Swift async/await: Re-inventing URLSession's `func data(for:) async throws` https://twitter.com/inamiy/status/1428378193170288645

withTaskCancellationHandler.swift

import Foundation

let url = URL(string: "https://httpbin.org/get")!

@main
struct Main {
    static func main() async throws {
        print("1")

        let task1 = Task.init { () -> Data? in
            do {
                return try await download(url: url)
//                return try await uncancellableDownload(url: url)
            } catch {
                print("download error: ", error)
                return nil
            }
        }

        // NOTE: Requires small delay to start running `URLSession.shared.dataTask`
        await Task.sleep(1)

        print("2. will cancel")
        task1.cancel()

        await Task.sleep(2_000_000_000)
        print("done")

        let data = try await task1.value
        print(String(data: data ?? Data(), encoding: .utf8)!)
    }
}

// https://twitter.com/inamiy/status/1428369626631524367
class Box<T> {
    var value: T
    init(_ value: T) { self.value = value }
}

// https://github.com/apple/swift-evolution/blob/main/proposals/0304-structured-concurrency.md#cancellation-handlers
func download(url: URL) async throws -> Data? {
    let urlSessionTask: Box<URLSessionTask?> = .init(nil)

    return try await withTaskCancellationHandler {
        let value: Data? = try await withUnsafeThrowingContinuation { continuation in
            urlSessionTask.value = URLSession.shared.dataTask(with: url) { data, _, error in
                if let error = error {
                    continuation.resume(throwing: error)
                } else {
                    continuation.resume(returning: data)
                }
            }
            urlSessionTask.value?.resume()
        }
        return value

    } onCancel: {
        print("cancelling", urlSessionTask.value)
        urlSessionTask.value?.cancel()
    }
}

func uncancellableDownload(url: URL) async throws -> Data? {
    var urlSessionTask: URLSessionTask?

    return try await withTaskCancellationHandler {
        let value: Data? = try await withUnsafeThrowingContinuation { continuation in
            urlSessionTask = URLSession.shared.dataTask(with: url) { data, _, error in
                if let error = error {
                    continuation.resume(throwing: error)
                } else {
                    continuation.resume(returning: data)
                }
            }
            urlSessionTask?.resume()
        }
        return value

    } onCancel: { [urlSessionTask] in // WARNING: strong capture is BAD which is always nil
        print("cancelling", urlSessionTask)
        urlSessionTask?.cancel()
    }
}