iOS Concurrency Router

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:

1. Try ios-concurrency FIRST — Main thread blocking is the #1 cause of UI freezes. Check for synchronous work on @MainActor before profiling.
2. 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

User asks about concurrency
  ├─ Concurrency errors?
  │  ├─ Data races? → swift-concurrency
  │  ├─ Actor isolation? → swift-concurrency
  │  ├─ @MainActor? → swift-concurrency
  │  └─ Sendable? → swift-concurrency
  │
  ├─ Writing async code? → swift-concurrency
  │
  ├─ assumeIsolated questions? → assume-isolated
  │
  ├─ Mutex/​lock/​synchronization? → synchronization
  │
  ├─ borrowing/​consuming/​ownership? → ownership-conventions
  │
  ├─ Profile async performance? → concurrency-profiling
  │
  └─ Performance optimization? → swift-performance

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