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:

Stay single-threaded if UI is responsive and operations are fast
Add async/await when high-latency operations (network, file I/O) block UI
Add concurrency when CPU-intensive work (image processing, parsing) freezes UI
Add actors when too much main actor code causes contention

Key insight: Concurrent code is more complex. Only introduce concurrency when profiling shows it's needed.

Step 1: Single-Threaded Code (Start Here)

All code runs on the main thread by default in Swift 6.

// ✅ Simple, single-threaded
class ImageModel {
    var imageCache: [URL: Image] = [:]

    func fetchAndDisplayImage(url: URL) throws {
        let data = try Data(contentsOf: url)  // Reads local file
        let image = decodeImage(data)
        view.displayImage(image)
    }

    func decodeImage(_ data: Data) -> Image {
        // Decode image data
        return Image()
    }
}

Main Actor Mode (Xcode 26+):

Enabled by default for new projects
All code protected by @MainActor unless explicitly marked otherwise
Access shared state safely without worrying about concurrent access

Build Setting (Xcode 26+):

Build Settings → Swift Compiler — Language
→ "Default Actor Isolation" = Main Actor

Build Settings → Swift Compiler — Upcoming Features
→ "Approachable Concurrency" = Yes

When this is enough: If all operations are fast (<16ms for 60fps), stay single-threaded!

Step 2: Asynchronous Tasks (Hide Latency)

Add async/await when waiting on data (network, file I/O) would freeze UI.

Problem: Network Access Blocks UI

// ❌ Blocks main thread until network completes
func fetchAndDisplayImage(url: URL) throws {
    let (data, _) = try URLSession.shared.data(from: url)  // ❌ Freezes UI!
    let image = decodeImage(data)
    view.displayImage(image)
}

Solution: Async/Await

// ✅ Suspends without blocking main thread
func fetchAndDisplayImage(url: URL) async throws {
    let (data, _) = try await URLSession.shared.data(from: url)  // ✅ Suspends here
    let image = decodeImage(data)  // ✅ Resumes here when data arrives
    view.displayImage(image)
}

What happens:

Function starts on main thread
await suspends function without blocking main thread
URLSession fetches data on background thread (library handles this)
Function resumes on main thread when data arrives
UI stays responsive the entire time

Task Creation

Create tasks in response to user events:

class ImageModel {
    var url: URL = URL(string: "https://swift.org")!

    func onTapEvent() {
        Task {  // ✅ Create task for user action
            do {
                try await fetchAndDisplayImage(url: url)
            } catch {
                displayError(error)
            }
        }
    }
}

Task Interleaving (Important Concept)

Multiple async tasks can run on the same thread by taking turns:

Task 1: [Fetch Image] → (suspend) → [Decode] → [Display]
Task 2: [Fetch News]  → (suspend) → [Display News]

Main Thread Timeline:
[Fetch Image] → [Fetch News] → [Decode Image] → [Display Image] → [Display News]

Benefits:

Main thread never sits idle
Tasks make progress as soon as possible
No concurrency yet—still single-threaded!

When to use tasks:

High-latency operations (network, file I/O)
Library APIs handle background work for you (URLSession, FileManager)
Your own code stays on main thread

Step 3: Concurrent Code (Background Threads)

Add concurrency when CPU-intensive work blocks UI.

Problem: Decoding Blocks UI

Profiling shows decodeImage() takes 200ms, causing UI glitches:

func fetchAndDisplayImage(url: URL) async throws {
    let (data, _) = try await URLSession.shared.data(from: url)
    let image = decodeImage(data)  // ❌ 200ms on main thread!
    view.displayImage(image)
}

Solution 1: `@concurrent` Attribute (Swift 6.2+)

Forces function to always run on background thread:

func fetchAndDisplayImage(url: URL) async throws {
    let (data, _) = try await URLSession.shared.data(from: url)
    let image = await decodeImage(data)  // ✅ Runs on background thread
    view.displayImage(image)
}

@concurrent
func decodeImage(_ data: Data) async -> Image {
    // ✅ Always runs on background thread pool
    // Good for: image processing, file I/O, parsing
    return Image()
}

What @concurrent does:

Function always switches to background thread pool
Compiler highlights main actor data access (shows what you need to fix)
Cannot access @MainActor properties without await

Requirements: Swift 6.2, Xcode 16.2+, iOS 18.2+

Solution 2: `nonisolated` (Library APIs)

If providing a general-purpose API, use nonisolated instead:

// ✅ Stays on caller's actor
nonisolated
func decodeImage(_ data: Data) -> Image {
    // Runs on whatever actor called it
    // Main actor → stays on main actor
    // Background → stays on background
    return Image()
}

When to use nonisolated:

Library APIs where caller decides where work happens
Small operations that might be OK on main thread
General-purpose code used in many contexts

When to use @concurrent:

Operations that should always run on background (image processing, parsing)
Performance-critical work that shouldn't block UI

Breaking Ties to Main Actor

When you mark a function @concurrent, compiler shows main actor access:

@MainActor
class ImageModel {
    var cachedImage: [URL: Image] = [:]  // Main actor data

    @concurrent
    func decodeImage(_ data: Data, at url: URL) async -> Image {
        if let image = cachedImage[url] {  // ❌ Error: main actor access!
            return image
        }
        // decode...
    }
}

Strategy 1: Move to caller (keep work synchronous):

func fetchAndDisplayImage(url: URL) async throws {
    // ✅ Check cache on main actor BEFORE async work
    if let image = cachedImage[url] {
        view.displayImage(image)
        return
    }

    let (data, _) = try await URLSession.shared.data(from: url)
    let image = await decodeImage(data)  // No URL needed now
    view.displayImage(image)
}

@concurrent
func decodeImage(_ data: Data) async -> Image {
    // ✅ No main actor access needed
    return Image()
}

Strategy 2: Use await (access main actor asynchronously):

@concurrent
func decodeImage(_ data: Data, at url: URL) async -> Image {
    // ✅ Await to access main actor data
    if let image = await cachedImage[url] {
        return image
    }
    // decode...
}

Strategy 3: Make nonisolated (if doesn't need actor):

nonisolated
func decodeImage(_ data: Data) -> Image {
    // ✅ No actor isolation, can call from anywhere
    return Image()
}

Concurrent Thread Pool

When work runs on background:

Main Thread:    [UI] → (suspend) → [UI Update]
                         ↓
Background Pool: [Task A] → [Task B] → [Task A resumes]
                 Thread 1    Thread 2    Thread 3

Key points:

System manages thread pool size (1-2 threads on Watch, many on Mac)
Task can resume on different thread than it started
You never specify which thread—system optimizes automatically

Step 4: Actors (Move Data Off Main Thread)

Add actors when too much code runs on main actor causing contention.

Problem: Main Actor Contention

@MainActor
class ImageModel {
    var cachedImage: [URL: Image] = [:]
    let networkManager: NetworkManager = NetworkManager()  // ❌ Also @MainActor

    func fetchAndDisplayImage(url: URL) async throws {
        // ✅ Background work...
        let connection = await networkManager.openConnection(for: url)  // ❌ Hops to main!
        let data = try await connection.data(from: url)
        await networkManager.closeConnection(connection, for: url)  // ❌ Hops to main!

        let image = await decodeImage(data)
        view.displayImage(image)
    }
}

Issue: Background task keeps hopping to main actor for network manager access.

Solution: Network Manager Actor

// ✅ Move network state off main actor
actor NetworkManager {
    var openConnections: [URL: Connection] = [:]

    func openConnection(for url: URL) -> Connection {
        if let connection = openConnections[url] {
            return connection
        }
        let connection = Connection()
        openConnections[url] = connection
        return connection
    }

    func closeConnection(_ connection: Connection, for url: URL) {
        openConnections.removeValue(forKey: url)
    }
}

@MainActor
class ImageModel {
    let networkManager: NetworkManager = NetworkManager()

    func fetchAndDisplayImage(url: URL) async throws {
        // ✅ Now runs mostly on background
        let connection = await networkManager.openConnection(for: url)
        let data = try await connection.data(from: url)
        await networkManager.closeConnection(connection, for: url)

        let image = await decodeImage(data)
        view.displayImage(image)
    }
}

What changed:

NetworkManager is now an actor instead of @MainActor class
Network state isolated in its own actor
Background code can access network manager without hopping to main actor
Main thread freed up for UI work

When to Use Actors

✅ Use actors for:

Non-UI subsystems with independent state (network manager, cache, database)
Data that's causing main actor contention
Separating concerns from UI code

❌ Do NOT use actors for:

UI-facing classes (ViewModels, View Controllers) → Use @MainActor
Model classes used by UI → Keep @MainActor or non-Sendable
Every class in your app (actors add complexity)

Guideline: Profile first. If main actor has too much state causing bottlenecks, extract one subsystem at a time into actors.

Sendable Types (Data Crossing Actor Boundaries)

When data passes between actors or tasks, Swift checks it's Sendable (safe to share).

Value Types Are Sendable

// ✅ Value types copy when passed
let url = URL(string: "https://swift.org")!

Task {
    // ✅ This is a COPY of url, not the original
    // URLSession.shared.data runs on background automatically
    let data = try await URLSession.shared.data(from: url)
}

// ✅ Original url unchanged by background task

Why safe: Each actor gets its own independent copy. Changes don't affect other copies.

What's Sendable?

// ✅ Basic types
extension URL: Sendable {}
extension String: Sendable {}
extension Int: Sendable {}
extension Date: Sendable {}

// ✅ Collections of Sendable elements
extension Array: Sendable where Element: Sendable {}
extension Dictionary: Sendable where Key: Sendable, Value: Sendable {}

// ✅ Structs/enums with Sendable storage
struct Track: Sendable {
    let id: String
    let title: String
    let duration: TimeInterval
}

enum PlaybackState: Sendable {
    case stopped
    case playing
    case paused
}

// ✅ Main actor types
@MainActor class ImageModel {}  // Implicitly Sendable (actor protects state)

// ✅ Actor types
actor NetworkManager {}  // Implicitly Sendable (actor protects state)

Reference Types (Classes) and Sendable

// ❌ Classes are NOT Sendable by default
class MyImage {
    var width: Int
    var height: Int
    var pixels: [Color]

    func scale(by factor: Double) {
        // Mutates shared state
    }
}

let image = MyImage()
let otherImage = image  // ✅ Both reference SAME object

image.scale(by: 0.5)  // ✅ Changes visible through otherImage!

Problem with concurrency:

func scaleAndDisplay(imageName: String) {
    let image = loadImage(imageName)

    Task {
        image.scale(by: 0.5)  // Background task modifying
    }

    view.displayImage(image)  // Main thread reading
    // ❌ DATA RACE! Both threads could touch same object!
}

Solution 1: Finish modifications before sending:

@concurrent
func scaleAndDisplay(imageName: String) async {
    let image = loadImage(imageName)
    image.scale(by: 0.5)  // ✅ All modifications on background
    image.applyAnotherEffect()  // ✅ Still on background

    await view.displayImage(image)  // ✅ Send to main actor AFTER modifications done
    // ✅ Main actor now owns image exclusively
}

Solution 2: Don't share classes concurrently:

Keep model classes @MainActor or non-Sendable to prevent concurrent access.

Sendable Checking

Happens automatically when:

Passing data into/out of actors
Passing data into/out of tasks
Crossing actor boundaries with await

func fetchAndDisplayImage(url: URL) async throws {
    let (data, _) = try await URLSession.shared.data(from: url)
    //  ↑ Sendable    ↑ Sendable (crosses to background)

    let image = await decodeImage(data)
    //          ↑ data crosses to background (must be Sendable)
    //                            ↑ image returns to main (must be Sendable)
}

Common Patterns (Copy-Paste Templates)

Pattern 1: Sendable Enum/Struct

When: Type crosses actor boundaries

// ✅ Enum (no associated values)
private enum PlaybackState: Sendable {
    case stopped
    case playing
    case paused
}

// ✅ Struct (all properties Sendable)
struct Track: Sendable {
    let id: String
    let title: String
    let artist: String?
}

// ✅ Enum with Sendable associated values
enum Result: Sendable {
    case success(data: Data)
    case failure(error: Error)  // Error is Sendable
}

Pattern 2: Delegate Value Capture (CRITICAL)

When: nonisolated delegate method needs to update @MainActor state

nonisolated func delegate(_ param: SomeType) {
    // ✅ Step 1: Capture delegate parameter values BEFORE Task
    let value = param.value
    let status = param.status

    // ✅ Step 2: Task hop to MainActor
    Task { @MainActor in
        // ✅ Step 3: Safe to access self (we're on MainActor)
        self.property = value
        print("Status: \(status)")
    }
}

Why: Delegate methods are nonisolated (called from library's threads). Capture parameters before Task. Accessing self inside Task { @MainActor in } is safe.

Pattern 3: Weak Self in Tasks

When: Task is stored as property OR runs for long time

class MusicPlayer {
    private var progressTask: Task<Void, Never>?

    func startMonitoring() {
        progressTask = Task { [weak self] in  // ✅ Weak capture
            guard let self = self else { return }

            while !Task.isCancelled {
                await self.updateProgress()
            }
        }
    }

    deinit {
        progressTask?.cancel()
    }
}

Note: Short-lived Tasks (not stored) can use strong captures.

Pattern 4: Background Work with @concurrent

When: CPU-intensive work should always run on background (Swift 6.2+)

@concurrent
func decodeImage(_ data: Data) async -> Image {
    // ✅ Always runs on background thread pool
    // Good for: image processing, file I/O, JSON parsing
    return Image()
}

// Usage
let image = await decodeImage(data)  // Automatically offloads

Requirements: Swift 6.2, Xcode 16.2+, iOS 18.2+

Pattern 5: Isolated Protocol Conformances (Swift 6.2+)

When: Type needs to conform to protocol with specific actor isolation

protocol Exportable {
    func export()
}

class PhotoProcessor {
    @MainActor
    func exportAsPNG() {
        // Export logic requiring UI access
    }
}

// ✅ Conform with explicit isolation
extension StickerModel: @MainActor Exportable {
    func export() {
        photoProcessor.exportAsPNG()  // ✅ Safe: both on MainActor
    }
}

When to use: Protocol methods need specific actor context (main actor for UI, background for processing)

Pattern 6: Atomic Snapshots

When: Reading multiple properties that could change mid-access

var currentTime: TimeInterval {
    get async {
        // ✅ Cache reference for atomic snapshot
        guard let player = player else { return 0 }
        return player.currentTime
    }
}

Pattern 7: MainActor for UI Code

When: Code touches UI

@MainActor
class PlayerViewModel: ObservableObject {
    @Published var currentTrack: Track?
    @Published var isPlaying: Bool = false

    func play(_ track: Track) async {
        // Already on MainActor
        self.currentTrack = track
        self.isPlaying = true
    }
}

Data Persistence Concurrency Patterns

Pattern 8: Background SwiftData Access

actor DataFetcher {
    let modelContainer: ModelContainer

    func fetchAllTracks() async throws -> [Track] {
        let context = ModelContext(modelContainer)
        let descriptor = FetchDescriptor<Track>(
            sortBy: [SortDescriptor(\.title)]
        )
        return try context.fetch(descriptor)
    }
}

@MainActor
class TrackViewModel: ObservableObject {
    @Published var tracks: [Track] = []

    func loadTracks() async {
        let fetchedTracks = try await fetcher.fetchAllTracks()
        self.tracks = fetchedTracks  // Back on MainActor
    }
}

Pattern 9: Core Data Thread-Safe Fetch

actor CoreDataFetcher {
    func fetchTracksID(genre: String) async throws -> [String] {
        let context = persistentContainer.newBackgroundContext()
        var trackIDs: [String] = []

        try await context.perform {
            let request = NSFetchRequest<CDTrack>(entityName: "Track")
            request.predicate = NSPredicate(format: "genre = %@", genre)
            let results = try context.fetch(request)
            trackIDs = results.map { $0.id }  // Extract IDs before leaving context
        }

        return trackIDs  // Lightweight, Sendable
    }
}

Pattern 10: Batch Import with Progress

actor DataImporter {
    func importRecords(_ records: [RawRecord], onProgress: @MainActor (Int, Int) -> Void) async throws {
        let chunkSize = 1000
        let context = ModelContext(modelContainer)

        for (index, chunk) in records.chunked(into: chunkSize).enumerated() {
            for record in chunk {
                context.insert(Track(from: record))
            }
            try context.save()

            let processed = (index + 1) * chunkSize
            await onProgress(min(processed, records.count), records.count)

            if Task.isCancelled { throw CancellationError() }
        }
    }
}

Pattern 11: GRDB Background Execution

actor DatabaseQueryExecutor {
    let dbQueue: DatabaseQueue

    func fetchUserWithPosts(userId: String) async throws -> (user: User, posts: [Post]) {
        return try await dbQueue.read { db in
            let user = try User.filter(Column("id") == userId).fetchOne(db)!
            let posts = try Post
                .filter(Column("userId") == userId)
                .order(Column("createdAt").desc)
                .limit(100)
                .fetchAll(db)
            return (user, posts)
        }
    }
}

Quick Decision Tree

Starting new feature?
└─ Is UI responsive with all operations on main thread?
   ├─ YES → Stay single-threaded (Step 1)
   └─ NO → Continue...
       └─ Do you have high-latency operations? (network, file I/O)
          ├─ YES → Add async/await (Step 2)
          └─ NO → Continue...
              └─ Do you have CPU-intensive work? (Instruments shows main thread busy)
                 ├─ YES → Add @concurrent or nonisolated (Step 3)
                 └─ NO → Continue...
                     └─ Is main actor contention causing slowdowns?
                        └─ YES → Extract subsystem to actor (Step 4)

Error: "Main actor-isolated property accessed from nonisolated context"
├─ In delegate method?
│  └─ Pattern 2: Value Capture Before Task
├─ In async function?
│  └─ Add @MainActor or call from Task { @MainActor in }
└─ In @concurrent function?
   └─ Move access to caller, use await, or make nonisolated

Error: "Type does not conform to Sendable"
├─ Enum/struct with Sendable properties?
│  └─ Add `: Sendable`
└─ Class?
   └─ Make @MainActor or keep non-Sendable (don't share concurrently)

Want to offload work to background?
├─ Always background (image processing)?
│  └─ Use @concurrent (Swift 6.2+)
├─ Caller decides?
│  └─ Use nonisolated
└─ Too much main actor state?
   └─ Extract to actor

Build Settings (Xcode 16+)

Build Settings → Swift Compiler — Language
→ "Default Actor Isolation" = Main Actor
→ "Approachable Concurrency" = Yes

Build Settings → Swift Compiler — Concurrency
→ "Strict Concurrency Checking" = Complete

What this enables:

Main actor mode (all code @MainActor by default)
Compile-time data race prevention
Progressive concurrency adoption

Anti-Patterns (DO NOT DO THIS)

❌ Using Concurrency When Not Needed

// ❌ Premature optimization
@concurrent
func addNumbers(_ a: Int, _ b: Int) async -> Int {
    return a + b  // ❌ Trivial work, concurrency adds overhead
}

// ✅ Keep simple
func addNumbers(_ a: Int, _ b: Int) -> Int {
    return a + b
}

❌ Strong Self in Stored Tasks

// ❌ Memory leak
progressTask = Task {
    while true {
        await self.update()  // ❌ Strong capture
    }
}

// ✅ Weak capture
progressTask = Task { [weak self] in
    guard let self = self else { return }
    // ...
}

❌ Making Every Class an Actor

// ❌ Don't do this
actor MyViewModel: ObservableObject {  // ❌ UI code should be @MainActor!
    @Published var state: State  // ❌ Won't work correctly
}

// ✅ Do this
@MainActor
class MyViewModel: ObservableObject {
    @Published var state: State
}

Code Review Checklist

Before Adding Concurrency

Profiled and confirmed UI unresponsiveness
Identified specific slow operations (network, CPU, contention)
Started with simplest solution (async → concurrent → actors)

Async/Await

Used for high-latency operations only
Task creation in response to events
Error handling with do-catch

Background Work

@concurrent for always-background work (Swift 6.2+)
nonisolated for library APIs
No blocking operations on main actor

Sendable

Value types for data crossing actors
Classes stay @MainActor or non-Sendable
No concurrent modification of shared classes

Actors

Only for non-UI subsystems
UI code stays @MainActor
Model classes stay @MainActor or non-Sendable

Real-World Impact

Before: Random crashes, data races, "works on my machine" bugs, premature complexity After: Compile-time guarantees, progressive adoption, only use concurrency when needed

Key insight: Swift 6's approach makes you prove code is safe before compilation succeeds. Start simple, add complexity only when profiling proves it's needed.

Resources

WWDC: 2025-268, 2025-245, 2022-110351, 2021-10133

Docs: /swift/adoptingswift6, /swift/sendable

Last Updated: 2025-12-01 Status: Enhanced with WWDC 2025-268 progressive journey, @concurrent attribute, isolated conformances, and approachable concurrency patterns

axiom-swift-concurrency