Swift Concurrency Reference

An API reference for Swift's concurrency model: actors, Sendable, tasks, async sequences, continuations, and migration from GCD. It is the syntax lookup your assistant reaches for once you know which tool you need.

When to Use This Reference

Use this reference when:

• Looking up actor definition syntax or reentrancy rules
• Implementing Sendable conformance for your types
• Creating task groups for parallel work
• Working with AsyncStream or a custom AsyncSequence
• Bridging callback-based APIs with continuations
• Migrating from DispatchQueue and DispatchGroup to actors and task groups
• Replacing userInfo-based notifications with typed messages

Example Prompts

Questions you can ask your assistant that will draw from this reference:

• "How do I create a TaskGroup in Swift?"
• "What's the AsyncStream continuation API?"
• "How do I make a custom global actor?"
• "How do I convert a completion handler to async/await?"
• "What are the actor reentrancy rules?"
• "How do I make my class Sendable?"
• "What's the difference between Task and Task.detached?"
• "How do I run a block that ignores task cancellation in iOS 27?"
• "Why does my loop never exit inside withTaskCancellationShield?"

What's Covered

Each part ends with a gotcha table of symptoms and fixes.

Actors

• Actor definition, isolation rules, nonisolated, reentrancy
• Global actors, @MainActor, actor initializers
• Custom executors (SerialExecutor, unownedExecutor)

Sendable

• Automatic conformance, @Sendable closures, @unchecked Sendable, conditional conformance
• sending parameters (SE-0430)
• SWIFT_STRICT_CONCURRENCY build settings
• Extracting Sendable snapshots from non-Sendable types

Task Management

• Task, Task.detached, cancellation, Task.sleep, Task.yield, priority
• withTaskCancellationShield and hasActiveCancellationShield (Swift 6.4, 27 cycle)
• @TaskLocal, the timeout pattern, task retain cycles, Thread.current in Swift 6

Structured Concurrency

• async let, withTaskGroup, withThrowingTaskGroup
• withDiscardingTaskGroup (iOS 17+)
• Task tree semantics, batching with bounded concurrency

Async Sequences

• AsyncStream, AsyncThrowingStream, the continuation API, buffering policies
• Custom AsyncSequence, operators, built-in sequences

Isolation

• @MainActor functions, MainActor.run, MainActor.assumeIsolated
• nonisolated(unsafe), @preconcurrency
• #isolation and isolation capture in Task closures (SE-0420)

Continuations

• withCheckedContinuation, withCheckedThrowingContinuation, the resume-exactly-once rule
• Bridging delegates, unsafe continuations
• Single-resume Continuation (Swift 6.4), with its 27.0–27.1 limitation

Migration Patterns

• DispatchQueue to actor, DispatchGroup to TaskGroup, completion handler to async
• @objc delegates with @MainActor
• NotificationCenter, Timer and DispatchSemaphore to async equivalents
• Typed notifications (MainActorMessage / AsyncMessage, iOS 26+): replacing userInfo casting, observation-token lifetime, per-framework coverage

Swift 6.4 Changes

• await inside defer
• The diagnostic for unstructured Tasks that swallow thrown errors

API Quick Reference

• A table of common tasks and the Swift version each API requires

Documentation Scope

This page documents the swift-concurrency-ref skill (in the axiom-concurrency suite), the API reference your assistant uses for Swift concurrency syntax and patterns. For when and why to reach for concurrency — the progressive journey, decision trees, @concurrent and isolated conformances — see swift-concurrency.

Related

• synchronization – Mutex, OSAllocatedUnfairLock and atomics, for when you need locks instead of actors
• assume-isolated – synchronous actor access patterns
• concurrency-profiling – Instruments workflows for async performance
• concurrency-auditor – scans your code for Sendable violations, missing isolation and other concurrency anti-patterns

Resources

WWDC: 2021-10132, 2021-10134, 2022-110350, 2025-268

Docs: /swift/concurrency, /swift/actor, /swift/sendable, /swift/taskgroup