Swift Concurrency

Review, fix, and write concurrent Swift code targeting Swift 6.3+. Gate Swift 6.4 / Xcode 27 beta cleanup APIs behind explicit toolchain and availability checks. Apply actor isolation, Sendable safety, and modern concurrency patterns with minimal behavior changes.

Contents

• Triage Workflow
• Swift 6.2 Language Changes
• Actor Isolation Rules
• Sendable Rules
• Structured Concurrency Patterns
• Task Cancellation
• Actor Reentrancy
• AsyncSequence and AsyncStream
• @Observable and Concurrency
• Synchronization Primitives
• Common Mistakes
• Review Checklist
• References

Triage Workflow

When diagnosing a concurrency issue, follow this sequence:

Step 1: Capture context

• Copy the exact compiler diagnostic(s) and the offending symbol(s).
• Identify the project's concurrency settings:
  • Swift language version (must be 6.2+).
  • Xcode/toolchain version for version-specific features and release-note workarounds.
  • Whether Approachable Concurrency is enabled.
  • Whether Default Actor Isolation is set to MainActor.
  • Swift 6 strict concurrency status: complete/errors in Swift 6 language mode; Complete / Targeted / Minimal only when auditing Swift 5 migration settings.
• Determine the current actor context of the code (@MainActor, custom actor, nonisolated) and whether a default isolation mode is active.
• Confirm whether the code is UI-bound or intended to run off the main actor.

Step 2: Apply the smallest safe fix

Prefer edits that preserve existing behavior while satisfying data-race safety.

Step 3: Verify

• Rebuild and confirm the diagnostic is resolved.
• Check for new warnings introduced by the fix.
• Ensure no unnecessary @unchecked Sendable or nonisolated(unsafe) was added.
• For build-setting reviews, stop at settings plus the smallest code-level remediation. Do not add Thread Sanitizer, broad migration ordering, or architecture advice unless the prompt asks for diagnostics or migration.

Swift 6.2 Language Changes

Swift 6.2 introduces "approachable concurrency" -- a set of language changes that make concurrent code safer by default while reducing annotation burden. In Xcode, Approachable Concurrency and Default Actor Isolation are separate build settings: use Approachable Concurrency for the bundled upcoming-feature flags, and set Default Actor Isolation to MainActor when you want unannotated code inferred as @MainActor.

SE-0466: Default MainActor Isolation

With the -default-isolation MainActor compiler flag, SwiftPM .defaultIsolation(MainActor.self), or Xcode's Default Actor Isolation setting set to MainActor, unannotated declarations in the module are inferred as @MainActor unless explicitly opted out.

Effect: Eliminates most data-race safety errors for UI-bound code and global/static state without writing @MainActor everywhere.

// With default MainActor isolation enabled, these are implicitly @MainActor:
final class StickerLibrary {
    static let shared = StickerLibrary()  // safe -- on MainActor
    var stickers: [Sticker] = []
}

final class StickerModel {
    let photoProcessor = PhotoProcessor()
    var selection: [PhotosPickerItem] = []
}

// Conformances are also implicitly isolated:
extension StickerModel: Exportable {
    func export() {
        photoProcessor.exportAsPNG()
    }
}

When to use: Recommended for apps, scripts, and other executable targets where most code is UI-bound. Not recommended for library targets that should remain actor-agnostic.

SE-0461: nonisolated(nonsending)

Nonisolated async functions now stay on the caller's actor by default instead of hopping to the global concurrent executor. This is the nonisolated(nonsending) behavior.

class PhotoProcessor {
    func extractSticker(data: Data, with id: String?) async -> Sticker? {
        // In Swift 6.2+, this runs on the caller's actor (e.g., MainActor)
        // instead of hopping to a background thread.
        // ...
    }
}

@MainActor
final class StickerModel {
    let photoProcessor = PhotoProcessor()

    func extractSticker(_ item: PhotosPickerItem) async throws -> Sticker? {
        guard let data = try await item.loadTransferable(type: Data.self) else {
            return nil
        }
        // No data race -- photoProcessor stays on MainActor
        return await photoProcessor.extractSticker(data: data, with: item.itemIdentifier)
    }
}

Use @concurrent to explicitly request background execution when needed.

@concurrent Attribute

@concurrent ensures a function always runs on the concurrent thread pool, freeing the calling actor to run other tasks.

class PhotoProcessor {
    var cachedStickers: [String: Sticker] = [:]

    func extractSticker(data: Data, with id: String) async -> Sticker {
        if let sticker = cachedStickers[id] { return sticker }

        let sticker = await Self.extractSubject(from: data)
        cachedStickers[id] = sticker
        return sticker
    }

    @concurrent
    static func extractSubject(from data: Data) async -> Sticker {
        // Expensive image processing -- runs on background thread pool
        // ...
    }
}

To move a function to a background thread, show both opt-outs together:

1. Ensure the containing type is nonisolated or the function can be called from a nonisolated context.
2. Add @concurrent to the offloaded function. nonisolated alone does not move CPU-heavy work off the caller's actor.
3. Add async if not already asynchronous.
4. Add await at call sites.

nonisolated struct PhotoProcessor {
    @concurrent
    func process(data: Data) async -> ProcessedPhoto? { /* ... */ }
}

// Caller:
processedPhotos[item.id] = await PhotoProcessor().process(data: data)

SE-0472: Task.immediate

Task.immediate starts executing synchronously on the current actor before any suspension point, rather than being enqueued.

Task.immediate { await handleUserInput() }

Use for latency-sensitive work that should begin without delay. There is also Task.immediateDetached which combines immediate start with detached semantics.

SE-0475: Transactional Observation (Observations)

Observations { } provides async observation of @Observable types via AsyncSequence, enabling transactional change tracking.

for await _ in Observations { model.count } {
    print("Count changed to \(model.count)")
}

Isolated Conformances

A conformance that needs MainActor state is called an isolated conformance. The compiler ensures it is only used in a matching isolation context.

protocol Exportable {
    func export()
}

// Isolated conformance: only usable on MainActor
extension StickerModel: @MainActor Exportable {
    func export() {
        photoProcessor.exportAsPNG()
    }
}

@MainActor
struct ImageExporter {
    var items: [any Exportable]

    mutating func add(_ item: StickerModel) {
        items.append(item)  // OK -- ImageExporter is on MainActor
    }
}

If ImageExporter were nonisolated, adding a StickerModel would fail: "Main actor-isolated conformance of 'StickerModel' to 'Exportable' cannot be used in nonisolated context."

Clock Epochs

ContinuousClock and SuspendingClock now expose .epoch (SE-0473), enabling instant comparison and conversion between clock types.

let continuous = ContinuousClock()
let elapsed = continuous.now - continuous.epoch  // Duration since system boot

Actor Isolation Rules

1. All mutable shared state MUST be protected by an actor or global actor.
2. @MainActor for all UI-touching code. No exceptions. Global actors are actor isolation; use @MainActor as the standard pattern for UI-bound shared state.
3. Use nonisolated only for methods that access immutable (let) properties or are pure computations.
4. Use @concurrent to explicitly move work off the caller's actor.
5. Never use nonisolated(unsafe) unless you have proven internal synchronization and exhausted all other options. It is an unsafe audit boundary, not a synchronization primitive.
6. Never add manual locks (NSLock, DispatchSemaphore) inside actors.

Sendable Rules

1. Value types (structs, enums) are automatically Sendable when all stored properties are Sendable. For diagnostics on mutable reference types, first extract an immutable Sendable value snapshot or DTO instead of sharing the reference.
2. Actors are implicitly Sendable.
3. @MainActor classes are implicitly Sendable. Do NOT add redundant Sendable conformance.
4. Non-actor classes: must be final with all stored properties let and Sendable.
5. @unchecked Sendable is a last resort. Document why the compiler cannot prove safety.
6. Use sending parameters (SE-0430) for finer-grained isolation control.
7. Use @preconcurrency import only for third-party libraries you cannot modify. Plan to remove it.

Structured Concurrency Patterns

Async Defer (Swift 6.4+)

defer blocks in async contexts can contain await in Swift 6.4+ (SE-0493). Use for async cleanup: closing connections, flushing buffers, or releasing resources that require an async call.

The defer body inherits the surrounding isolation and is implicitly awaited at scope exit. It does not suppress cancellation; cleanup that checks Task.isCancelled or Task.checkCancellation() still observes cancellation.

func fetchData() async throws -> Data {
    let connection = try await openConnection()
    defer { await connection.close() }
    return try await connection.read()
}

Task: Unstructured, inherits caller context.

Task { await doWork() }

Task.detached: No inherited context. Use only when you explicitly need to break isolation inheritance.

Task.immediate: Starts immediately on current actor. Use for latency-sensitive work.

Task.immediate { await handleUserInput() }

async let: Fixed number of concurrent operations.

async let a = fetchA()
async let b = fetchB()
let result = try await (a, b)

TaskGroup: Dynamic number of concurrent operations.

try await withThrowingTaskGroup(of: Item.self) { group in
    for id in ids {
        group.addTask { try await fetch(id) }
    }
    for try await item in group { process(item) }
}

Task Cancellation

• Cancellation is cooperative. Check Task.isCancelled or call try Task.checkCancellation() in loops.
• Use .task modifier in SwiftUI -- it handles cancellation on view disappear.
• Use withTaskCancellationHandler for cleanup.
• Swift 6.4 / iOS 27+ beta: use withTaskCancellationShield only for short cleanup or rollback that must complete after cancellation. Inside the shield, Task.isCancelled is false and Task.checkCancellation() does not throw; cancellation is observable again after the scope exits.
• Cancel stored tasks in deinit or onDisappear.

Actor Reentrancy

Actors are reentrant. State can change across suspension points.

// WRONG: State may change during await
actor Counter {
    var count = 0
    func increment() async {
        let current = count
        await someWork()
        count = current + 1  // BUG: count may have changed
    }
}

// CORRECT: Mutate synchronously, no reentrancy risk
actor Counter {
    var count = 0
    func increment() { count += 1 }
}

AsyncSequence and AsyncStream

Use AsyncStream to bridge callback/delegate APIs:

let stream = AsyncStream<Location> { continuation in
    let delegate = LocationDelegate { location in
        continuation.yield(location)
    }
    continuation.onTermination = { _ in delegate.stop() }
    delegate.start()
}

Use withCheckedContinuation / withCheckedThrowingContinuation for single-value callbacks. Resume exactly once.

@Observable and Concurrency

• @Observable classes should be @MainActor for view models.
• Use @State to own an @Observable instance (replaces @StateObject).
• Use Observations { } (SE-0475) for async observation of @Observable properties as an AsyncSequence.

Synchronization Primitives

When actors are not the right fit — synchronous access, performance-critical paths, or bridging C/ObjC — use low-level synchronization primitives:

• Actors remain the default for async shared state when callers can suspend; they give compiler-enforced isolation and structured-concurrency integration, but outside calls are async actor hops, require reentrancy care across await, and do not fit synchronous C callbacks. Use global actors such as @MainActor for UI-bound shared state; never use nonisolated(unsafe) as a synchronization substitute.
• Mutex<Value> (iOS 18+, Synchronization module): Preferred lock for new code. Stores protected state inside the lock. withLock { } pattern.
• OSAllocatedUnfairLock (iOS 16+, os module): Use when targeting older iOS versions. Supports ownership assertions for debugging.
• Atomic<Value> (iOS 18+, Synchronization module): Lock-free atomics for independent counters and flags. Atomic is Sendable and can be stored in Sendable holder types. Use .relaxed only for standalone metrics; use acquire/release ordering or a lock when coordinating other data.

Key rule: Never put locks inside actors (double synchronization), and never hold a lock across await (blocks a thread through suspension and can starve the cooperative pool or deadlock). See references/synchronization-primitives.md for full API details, code examples, and a decision guide for choosing locks vs actors. Mutex.withLock and OSAllocatedUnfairLock.withLock use synchronous closures; that API shape is what keeps critical sections non-suspending. Gate Mutex and Atomic with runtime if #available(iOS 18, *), never #if swift(...) or platform compile-time checks. For NSLock, correct only the false Sendable premise and avoid explaining conformance mechanics. If a legacy lock wrapper truly needs @unchecked Sendable, name the invariant: all mutable state is private, all access uses one lock, no mutable references escape, and no lock is held across await.

Common Mistakes

1. Blocking the main actor. Heavy computation on @MainActor freezes UI. Move to a @concurrent function.
2. Unnecessary @MainActor. Network layers, data processing, and model code do not need @MainActor. Only UI-touching code does.
3. Actors for stateless code. No mutable state means no actor needed. Use a plain struct or function.
4. Actors for immutable data. Use a Sendable struct, not an actor.
5. Task.detached without good reason. Loses priority, task-local values, and cancellation propagation.
6. Forgetting task cancellation. Store Task references and cancel them, or use the .task view modifier.
7. Retain cycles in Tasks. Use [weak self] when capturing self in long-lived stored tasks.
8. Semaphores in async context. DispatchSemaphore.wait() in async code will deadlock. Use structured concurrency instead.
9. Split isolation. Mixing @MainActor and nonisolated properties in one type. Isolate the entire type consistently.
10. MainActor.run instead of static isolation. Prefer @MainActor func over await MainActor.run { }.
11. Using GCD APIs. Never use DispatchQueue, DispatchGroup, DispatchSemaphore, or any GCD API. Use async/await, actors, and TaskGroups instead. GCD has no data-race safety guarantees.

Review Checklist

• All mutable shared state is actor-isolated
• No data races (no unprotected cross-isolation access)
• Tasks are cancelled when no longer needed
• No blocking calls on @MainActor
• No manual locks inside actors
• Sendable conformance is correct (no unjustified @unchecked)
• Actor reentrancy is handled (no state assumptions across awaits)
• @preconcurrency imports are documented with removal plan
• Heavy work uses @concurrent, not @MainActor
• .task modifier used in SwiftUI instead of manual Task management

References

• references/concurrency-patterns.md — detailed concurrency patterns and migration examples
• references/approachable-concurrency.md — approachable concurrency mode quick-reference
• references/swiftui-concurrency.md — SwiftUI-specific concurrency guidance
• references/synchronization-primitives.md — Mutex, OSAllocatedUnfairLock, locks vs actors
• references/bridging-interop.md — checked continuations, delegate bridging, GCD migration table
• references/diagnostics.md — compiler diagnostic → fix reference, strict concurrency adoption
• references/async-algorithms.md — swift-async-algorithms: debounce, throttle, merge, combineLatest, chunks