Swift Concurrency

This skill covers Swift’s modern concurrency features from Swift 5.5 through Swift 6, including async/await, structured concurrency, actors, and Swift 6’s strict data-race safety.

Overview

Swift’s modern concurrency model provides a safer, more intuitive way to write asynchronous code. Swift 6 takes this further with compile-time data-race safety, turning potential concurrency bugs into compiler errors.

Available References

Swift 6 & Strict Concurrency

• Swift 6 Strict Mode – Complete concurrency checking, data race prevention
• Sendable Protocol – Sendable conformance, @Sendable closures, thread safety
• Isolation & Actors – @MainActor, global actors, region-based isolation

Core Concurrency

• Async/Await – Asynchronous functions, Task, TaskGroup, Actor basics

Swift 6 Highlights

Strict Concurrency by Default

Swift 6 enforces data-race safety at compile time:

// ❌ Compile-time error in Swift 6
var globalCounter = 0

func increment() {
    globalCounter += 1  // Error: concurrent access
}

// ✅ Safe with actor isolation
actor Counter {
    private var value = 0
    func increment() { value += 1 }
}

Sendable Protocol

Mark types safe to share across concurrency boundaries:

struct User: Sendable {
    let id: Int
    let name: String
}

@Sendable func process() async {
    // Captures must be Sendable
}

MainActor & Global Actors

@MainActor
class ViewModel: ObservableObject {
    @Published var items = [Item]()
}

@globalActor
actor DatabaseActor {
    static let shared = DatabaseActor()
    private init() {}
}

Quick Reference

Async/Await Basics

func fetchData() async throws -> Data {
    let (data, _) = try await URLSession.shared.data(from: url)
    return data
}

Swift 6 Migration Checklist

• Enable Swift 6 language mode
• Enable strict concurrency checking
• Wrap global mutable state in actors
• Add Sendable conformance to types
• Fix non-Sendable captures in @Sendable closures
• Isolate UI code with @MainActor
• Audit third-party dependencies

Running Async Code

// Fire-and-forget
Task {
    let result = await asyncOperation()
}

// With priority
Task(priority: .background) {
    await heavyComputation()
}

// With cancellation
let task = Task {
    try await longRunningOperation()
}
task.cancel()

Concurrent Operations

// Async let (parallel await)
async let task1 = fetchUser()
async let task2 = fetchSettings()
let (user, settings) = try await (task1, task2)

// TaskGroup
try await withThrowingTaskGroup(of: Item.self) { group in
    for id in ids {
        group.addTask { try await fetchItem(id: id) }
    }
    return try await group.reduce(into: []) { $0.append($1) }
}

Actor Thread Safety

actor BankAccount {
    private var balance: Double = 0
    
    func deposit(_ amount: Double) {
        balance += amount
    }
    
    func getBalance() -> Double {
        return balance
    }
}

let account = BankAccount()
await account.deposit(100)

Isolation Boundaries

// Crossing isolation boundaries
@MainActor
func updateUI() async {
    // On main thread
    let data = await fetchData()  // Switch to non-isolated
    label.text = data  // Back to main thread
}

// Region transfer
func process() async {
    let data = Data()  // Disconnected
    await save(data)   // Transfer to actor
    // ❌ Can't use data here anymore
}

Swift 6 vs Swift 5.x

Best Practices

Swift 6 Best Practices

1. Enable Swift 6 mode early – Start migration now
2. Use actors for shared state – Default to actors over locks
3. Design Sendable types – Make types Sendable from the start
4. Isolate UI with @MainActor – All UI code on main thread
5. Respect isolation regions – Don’t use values after transfer
6. Leverage compile-time safety – Let compiler catch data races
7. Create domain actors – Custom global actors for heavy work

General Concurrency

1. Prefer async/await – Over completion handlers
2. Use structured concurrency – Clear task hierarchies
3. Handle cancellation – Check Task.isCancelled
4. Use value types – Immutable data is thread-safe
5. Avoid shared mutable state – Or protect with actors

Migration from Completion Handlers

// Before (Swift 5)
func fetchUser(completion: @escaping (Result<User, Error>) -> Void) {
    URLSession.shared.dataTask(with: url) { data, response, error in
        // Handle result
        completion(result)
    }.resume()
}

// After (Swift 6)
func fetchUser() async throws -> User {
    let (data, _) = try await URLSession.shared.data(from: url)
    return try JSONDecoder().decode(User.self, from: data)
}

Common Swift 6 Errors

Resources

• Swift Evolution: https://github.com/apple/swift-evolution
• SE-0412: Strict concurrency for global variables
• SE-0471: SerialExecutor isolation checking
• SE-0430: Sending parameters
• SE-0414: Region-based isolation
• Apple Docs: https://developer.apple.com/documentation/swift/concurrency
• SwiftZilla: https://swiftzilla.dev

For More Information

Each reference file contains detailed information, code examples, and best practices for specific topics. Visit https://swiftzilla.dev for comprehensive Swift concurrency documentation.