Use when writing ANY code with async, actors, threads, or seeing ANY concurrency error. Covers Swift 6 concurrency, @MainActor, Sendable, data races, async/await patterns, performance optimization.
iOS Concurrency Router
You MUST use this skill for ANY concurrency, async/await, threading, or Swift 6 concurrency work.
When to Use
Use this router when:
- Writing async/await code
- Seeing concurrency errors (data races, actor isolation)
- Working with @MainActor
- Dealing with Sendable conformance
- Optimizing Swift performance
- Migrating to Swift 6 concurrency
- App freezes during loading (likely main thread blocking)
Conflict Resolution
ios-concurrency vs ios-performance: When app freezes or feels slow:
- Try ios-concurrency FIRST — Main thread blocking is the #1 cause of UI freezes. Check for synchronous work on @MainActor before profiling.
- Only use ios-performance if concurrency fixes don't help — Profile after ruling out obvious blocking.
ios-concurrency vs ios-build: When seeing Swift 6 concurrency errors:
- Use ios-concurrency, NOT ios-build — Concurrency errors are CODE issues, not environment issues
- ios-build is for "No such module", simulator issues, build failures unrelated to Swift language errors
Rationale: A 2-second freeze during data loading is almost always await on main thread or missing background dispatch. Domain knowledge solves this faster than Time Profiler.
Routing Logic
Swift Concurrency Issues
Swift 6 concurrency patterns → /skill axiom-swift-concurrency
- async/await patterns
- @MainActor usage
- Actor isolation
- Sendable conformance
- Data race prevention
- Swift 6 migration
Swift performance → /skill axiom-swift-performance
- Value vs reference types
- Copy-on-write optimization
- ARC overhead
- Generic specialization
- Collection performance
Synchronous actor access → /skill axiom-assume-isolated
- MainActor.assumeIsolated
- @preconcurrency protocol conformances
- Legacy delegate callbacks
- Testing MainActor code synchronously
Thread-safe primitives → /skill axiom-synchronization
- Mutex (iOS 18+)
- OSAllocatedUnfairLock (iOS 16+)
- Atomic types
- Lock vs actor decision
Parameter ownership → /skill axiom-ownership-conventions
- borrowing/consuming modifiers
- Noncopyable types (~Copyable)
- ARC traffic reduction
- consume operator
Concurrency profiling → /skill axiom-concurrency-profiling
- Swift Concurrency Instruments template
- Actor contention diagnosis
- Thread pool exhaustion
- Task visualization
Decision Tree
- Data races / actor isolation / @MainActor / Sendable? → swift-concurrency
- Writing async/await code? → swift-concurrency
- Swift 6 migration? → swift-concurrency
- assumeIsolated / @preconcurrency? → assume-isolated
- Mutex / lock / synchronization? → synchronization
- borrowing / consuming / ~Copyable? → ownership-conventions
- Profile async performance / actor contention? → concurrency-profiling
- Value type / ARC / generic optimization? → swift-performance
Anti-Rationalization
| Thought | Reality |
|---|---|
| "Just add @MainActor and it'll work" | @MainActor has isolation inheritance rules. swift-concurrency covers all patterns. |
| "I'll use nonisolated(unsafe) to silence the warning" | Silencing warnings hides data races. swift-concurrency shows the safe pattern. |
| "It's just one async call" | Even single async calls have cancellation and isolation implications. swift-concurrency covers them. |
| "I know how actors work" | Actor reentrancy and isolation rules changed in Swift 6.2. swift-concurrency is current. |
| "I'll fix the Sendable warnings later" | Sendable violations cause runtime crashes. swift-concurrency fixes them correctly now. |
Critical Patterns
Swift 6 Concurrency (swift-concurrency):
- Progressive journey: single-threaded → async → concurrent → actors
- @concurrent attribute for forced background execution
- Isolated conformances
- Main actor mode for approachable concurrency
- 11 copy-paste patterns
Swift Performance (swift-performance):
- ~Copyable for non-copyable types
- Copy-on-write (COW) patterns
- Value vs reference type decisions
- ARC overhead reduction
- Generic specialization
Example Invocations
User: "I'm getting 'data race' errors in Swift 6"
→ Invoke: /skill axiom-swift-concurrency
User: "How do I use @MainActor correctly?"
→ Invoke: /skill axiom-swift-concurrency
User: "My app is slow due to unnecessary copying"
→ Invoke: /skill axiom-swift-performance
User: "Should I use async/await for this network call?"
→ Invoke: /skill axiom-swift-concurrency
User: "How do I use assumeIsolated?"
→ Invoke: /skill axiom-assume-isolated
User: "My delegate callback runs on main thread, how do I access MainActor state?"
→ Invoke: /skill axiom-assume-isolated
User: "Should I use Mutex or actor?"
→ Invoke: /skill axiom-synchronization
User: "What's the difference between os_unfair_lock and OSAllocatedUnfairLock?"
→ Invoke: /skill axiom-synchronization
User: "What does borrowing do in Swift?"
→ Invoke: /skill axiom-ownership-conventions
User: "How do I use ~Copyable types?"
→ Invoke: /skill axiom-ownership-conventions
User: "My async code is slow, how do I profile it?"
→ Invoke: /skill axiom-concurrency-profiling
User: "I think I have actor contention, how do I diagnose it?"
→ Invoke: /skill axiom-concurrency-profiling
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