Swift Async let with loop

Question

我想并行获取数据。 我找到了一个并行调用 API 的例子，但我想用循环存储async let variables 。

异步让示例。 但是，此示例不使用循环。

async let firstPhoto = downloadPhoto(named: photoNames[0])
async let secondPhoto = downloadPhoto(named: photoNames[1])
async let thirdPhoto = downloadPhoto(named: photoNames[2])

let photos = await [firstPhoto, secondPhoto, thirdPhoto]
show(photos)

我想做类似以下的事情。

let items = photoNames.map({ photo in
    async let item = downloadPhoto(named: photo)
    return item
}) 
let photos = await items
show(photos)

Answer 1

let photos = await photoNames.map(downloadPhoto)

public extension Sequence {  
  func map<Transformed>(
    priority: TaskPriority? = nil,
    _ transform: @escaping (Element) async throws -> Transformed
  ) async rethrows -> [Transformed] {
    try await withThrowingTaskGroup(of: Transformed.self) { group in
      for element in self {
        group.addTask(priority: priority) {
          try await transform(element)
        }
      }
      
      return try await .init(group)
    }
  }
}

public extension Array {
  init<AsyncSequence: _Concurrency.AsyncSequence>(_ asyncSequence: AsyncSequence) async rethrows
  where AsyncSequence.Element == Element {
    self = try await asyncSequence.reduce(into: []) { $0.append($1) }
  }
}

CollectionConcurrencyKit对此提供了一个变体。

Answer 2

如果结果的顺序在这里无关紧要，请改用TaskGroup 。

Answer 3

您可以使用任务组。 请参阅The Swift Programming Language: Concurrency 的Tasks and Task Groups部分（这似乎是您获得示例的地方）。

可以使用withTaskGroup(of:returning:body:)创建一个任务组来并行运行任务，但最后将所有结果整理在一起。

例如，这是一个创建子任务的示例，该任务返回“名称”和“图像”的元组，并且

func downloadImages(with names: [String]) async -> [String: UIImage] {
    await withTaskGroup(
        of: (String, UIImage).self,
        returning: [String: UIImage].self
    ) { [self] group in
        for name in names {
            group.addTask { await (name, downloadPhoto(named: name)) }
        }

        var images: [String: UIImage] = [:]

        for await result in group {
            images[result.0] = result.1
        }

        return images
    }
}

或者，更简洁地说：

func downloadImages(with names: [String]) async -> [String: UIImage] {
    await withTaskGroup(
        of: (String, UIImage).self,
        returning: [String: UIImage].self
    ) { [self] group in
        for name in names {
            group.addTask { await (name, downloadPhoto(named: name)) }
        }

        return await group.reduce(into: [:]) { $0[$1.0] = $1.1 }
    }
}

它们并行运行：

但是您可以从结果字典中提取它们：

let stoodges = ["mo", "larry", "curly"]
let images = await downloadImages(with: stoodges)

imageView1.image = images["mo"]
imageView2.image = images["larry"]
imageView3.image = images["curly"]

Answer 4

对于遍历数组并并行响应，您不使用async let 。 你想象中的代码...

let items = photoNames.map({ photo in
    async let item = downloadPhoto(named: photo)
    return item
}) 
let photos = await items
show(photos)

...只需转换为使用任务组（直接在浏览器中输入，因此准备根据需要进行补偿；另请注意，您提供的信息不足，因为我不知道您从downloadPhoto返回的是什么类型，所以我必须振作起来）：

var result = [YourPhotoType]()
await withTaskGroup(of:YourPhotoType.self) { group in
    photoNames.forEach { name in
        group.addTask {
            async downloadPhoto(named: name)
        }
    }
    for await photo in group {
        result.append(photo)
    }
}

有关更多信息，请参阅我的教程https://www.biteinteractive.com/swift-5-5-asynchronous-looping-with-async-await/ （特别是关于您现在丢失原始数组顺序的问题） ）。