swift-concurrency

# Swift Concurrency

## Overview

This skill provides guidance for writing thread-safe Swift code using modern concurrency patterns. It covers three main workflows: building new async code, auditing existing code for issues, and refactoring legacy patterns to Swift 6+.

**Core principle**: Isolation is inherited by default. With Approachable Concurrency, code starts on MainActor and propagates through the program automatically. Opt out explicitly when needed.

## Workflow Decision Tree

```
What are you doing?
│
├─► BUILDING new async code
│   └─► See "Building Workflow" below
│
├─► AUDITING existing code
│   └─► See "Auditing Checklist" below
│
└─► REFACTORING legacy code
    └─► See "Refactoring Workflow" below
```

## Building Workflow

When writing new async code, follow this decision process:

### Step 1: Determine Isolation Needs

```
Does this type manage UI state or interact with UI?
│
├─► YES → Mark with @MainActor
│
└─► NO → Does it have mutable state shared across contexts?
         │
         ├─► YES → Consider: Can it live on MainActor anyway?
         │         │
         │         ├─► YES → Use @MainActor (simpler)
         │         │
         │         └─► NO → Use a custom actor (requires justification)
         │
         └─► NO → Leave non-isolated (default with Approachable Concurrency)
```

### Step 2: Design Async Functions

```swift
// PREFER: Inherit caller's isolation (works everywhere)
func fetchData(isolation: isolated (any Actor)? = #isolation) async throws -> Data {
  // Runs on whatever actor the caller is on
}

// USE WHEN: CPU-intensive work that must run in background
@concurrent
func processLargeFile() async -> Result { }

// AVOID: Non-isolated async without explicit choice
func ambiguousAsync() async { } // Where does this run?
```

### Step 3: Handle Parallel Work

```swift
// For known number of independent operations
async let avatar = fetchImage("avatar.jpg")
async let banner = fetchImage("banner.jpg")
let (a, b) = await (ava