SWIFT并发：合并Taskgroup和AsyncStream

其 idea 是，通过为异步工作创建Task，您可以从AsyncStream的同步上下文过渡到SWIFT并发.还请记住添加onTermination结束，以便它将响应AsyncStream的取消:

func images(for urls: [URL]) -> AsyncStream<NSImage> {
    AsyncStream { continuation in
        let task = Task {
            await withTaskGroup(of: NSImage.self) { group in
                for url in urls {
                    group.addTask { await self.downloadImage(url: url) }
                }

                for await image in group {
                    continuation.yield(image)
                }

                continuation.finish()
            }
        }

        continuation.onTermination = { _ in
            task.cancel()
        }
    }
}

显然，由于您同时执行所有这些网络请求，您必须认识到这些请求很可能不会按照与原始URL数组对应的顺序完成.

因此，您可以返回原始URL和结果图像的元组:

func images(for urls: [URL]) -> AsyncStream<(URL, NSImage)> {
    AsyncStream { continuation in
        let task = Task {
            await withTaskGroup(of: (URL, NSImage).self) { group in
                for url in urls {
                    group.addTask { await (url, self.downloadImage(url: url)) }
                }

                for await tuple in group {
                    continuation.yield(tuple)
                }

                continuation.finish()
            }
        }

        continuation.onTermination = { _ in
            task.cancel()
        }
    }
}

或者可能是索引号和图像:

func images(for urls: [URL]) -> AsyncStream<(Int, NSImage)> {
    AsyncStream { continuation in
        let task = Task {
            await withTaskGroup(of: (Int, NSImage).self) { group in
                for (index, url) in urls.enumerated() {
                    group.addTask { await (index, self.downloadImage(url: url)) }
                }

                for await tuple in group {
                    continuation.yield(tuple)
                }

                continuation.finish()
            }
        }

        continuation.onTermination = { _ in
            task.cancel()
        }
    }
}