Utilities for working with Swift Concurrency

What's New


  • Lots of typo fixes (thanks @rex4539)
  • callback relay Task APIs
  • RelaxedDispatchQueue
  • OwnershipTransferring
  • SendableBox

Build Status License Platforms Documentation


Utilities for working with Swift Concurrency

This is a really small library with some types and extensions that may be useful when working with Swift's concurrency system.

  • A TaskQueue for queuing tasks in FIFO ordering
  • CheckedContinuation extensions for improved ergonomics
  • Task extensions for improved ergonomics when used to bridge to non-async code
  • NSXPCConnection extensions for safe async integration
  • MainActor.runUnsafely to help work around incorrectly- or insufficiently-annotated code not under your control
  • OwnershipTransferring to move a non-Sendable value across actor boundaries
  • SendableBox to lie to the compiler about Sendable conformance
  • RelaxedDispatchQueue a very thin DispatchQueue wrapper with relaxed argument sendability constraints


let queue = TaskQueue()

queue.addOperation {
    await asyncFunction()
    await anotherAsyncFunction()

// This can can also return the underlying Task, so you can cancel, or await a value
let task = await queue.addOperation {
    return await makeValue()

let value = try await task.value
// Without .ordered, the execution order of these tasks is not well-defined.
Task.ordered {

Task.ordered(priority: .background) {

Task.ordered {

Task Ergonomics

Some handy functions that ease integration with existing callbacks.

func callbackOptionalPair(_ block: @escaping (Int?, Error?) -> Void) {
    Task.relayResult(to: block) {
        // ... return async value or throw...

func callbackResult(_ block: @escaping (Result<Int, Error>) -> Void) {
    Task.relayResult(to: block) {
        // ... return async value or throw...

func callbackOptionalError(_ block: @escaping (Error?) -> Void) {
    Task.relayResult(to: block) {
        // ... possibly throw...


This is a tool for moving a value across actor boundaries in a way that will keep the compiler happy. It is reasonably unsafe. You have to be very careful about how the moved value is accessed.

actor MyActor {
    let nonSendable: UnsendableType

    init(_ transfer: OwnershipTransferring<UnsendableType>) {
        self.nonSendable = transfer.takeOwnership()

let nonSendable = UnsendableType()
let transfer = OwnershipTransferring(nonSendable)

let myActor = MyActor(transfer) // no warnings!

transfer.hasOwnershipBeenTransferred() // true
transfer.takeOwnership() // this will crash


DispatchQueue now has implicit @Sendable closure arguments. This is a highly-disruptive change, as it makes queues no longer feasible as a means of non-Sendable state protection. Wrap up that that queue and carry on.

let nonSendable = UnsendableType()
let queue = RelaxedDisptachQueue(label: "myqueue")

queue.async {
    nonSendable.doThing() // no warnings

Working with XPC

You might be tempted to make your XPC interface functions async. This approach does not handle connection failures and will violate the Structured Concurrency contract, resulting in hangs. See the post "ExtensionKit and XPC" for context.

This little NSXPCConnection extension provides a safe way to get into the async world.

func withContinuation<Service, T>(
    function: String = #function, 
    _ body: (Service, CheckedContinuation<T, Error>) -> Void
) async throws -> T

There are also some extensions on CheckedContinuation to make it easier to use in the context of XPC. These are really handy for resuming from common reply patterns.

Given an XPC service like this in your code:

protocol XPCService {
    func errorMethod(reply: (Error?) -> Void)
    func valueAndErrorMethod(reply: (String?, Error?) -> Void)
    func dataAndErrorMethod(reply: (Data?, Error?) -> Void)

The continuation helpers allow bridging like:

try await withContinuation { service, continuation in
    service.errorMethod(reply: continuation.resumingHandler)

try await withContinuation { service, continuation in
    service.valueAndErrorMethod(reply: continuation.resumingHandler)

// this one will try to use JSONDecoder on the resulting data
try await withContinuation { service, continuation in
    service.dataAndErrorMethod(reply: continuation.resumingHandler)

Other Useful Projects

Right now, it's still quite difficult to make use of AsyncSequence. These libraries might be useful and are definitely worth checking out as well.

  • AnyAsyncSequence: super-focused on addressing the lack of type-erased sequences
  • AsyncAlgorithms: Apple-owned reactive extensions to AsyncSequence
  • AsyncExtensions: Companion to AsyncAlgorithms to add additional reactive features
  • Asynchrone: Extensions to bring reactive features to AsyncSequence

Suggestions or Feedback

We'd love to hear from you! Please open up an issue or pull request.

Please note that this project is released with a Contributor Code of Conduct. By participating in this project you agree to abide by its terms.


  • Swift Tools 5.5.0
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Last updated: Sat May 25 2024 13:33:30 GMT-0900 (Hawaii-Aleutian Daylight Time)