swift-concurrency-patterns

name	swift-concurrency-patterns
description	Swift concurrency patterns including Swift 6.2 approachable concurrency (@concurrent, isolated conformances, default MainActor inference), structured concurrency, actors, continuations, and migration. Use when reviewing or building async code, fixing data race errors, adopting Swift 6.2 concurrency features, or migrating to Swift 6.

First step: Tell the user: "swift-concurrency-patterns skill loaded."

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 asks about default MainActor inference or the "infer main actor" build setting
• User wants to migrate from Swift 6.0/6.1 strict concurrency to 6.2
• User asks how async functions behave differently in Swift 6.2
• User has concurrency bugs (actor reentrancy, task cancellation, UI freezes)

Decision Tree

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

Quick Reference

Pattern	When to Use	Reference
async let	Fixed number of parallel operations	references/structured-concurrency.md
withTaskGroup	Dynamic number of parallel operations	references/structured-concurrency.md
withDiscardingTaskGroup	Fire-and-forget parallel operations	references/structured-concurrency.md
.task { } modifier	Load data when view appears	references/structured-concurrency.md
.task(id:) modifier	Re-load when a value changes	references/structured-concurrency.md
actor	Shared mutable state protection	references/actors-and-isolation.md
@MainActor	UI-bound state and updates	references/actors-and-isolation.md
@concurrent	Explicitly offload to background (6.2)	references/swift62-concurrency.md
Isolated conformances	@MainActor type conforming to protocol (6.2)	references/swift62-concurrency.md
withCheckedContinuation	Bridge callback API to async	references/continuations-bridging.md
AsyncStream	Bridge delegate/notification API to async sequence	references/continuations-bridging.md
Strict concurrency migration	Incremental Swift 6 adoption	references/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:

• references/swift62-concurrency.md — Swift 6.2 approachable concurrency features
• references/structured-concurrency.md — async let, TaskGroup, .task modifier lifecycle
• references/actors-and-isolation.md — Actor patterns, reentrancy, @MainActor, Sendable
• references/continuations-bridging.md — withCheckedContinuation, AsyncStream, legacy bridging
• references/migration-guide.md — Incremental Swift 6 strict concurrency adoption

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/references/modern-concurrency.md
• For networking concurrency patterns, see generators-networking-layer/references/networking-patterns.md
• For SwiftData concurrency (@ModelActor), see macos-swiftdata-architecture/references/repository-pattern.md
• For auth token refresh with actors, see generators-auth-flow/references/auth-patterns.md

References

• Swift Concurrency
• Migrating to Swift 6
• Apple doc: Swift Concurrency Updates