
}