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name: concurrency-patterns description: Swift concurrency patterns including Swift 6.2 approachable concurrency, structured concurrency, actors, continuations, and migration. Use when reviewing or building async code, fixing data race errors, or migrating to Swift 6. allowed-tools: [Read, Glob, Grep] last_verified: 2026-07-16 review_by: 2027-06-22 os_version: iOS 27 / macOS 27

Swift Concurrency Patterns

Comprehensive guide for Swift concurrency covering async/await, structured concurrency, actors, and the Swift 6.2 "Approachable Concurrency" features. Focuses on patterns that prevent data races and common mistakes that cause crashes.

When This Skill Activates

• User has data race errors or actor isolation compiler errors
• User is migrating to Swift 6 strict concurrency
• User asks about async/await, actors, Sendable, TaskGroup, or MainActor
• User needs to bridge legacy completion-handler APIs to async/await
• User is working with Swift 6.2 features (@concurrent, isolated conformances)
• User has concurrency bugs (actor reentrancy, task cancellation, UI freezes)

Decision Tree

What concurrency problem are you solving?
│
├─ Swift 6 compiler errors / migration
│  └─ migration-guide.md
│
├─ Swift 6.2 new features (@concurrent, isolated conformances)
│  └─ swift62-concurrency.md
│
├─ Running work in parallel (async let, TaskGroup)
│  └─ structured-concurrency.md
│
├─ Thread safety for shared mutable state
│  └─ actors-and-isolation.md
│
├─ Bridging old APIs (delegates, callbacks) to async/await
│  └─ continuations-bridging.md
│
├─ Hangs, slow parallelism, thread-pool problems, profiling
│  └─ concurrency-internals.md
│
└─ General async/await patterns
   └─ See macos/coding-best-practices/modern-concurrency.md for basics

Quick Reference

Pattern	When to Use	Reference
async let	Fixed number of parallel operations	structured-concurrency.md
withTaskGroup	Dynamic number of parallel operations	structured-concurrency.md
withDiscardingTaskGroup	Fire-and-forget parallel operations	structured-concurrency.md
.task { } modifier	Load data when view appears	structured-concurrency.md
.task(id:) modifier	Re-load when a value changes	structured-concurrency.md
actor	Shared mutable state protection	actors-and-isolation.md
@MainActor	UI-bound state and updates	actors-and-isolation.md
@concurrent	Explicitly offload to background (6.2)	swift62-concurrency.md
Isolated conformances	@MainActor type conforming to protocol (6.2)	swift62-concurrency.md
withCheckedContinuation	Bridge callback API to async	continuations-bridging.md
AsyncStream	Bridge delegate/notification API to async sequence	continuations-bridging.md
@TaskLocal	Context (IDs, tracing) down the task tree	structured-concurrency.md
Atomic / Mutex	Sendable wrappers for synchronous hot paths (Swift 6)	actors-and-isolation.md
Custom actor executors	Bridge existing DispatchSerialQueue synchronization	concurrency-internals.md
Swift Concurrency Instrument	Diagnose hangs, actor contention, continuation leaks	concurrency-internals.md
Strict concurrency migration	Incremental Swift 6 adoption	migration-guide.md

Process

1. Identify the Problem

Read the user's code or error messages to determine:

• Is this a compiler error (strict concurrency) or a runtime issue (data race, crash)?
• What Swift version and concurrency checking level are they using?
• Are they migrating existing code or writing new code?

2. Load Relevant Reference Files

Based on the problem, read from this directory:

• swift62-concurrency.md — Swift 6.2 approachable concurrency features, Apple's adoption doctrine, Swift 6.3/6.4 additions
• structured-concurrency.md — async let, TaskGroup, .task modifier lifecycle, task-locals, cancellation semantics
• actors-and-isolation.md — Actor patterns, reentrancy, @MainActor, Sendable, Atomic/Mutex
• concurrency-internals.md — cooperative pool, unsafe primitives, actor scheduling, Swift Concurrency Instrument
• continuations-bridging.md — withCheckedContinuation, AsyncStream, built-in async sequences, legacy bridging
• migration-guide.md — Incremental Swift 6 strict concurrency adoption (WWDC24 doctrine)

3. Review Checklist

• No blocking calls on @MainActor (use await for long operations)
• Shared mutable state protected by an actor (not locks or DispatchQueue)
• Sendable conformance correct for types crossing isolation boundaries
• Task cancellation handled (check Task.isCancelled or Task.checkCancellation())
• No unstructured Task {} where structured concurrency (.task, TaskGroup) would work
• Actor reentrancy considered at suspension points
• withCheckedContinuation called exactly once (not zero, not twice)
• .task(id:) used instead of manual onChange + cancel patterns

4. Cross-Reference

• For async/await basics and actor fundamentals, see macos/coding-best-practices/modern-concurrency.md
• For networking concurrency patterns, see generators/networking-layer/networking-patterns.md
• For SwiftData concurrency (@ModelActor), see macos/swiftdata-architecture/repository-pattern.md
• For auth token refresh with actors, see generators/auth-flow/auth-patterns.md

References

• Swift Concurrency
• Migrating to Swift 6
• Local captured doc (optional): ~/Downloads/docs/Swift-Concurrency-Updates.md — read if present; skip silently if absent.