axiom-swift-concurrency

# Swift 6 Concurrency Guide

**Purpose**: Progressive journey from single-threaded to concurrent Swift code
**Swift Version**: Swift 6.0+, Swift 6.2+ for `@concurrent`
**iOS Version**: iOS 17+ (iOS 18.2+ for `@concurrent`)
**Xcode**: Xcode 16+ (Xcode 16.2+ for `@concurrent`)
**Context**: WWDC 2025-268 "Embracing Swift concurrency" - approachable path to data-race safety

## When to Use This Skill

✅ **Use this skill when**:
- Starting a new project and deciding concurrency strategy
- Debugging Swift 6 concurrency errors (actor isolation, data races, Sendable warnings)
- Deciding when to introduce async/await vs concurrency
- Implementing `@MainActor` classes or async functions
- Converting delegate callbacks to async-safe patterns
- Deciding between `@MainActor`, `nonisolated`, `@concurrent`, or actor isolation
- Resolving "Sending 'self' risks causing data races" errors
- Making types conform to `Sendable`
- Offloading CPU-intensive work to background threads
- UI feels unresponsive and profiling shows main thread bottleneck

❌ **Do NOT use this skill for**:
- General Swift syntax (use Swift documentation)
- SwiftUI-specific patterns (use `axiom-swiftui-debugging` or `axiom-swiftui-performance`)
- API-specific patterns (use API documentation)

## Core Philosophy: Start Single-Threaded

> **Apple's Guidance (WWDC 2025-268)**: "Your apps should start by running all of their code on the main thread, and you can get really far with single-threaded code."

### The Progressive Journey

```
Single-Threaded → Asynchronous → Concurrent → Actors
     ↓                ↓             ↓           ↓
   Start here    Hide latency   Background   Move data
                 (network)      CPU work     off main
```

**When to advance**:
1. **Stay single-threaded** if UI is responsive and operations are fast
2. **Add async/await** when high-latency operations (network, file I/O) block UI
3. **Add concurrency** when CPU-intensive work (image processing, parsing) freezes UI
4. **Add acto