Swift 5.5 | Async throws/ Try await

简介

为了解决传统异步编程中回调block无限“套娃”的问题,苹果官方于Swift5.5版本引入了新的异步编程理念try await,类似于同步的异步(异步等待)方式,大大简化了异步编程的代码,提高了逻辑清晰性。

async throws可用于下列标识:

  • 属性(计算属性)
  • 方法
  • 初始化器

示例

异步下载一张图片

enum ImageDownloadError: Error {
    case failed
}

func downloadImage(url: String) async throws -> UIImage {
   try Task.checkCancellation()

    guard let aURL = URL(string: url) else {
        throw URLError(.badURL)
    }
    let request = URLRequest(url: aURL)
    print("Started downloading... \(url)")
    let (data, _) = try await URLSession.shared.data(for: request)
    guard data.count > 0, let image = UIImage(data: data) else {
        throw ImageDownloadError.failed
    }
    print("Finished downloading")
    return image
}


let task = Task {
    do {
        let idx = 1 + arc4random() % 20
        let image = try await downloadImage(url: "https://raw.githubusercontent.com/onevcat/Kingfisher-TestImages/master/DemoAppImage/HighResolution/\(idx).jpg")
        print("Success: ", image)
    } catch {
        print("Failure: \(error.localizedDescription)")
    }
}

// 0.1s后取消任务,这时候downloadImage异步任务直接会被thrown,
// 不会继续执行session网络请求
DispatchQueue.global().asyncAfter(deadline: .now() + 0.1) {
    task.cancel()
}
  • 如果不Task.checkCancellation的话,一旦URLSession任务发出去,就会被执行完(即使外部task已经被cancel)
  • 如果进行了Task.checkCancellation,那么当URLSession任务完成的时候,如果检测到所在task已经被cancel了,则不会返回URLSession的执行结果,而是直接throw cancelled error
  • 此外,如果不想在Task被cancel的时候抛出异常,而是当成正常操作,也可以如下操作:
if Task.isCancelled {
// 自定义默认返回值
  return someDefaultValue
}

或者,抛出自定义error:

if Task.isCancelled {
  throw MyError.some
}

打印:
Started downloading... image download url
Failure: cancelled

  • 异步串行任务
do {
        var getOneImageUrl: String {
            let idx = 1 + arc4random() % 20
            return "https://raw.githubusercontent.com/onevcat/Kingfisher-TestImages/master/DemoAppImage/HighResolution/\(idx).jpg"
        }
        let image1 = try await downloadImage(url: getOneImageUrl)
        print("Success image1: ", image1)
        
        let image2 = try await downloadImage(url: getOneImageUrl)
        print("Success image2: ", image2)
        
        let image3 = try await downloadImage(url: getOneImageUrl)
        print("Success image3: ", image3)
        
    } catch {
        print("Failure: \(error.localizedDescription)")
    }

打印:
Started: 3.jpg
Finished: 3.jpg
Success image1:
Started: 11.jpg
Finished: 11.jpg
Success image2:
Started: 1.jpg
Finished: 1.jpg
Success image3:

  • 主线程方法
@MainActor
func showImage(_ image: UIImage) {
    imageView.image = image
}
  • 也可以使用MainActorrun方法来包裹需要主线程执行的block,类似于GCD main queue:
/*
public static func run(
  resultType: T.Type = T.self, 
  body: @MainActor @Sendable () throws -> T
) async rethrows -> T
*/

var img: UIImage?
...
// give the `img` to some specific value
...
// need to capture the mutable value into capture list or compiles error
MainActor.run{ [img] in
  self.imageView.image = img
}

...
// or can also be replaced by this:
let img: UIImage
if xx {
  img = someValue
} else {
  img = nil
}
// and then, the `img` property will be immutable anymore, 
// so it is can be used in `MainActor` context
MainActor.run{
  self.imageView.image = img
}

@MainActor标识表示在主线程调用该方法(自动切换到)

  • 调用
let image1 = try await downloadImage(url: getOneImageUrl)
print("Success image1: ", image1)
await showImage(image1)

这里有两个新概念:Task和MainActor,使用Task的原因是在同步线程和异步线程之间,我们需要一个桥接,我们需要告诉系统开辟一个异步环境,否则编译器会报 'async' call in a function that does not support concurrency的错误。 另外Task表示开启一个任务。@MainActor表示让showImage方法在主线程执行。

使用 async-await并不会阻塞主线程,在同一个Task中,遇到await,后面的任务将会被挂起,等到await任务执行完后,会回到被挂起的地方继续执行。这样就做到了 异步串行。

异步并发 async-let

async let 和 let 类似,它定义一个本地常量,并通过等号右侧的表达式来初始化这个常量。区别在于,这个初始化表达式必须是一个异步函数的调用,通过将这个异步函数“绑定”到常量值上,Swift 会创建一个并发执行的子任务,并在其中执行该异步函数。async let 赋值后,子任务会立即开始执行。如果想要获取执行的结果 (也就是子任务的返回值),可以对赋值的常量使用 await 等待它的完成。

当 v0 任务完成后,它的结果将被暂存在它自身的续体栈上,等待执行上下文通过 await 切换到自己时,才会把结果返回。

如果没有 await,那么 Swift 并发会在被绑定的常量离开作用域时,隐式地将绑定的子任务取消掉,然后进行 await。

class ViewController: UIViewController {
    @IBOutlet weak var imageView: UIImageView!

    @IBAction func syncConcurrent(_ sender: UIControl) {
        Task {
            let image = try await downloadImageThumbnail(id: 1+arc4random()%20)
            self.imageView.image = image
        }
    }
    
    enum ThumbnailError: Error {
        case badImage
    }
    
    func downloadImageThumbnail(id: UInt32) async throws -> UIImage {
        try Task.checkCancellation()
        
        async let image = downloadImage(id: id)
        async let metadata = downloadImageMetadata(id: id)
        guard let thumbnail = try await image.preparingThumbnail(of: try await metadata) else {
            throw ThumbnailError.badImage
        }
        return thumbnail
    }

    func downloadImage(id: UInt32) async throws -> UIImage {
        try Task.checkCancellation()
        
        print("started download image...")
        guard let aURL = URL(string: getOneImageUrl(id: id)) else {
            throw URLError(.badURL)
        }
        let request = URLRequest(url: aURL)
        let (data, _) = try await URLSession.shared.data(for: request)
        guard let image = UIImage(data: data) else {
            throw ThumbnailError.badImage
        }
        print("ended download image")
        return image
    }
    func downloadImageMetadata(id: UInt32) async throws -> CGSize {
        try Task.checkCancellation()
        
        print("started download image metadata...")
        let image = try await downloadImage(id: id)
        let height: CGFloat = 200
        let width = image.size.width/image.size.height * height
        print("ended download image metadata")
        return .init(width: width, height: height)
    }
    
    func getOneImageUrl(id: UInt32? = nil) -> String {
        let idx = id ?? 1 + arc4random() % 20
        return "https://raw.githubusercontent.com/onevcat/Kingfisher-TestImages/master/DemoAppImage/HighResolution/\(idx).jpg"
    }
}

async let相当于对已存在的某个异步任务(方法)进行了二次封装,然后返回一个新的匿名异步任务,再将这个异步任务进行try await待其执行完成,就可使用结果值了。

Group Tasks

@available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
@frozen public struct ThrowingTaskGroup where Failure : Error {
...
}
  • group 满足 AsyncSequence,因此我们可以使用 for await 的语法来获取子任务的执行结果。group 中的某个任务完成时,它的结果将被放到异步序列的缓冲区中。每当 group 的 next 会被调用时,如果缓冲区里有值,异步序列就将它作为下一个值给出;如果缓冲区为空,那么就等待下一个任务完成,这是异步序列的标准行为。
  • for await 的结束意味着异步序列的 next 方法返回了 nil,此时group 中的子任务已经全部执行完毕了,withTaskGroup 的闭包也来到最后。接下来,外层的 “End” 也会被输出。整个结构化并发结束执行。
  • 即使我们没有明确 await 任务组,编译器在检测到结构化并发作用域结束时,会为我们自动添加上 await 并在等待所有任务结束后再继续控制流:
for i in 0 ..< 3 {
    group.addTask {
      await work(i)
    }
  }

// 编译器自动生成的代码
  for await _ in group { }

即使手动退出某个子任务的await行为,编译器也会自动加上如下的隐式操作:

for await result in group {
    print("Get result: \(result)")
    // 在首个子任务完成后就跳出
    break
  }
  print("Task ended")

  // 编译器自动生成的代码
  await group.waitForAll()

public mutating func addTask(priority: TaskPriority? = nil, operation: @escaping @Sendable () async throws -> ChildTaskResult)

  • 注意addTask的operation是一个返回值类型为ChildTaskResult@Sendable的block,这意味在多个异步的task之间可以进行数据send达到线程通信的目的,也保证了数据访问的线程安全

  • 也可以使用addTaskUnlessCancelled() -> Bool这个方法,如果group外层的task被cancel了,则不会再addTask了:

    • Adds a child task to the group, unless the group has been canceled.
    • This method doesn't throw an error, even if the child task does.
  • group.cancelAll()取消全部任务

func fetchThumbnails(ids: [UInt32]) async throws -> [UInt32: UIImage] {
        var result: [UInt32: UIImage] = [:]
        try await withThrowingTaskGroup(of: (UInt32, UIImage).self) { group in
            for id in ids {
                group.addTask(priority: .medium) { [self] in
                    return (id, try await downloadImageThumbnail(id: id))
                }
            }
            for try await (id, thumbnail) in group {
                result[id] = thumbnail
            }
        }
        return result
    }
  • 调用
@IBAction func btnTapped(_ sender: UIControl) {
        Task {
            let images = try await fetchThumbnails(ids: [1, 3, 5, 7])
            print("All thumbnail images downloaded")
            for (id,img) in images {
                DispatchQueue.main.asyncAfter(deadline: .now() + Double(id), qos: .userInteractive) {
                    self.imageView.image = img
                }
            }
        }
    }

Unstructured Tasks

如果将非结构化的异步方法调用和结构化的异步任务结合起来,可以利用Task{}包裹,并且将其存储,在合适的时机进行cancel和置nil

@MainActor
class MyDelegate: UICollectionViewDelegate {
    var thumbnailTasks: [IndexPath: Task] = [:]
    
    func collectionView(_ view: UICollectionView, willDisplay cell: UICollectionViewCell, forItemAt item: IndexPath) {
        let ids = getThumbnailIDs(for: item)
        thumbnailTasks[item] = Task {
            defer { thumbnailTasks[item] = nil }
            let thumbnails = await fetchThumbnails(for: ids)
            display(thumbnails, in: cell)
        }
    }
    
    func collectionView(_ view: UICollectionView, didEndDisplay cell: UICollectionViewCell, forItemAt item: IndexPath) {
        thumbnailTasks[item]?.cancel()
    }
}

Unstructured Detached Tasks

任务嵌套的异步子任务,可以通过group进行组合使其并发执行

@MainActor
class MyDelegate: UICollectionViewDelegate {
    var thumbnailTasks: [IndexPath: Task] = [:]
    
    func collectionView(_ view: UICollectionView, willDisplay cell: UICollectionViewCell, forItemAt item: IndexPath) {
        let ids = getThumbnailIDs(for: item)
        thumbnailTasks[item] = Task {
            defer { thumbnailTasks[item] = nil }
            let thumbnails = await fetchThumbnails(for: ids)
            Task.detached(priority: .background) {
                withTaskGroup(of: Void.self) { g in
                    g.async { writeToLocalCache(thumbnails) }
                    g.async { log(thumbnails) }
                    g.async { ... }
                }
            }
            display(thumbnails, in: cell)
        }
    }
}

异步计算属性

  var asyncCover: UIImage? {
        get async {
            return await getRemoteCoverImage()
        }
    }
    func getRemoteCoverImage() async -> UIImage? {
        //do some network requests
        return nil
    }

异步函数

在函数声明的返回箭头前面,加上 async 关键字,就可以把一个函数声明为异步函数:

func loadSignature() async throws -> String {
  fatalError("暂未实现")
}

异步函数的 async 关键字会帮助编译器确保两件事情:

  • 它允许我们在函数体内部使用 await 关键字;
  • 它要求其他人在调用这个函数时,使用 await 关键字。

这和与它处于类似位置的 throws 关键字有点相似。在使用 throws 时,它允许我们在函数内部使用 throw 抛出错误,并要求调用者使用 try 来处理可能的抛出。

结构化并发

  • 基于 Task 的结构化并发模型

在 Swift 并发编程中,结构化并发需要依赖异步函数,而异步函数又必须运行在某个任务上下文中,因此可以说,想要进行结构化并发,必须具有任务上下文。实际上,Swift 结构化并发就是以任务为基本要素进行组织的。

  • 当前任务状态
    Swift 并发编程把异步操作抽象为任务,在任意的异步函数中,我们总可是使用 withUnsafeCurrentTask 来获取和检查当前任务:
func foo() async {
  withUnsafeCurrentTask { task in
    // 3
    if let task = task {
      // 4
      print("Cancelled: \(task.isCancelled)")
      // => Cancelled: false

      print(task.priority)
      // TaskPriority(rawValue: 33)
    } else {
      print("No task")
    }
  }
}

actor模型

解决多线程数据访问安全问题,类似于lock的作用,保证了数据的安全访问

actor Holder {
  var results: [String] = []
  func setResults(_ results: [String]) {
    self.results = results
  }
    
  func append(_ value: String) {
    results.append(value)
  }
}

actor 内部会提供一个隔离域:在 actor 内部对自身存储属性或其他方法的访问,比如在 append(_:) 函数中使用 results 时,可以不加任何限制,这些代码都会被自动隔离在被封装的“私有队列”里。但是从外部对 actor 的成员进行访问时,编译器会要求切换到 actor 的隔离域,以确保数据安全。在这个要求发生时,当前执行的程序可能会发生暂停。编译器将自动把要跨隔离域的函数转换为异步函数,并要求我们使用 await 来进行调用。

  • 调用:由于是以类似异步队列-线程的方式进行了内部封装/隔离,所以访问这些数据需要使用await标识,表示线程的调度
// holder.setResults([])
await holder.setResults([])

// holder.append(data.appending(signature))
await holder.append(data.appending(signature))

// print("Done: \(holder.getResults())")
print("Done: \(await holder.results)")

当然,这种数据隔离只解决同时访问的造成的内存问题 (在 Swift 中,这种不安全行为大多数情况下表现为程序崩溃),并不会解决多个异步让数据增加/减少导致数据错乱不同步问题。

我们可以使用 @MainActor 来确保 UI 线程的隔离。

如果你是在一个没有“完全迁移”到 Swift Concurrency Safe 的项目的话,可能需要在 class 申明上也加上 @MainActor 来让它生效。

另外,需要指出的是,@MainActor 需要 async 环境来完成 actor 的切换。

Group抽象封装(简化)

封装一个MyAsyncTaskGroup泛型化的group

class MyAsyncTaskGroup {
    typealias Operation = (Data) async throws -> (ChildTaskResult)
    
    let datas: [Data]
    let operation: Operation
    
    init(children datas: [Data], child operation: @escaping Operation) {
        self.datas = datas
        self.operation = operation
    }
    
    func throwingStart() async throws -> [ChildTaskResult] {
        var results: [ChildTaskResult] = []
        try await withThrowingTaskGroup(of: ChildTaskResult.self) { group in
            for data in datas {
                group.addTask{ [self] in
                    try await self.operation(data)
                }
            }
            for try await result in group {
                results.append(result)
            }
        }
        return results
    }
}
  • 自定义具体操作的group
class AsyncImageDownloadGroup: MyAsyncTaskGroup {
    let URLs: [URL]
    init(urls: [String]) {
        self.URLs = urls.compactMap{ .init(string: $0) }
        super.init(children: self.URLs) {
            try await API.shared.downloadImage(withURL: $0)
        }
    }
}
  • 调用
@IBAction func syncConcurrent(_ sender: UIControl) {
        Task {
            let images = try await AsyncImageDownloadGroup(
                urls: [
                    API.shared.getOneImageUrl(),
                    API.shared.getOneImageUrl(),
                    API.shared.getOneImageUrl(),
                    API.shared.getOneImageUrl()
                ]
            ).throwingStart()
            
            print("All thumbnail images downloaded")
            for (idx,img) in images.enumerated() {
                DispatchQueue.main.asyncAfter(deadline: .now() + Double(idx), qos: .userInteractive) {
                    self.imageView.image = img
                }
            }
        }
    }

也可以这样使用:

Task {
            let images = try await MyAsyncTaskGroup(
                childrenDatas: [
                    API.shared.getOneImageUrl(),
                    API.shared.getOneImageUrl(),
                    API.shared.getOneImageUrl(),
                    API.shared.getOneImageUrl()
                ].compactMap{ URL(string: $0) }, childOperation: {
                    try await API.shared.downloadImage(withURL: $0)
                }
            ).throwingStart()
            
            print("All thumbnail images downloaded")
            for (idx,img) in images.enumerated() {
                DispatchQueue.main.asyncAfter(deadline: .now() + Double(idx), qos: .userInteractive) {
                    self.imageView.image = img
                }
            }
        }

这样一封装,是不是感觉比系统原生的简洁多了。

Actor Reentrancy

核心代码

do {
            let image = try await task.value
            cache[url] = .ready(image)
            return image
        } catch {
            cache[url] = nil
            throw error
        }
  • 此处执行reentrancy操作(等待task完成),目的是当downloadImage完成时,
  • 立即将对应的image进行缓存,并返回给外部调用者
  • 如果下载失败,则将对应的task存储置为nil
actor ImageDownloader {

    private enum CacheEntry {
        case inProgress(Task)
        case ready(Image)
    }

    private var cache: [URL: CacheEntry] = [:]

    func image(from url: URL) async throws -> Image? {
        if let cached = cache[url] {
            switch cached {
            case .ready(let image):
                return image
            case .inProgress(let task):
                return try await task.value
            }
        }

        let task = Task {
            try await downloadImage(from: url)
        }

        cache[url] = .inProgress(task)

        do {
            let image = try await task.value
            cache[url] = .ready(image)
            return image
        } catch {
            cache[url] = nil
            throw error
        }
    }
}

Actor Isolate

actor LibraryAccount {
    let idNumber: Int
    var booksOnLoan: [Book] = []
}

extension LibraryAccount: Hashable {
    nonisolated func hash(into hasher: inout Hasher) {
        hasher.combine(idNumber)
    }
}

Sendable & @Sendable

Sendable是一个协议,它标识的数据模型实例可以在actor的环境中被安全的访问

struct Book: Sendable {
    var title: String
    var authors: [Author]
}

Sendable协议下的模型内部也要求所有自定义类型均实现Sendable协议,否则就会编译报错,但是我们可以实现一个类似于包裹器的泛型结构体Pair,让其实现Sendable协议,就可以了:

struct Pair {
    var first: T
    var second: U
}

extension Pair: Sendable where T: Sendable, U: Sendable {
}

@sendable可以标识一个func或closure的类型,表示自动实现Sendable协议

public mutating func addTask(
  priority: TaskPriority? = nil, 
  operation: @escaping @Sendable () async throws -> ChildTaskResult
)

@MainActor

该标识的方法,将会在主线程执行,但同样的也要在调用的地方用await标识

@MainActor func checkedOut(_ booksOnLoan: [Book]) {
    booksView.checkedOutBooks = booksOnLoan
}

// Swift ensures that this code is always run on the main thread.
await checkedOut(booksOnLoan)

同样的,自定义类型也可以用@MainActor标识,表示其中的属性、方法等均在主线程执行,常用于UI类的标识。当然,如果用nonisolated标识某个方法、属性,表示其可以脱离于当前类型的main thread的context。

@MainActor class MyViewController: UIViewController {
    func onPress(...) { ... } // implicitly @MainActor

// 这个方法可以脱离主线程运行
    nonisolated func fetchLatestAndDisplay() async { ... } 
}

Async Sequence

@main
struct QuakesTool {
    static func main() async throws {
        let endpointURL = URL(string: "https://earthquake.usgs.gov/earthquakes/feed/v1.0/summary/all_month.csv")!

        // skip the header line and iterate each one 
        // to extract the magnitude, time, latitude and longitude
        for try await event in endpointURL.lines.dropFirst() {
            let values = event.split(separator: ",")
            let time = values[0]
            let latitude = values[1]
            let longitude = values[2]
            let magnitude = values[4]
            print("Magnitude \(magnitude) on \(time) at \(latitude) \(longitude)")
        }
    }
}

上面的for-await-in类似于:

var iterator = quakes.makeAsyncIterator()
while let quake = await iterator.next() {
    if quake.magnitude > 3 {
        displaySignificantEarthquake(quake)
    }
}

上述for-try await-in可以正常配合breakcontinuedo{} catch{}使用

Bytes from a FileHandle

也可以用于以异步序列的方式读取本地/网络文件:

let url = URL(fileURLWithPath: "/tmp/somefile.txt")
for try await line in url.lines {
    ...
}

Bytes from a URLSession

let (bytes, response) = try await URLSession.shared.bytes(from: url)

guard let httpResponse = response as? HTTPURLResponse,
      httpResponse.statusCode == 200 /* OK */
else {
    throw MyNetworkingError.invalidServerResponse
}

for try await byte in bytes {
    ...
}

Async Notifications

let center = NotificationCenter.default
let notification = await center.notifications(named: .NSPersistentStoreRemoteChange).first {
    $0.userInfo[NSStoreUUIDKey] == storeUUID
}

Custom AsyncSequence

class QuakeMonitor {
    var quakeHandler: (Quake) -> Void
    func startMonitoring()
    func stopMonitoring()
}

let quakes = AsyncStream(Quake.self) { continuation in
    let monitor = QuakeMonitor()
    monitor.quakeHandler = { quake in
        continuation.yield(quake)
    }
    continuation.onTermination = { @Sendable _ in
        monitor.stopMonitoring()
    }
    monitor.startMonitoring()
}

let significantQuakes = quakes.filter { quake in
    quake.magnitude > 3
}

for await quake in significantQuakes {
    ...
}

对应的,也可以使用AsyncThrowingStream包裹对应的Sequence数据流。

Continuation

由于async/throws是需要await然后立即return对应的结果的,那么如果在一个新的async方法里,想要嵌套原有的基于handler的异步方法,那么就没法return了,因为在handler里边才能进行结果的对错校验。为了搭配这2种方式,引入了Continuation异步转发handler返回的数据,然后return

问题:

func asyncRequest(_ req: URLRequest) async throws -> T {
        Network.request(req) { (error, data) in
            if let error = error {
                throw error
            }
            else if let data = data {
                // so how to `return` the data?
            }
        }
        
    }
    struct Network {
        static func request(_ req: URLRequest, completion: @escaping (Error?, Data?)->Void) {
            ...
        }
    }

解决:

func withCheckedThrowingContinuation(
  function: String = #function, 
  _ body: (CheckedContinuation) -> Void
) async throws -> T

// resume
public func resume(returning x: T)
public func resume(throwing x: E)

CheckedContinuation

    func asyncRequest(_ req: URLRequest) async throws -> T {
        typealias RequestContinuation = CheckedContinuation
        return try await withCheckedThrowingContinuation{ (continuation: RequestContinuation) in
            Network.request(req) { (error, data: T?) -> Void in
                if let error = error {
                    continuation.resume(throwing: error)
                }
                else if let data = data {
                    continuation.resume(returning: data)
                }
            }
        }
        
    }
    struct Network {
        enum Error: Swift.Error {
            case noData
        }
        
        static func request(
            _ req: URLRequest,
            completion: @escaping (Swift.Error?, T?)->Void
        ) {
            let handler: (Foundation.Data?, URLResponse?, Swift.Error?) -> Void = { data,_,error in
                if let error = error {
                    return completion(error, nil)
                }
                guard let data = data else {
                    return completion(Error.noData, nil)
                }
                do {
                    let object = try JSONDecoder().decode(T.self, from: data)
                    return completion(nil, object)
                }
                catch {
                    return completion(error, nil)
                }
            }
            let task = URLSession.shared.dataTask(with: req, completionHandler: handler)
            task.resume()
        }
    }

handler回调类似的,还有基于delegate的方式,同样可以使用Continuation来异步转发:

class ViewController: UIViewController {
    private var activeContinuation: CheckedContinuation<[Post], Error>?
    func sharedPostsFromPeer() async throws -> [Post] {
        try await withCheckedThrowingContinuation { continuation in
            self.activeContinuation = continuation
            self.peerManager.syncSharedPosts()
        }
    }
}

extension ViewController: PeerSyncDelegate {
    func peerManager(_ manager: PeerManager, received posts: [Post]) {
        self.activeContinuation?.resume(returning: posts)
 // guard against multiple calls to resume
        self.activeContinuation = nil
    }

    func peerManager(_ manager: PeerManager, hadError error: Error) {
        self.activeContinuation?.resume(throwing: error)
 // guard against multiple calls to resume
        self.activeContinuation = nil
    }
}

参考

  • https://www.cnblogs.com/lxlx1798/articles/15919810.html
  • https://onevcat.com/2021/09/structured-concurrency/
  • WWDC21

你可能感兴趣的:(Swift 5.5 | Async throws/ Try await)