Async await list detail-CodePudding

I'm trying to fetch a detailed list of items through a multi step async call. First I fetch a list of identifiers, after which I fetch the details for each of these identifiers.

func fetchList() async -> [UUID] {
    // some async network call here
    await Task.sleep(2)
    return [UUID(), UUID(), UUID()]
}

func fetchDetail(forUUID uuid: UUID) async -> String {
    // some async network call here
    await Task.sleep(1)
    return "\(uuid.uuidString.reversed())"
}

Individually these function work fine, but I'm stuck when I want to try and use a .map on the list.

Task {
    let list = await fetchList()
    let details = await list.map { fetchDetail(forUUID: $0) } // Does not work
}

I did find out about the withTaskGroup function and I could of course append to an array of items asynchronously, but I was wondering if there is a better way to achieve this functionality.

CodePudding user response：

I believe you don't need to use 'await' in front of list.map, as the function you are calling already has 'await' in it. Plus, ".map" is synchronous.

CodePudding user response：

Map for async code needs AsyncSequence to work on.

struct DelayedUUIDs: AsyncSequence {
  typealias Element = UUID

  struct DelayedIterator: AsyncIteratorProtocol {
    private var internalIterator = [UUID(), UUID(), UUID()].makeIterator()

    mutating func next() async -> UUID? {
      await Task.sleep(1_000_000_000)
      return internalIterator.next()
    }
  }

  func makeAsyncIterator() -> DelayedIterator {
    DelayedIterator()
  }
}

func fetchList() async -> DelayedUUIDs {
    // some async network call here
    await Task.sleep(1)
    return DelayedUUIDs()
}

func fetchDetail(forUUID uuid: UUID) async -> String {
    // some async network call here
    await Task.sleep(1)
    return "ID: \(uuid.uuidString)"
}


Task {
    let list = await fetchList()
    let details = list.map({ id in await fetchDetail(forUUID: id) })
    for await value in list {
      print(Date())
      print(value)
    }
    for await value in details {
      print(Date())
      print(value as String)
    }
}