Swift Concurrency Reference

Complete API reference for Swift's concurrency model — actors, Sendable, Task management, async sequences, continuations, and migration patterns from GCD.

When to Use This Reference

Use this reference when:

• Looking up actor definition syntax or reentrancy rules
• Implementing Sendable conformance for your types
• Creating TaskGroups for parallel work
• Working with AsyncStream or custom AsyncSequence
• Bridging callback-based APIs with continuations
• Migrating from DispatchQueue/DispatchGroup to actors/TaskGroup

Example Prompts

• "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?"

What's Covered

• Actor patterns (definition, nonisolated, reentrancy, global actors, gotcha table)
• Sendable patterns (struct/enum, @Sendable closures, @unchecked, conditional, gotcha table)
• Task management (Task, Task.detached, cancellation, priority, @TaskLocal, gotcha table)
• Structured concurrency (async let, TaskGroup, task tree semantics, gotcha table)
• Async sequences (AsyncStream, continuation API, buffering, custom AsyncSequence, gotcha table)
• Isolation patterns (@MainActor, nonisolated, #isolation, sending, gotcha table)
• Continuations (withCheckedContinuation, resume-exactly-once, bridging, gotcha table)
• Migration patterns (DispatchQueue to actor, DispatchGroup to TaskGroup, callbacks to async, gotcha table)
• API quick reference table with Swift version requirements

Documentation Scope

This page documents the axiom-swift-concurrency-ref skill — the API reference Claude uses for Swift concurrency syntax and patterns.

For the progressive journey and decision trees: See Swift Concurrency for when and why to introduce concurrency, plus 11 copy-paste patterns.

Related

• Swift Concurrency — Progressive journey from single-threaded to concurrent code, decision trees, @concurrent and isolated conformances
• Synchronization — Mutex, OSAllocatedUnfairLock, atomic types (when you need locks instead of actors)
• assumeIsolated — Synchronous actor access patterns
• Concurrency Profiling — Instruments workflows for async performance

Resources

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

Skills: axiom-swift-concurrency, axiom-synchronization, axiom-assume-isolated, axiom-concurrency-profiling