/swift-concurrency

Concurrency Patterns

Approachable concurrency patterns introduced in Swift 6.2+ — a philosophy shift where code stays single-threaded by default until you choose to introduce concurrency.

Contents

• Core Problem Solved
• SE-0466: Default MainActor Isolation
• SE-0461: nonisolated(nonsending)
• @concurrent Attribute
• SE-0472: Task.immediate
• Swift 6.4 Cleanup APIs
• Isolated Conformances
• SE-0481: weak let
• SE-0475: Transactional Observation (Observations)
• Global and Static State
• Migration and Build Settings
• Summary

Core Problem Solved

In Swift 6.0/6.1, data-race safety was enforced at compile time, but the most natural code to write often produced data-race errors. Async functions on types with mutable state would implicitly hop to the global concurrent executor, causing send-safety violations even when no actual parallelism was intended.

// Swift 6.0/6.1: This produces a data-race error
class PhotoProcessor {
    func extractSticker(data: Data, with id: String?) async -> Sticker? { /* ... */ }
}

@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 }
        // Error: Sending 'self.photoProcessor' risks causing data races
        return await photoProcessor.extractSticker(data: data, with: item.itemIdentifier)
    }
}

// Swift 6.2: The same code compiles without error
// because extractSticker stays on the caller's actor
class PhotoProcessor {
    func extractSticker(data: Data, with id: String?) async -> Sticker? { /* ... */ }
}

@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 }
        return await photoProcessor.extractSticker(data: data, with: item.itemIdentifier)
    }
}

SE-0466: Default MainActor Isolation

Enable with the -default-isolation MainActor compiler flag, SwiftPM .defaultIsolation(MainActor.self), or Xcode's separate Default Actor Isolation build setting set to MainActor.

Do not confuse this with Xcode's Approachable Concurrency build setting, which enables a bundle of upcoming-feature flags such as nonisolated-nonsending by default, isolated-conformance inference, inferred Sendable captures, and related global-actor usability changes.

What it does:

• Unannotated declarations in the module are inferred as @MainActor unless opted out.
• Global and static variables are protected by the main actor by default.
• Protocol conformances are implicitly isolated to @MainActor.
• Eliminates most annotation burden for single-threaded UI code.

Recommended for: Apps, scripts, and executable targets. Not recommended for library targets that should remain actor-agnostic.

// With default MainActor isolation -- no @MainActor annotations needed:
final class StickerLibrary {
    static let shared = StickerLibrary()
}

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

extension StickerModel: Exportable {
    func export() { photoProcessor.exportAsPNG() }
}

SE-0461: nonisolated(nonsending)

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

Key implication: Values passed into an async function are never sent outside the actor, eliminating data races without annotation.

To explicitly opt into background execution, use @concurrent.

@concurrent Attribute

Ensures a function always runs on the concurrent thread pool, freeing the calling actor for other work.

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 { /* ... */ }
}

Steps to offload a function to background:

1. Ensure the containing type is nonisolated or the function can be called from a nonisolated context.
2. Add @concurrent to the 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? { /* ... */ }
}

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. There is also Task.immediateDetached which combines immediate start with detached semantics.

Task.immediate { await handleUserInput() }

Use for latency-sensitive work where enqueue delay is unacceptable.

Swift 6.4 Cleanup APIs

Swift 6.4 adds async defer (SE-0493) and cancellation shields (SE-0504). Gate both behind Swift 6.4 / Xcode 27 beta and the platform availability of withTaskCancellationShield (iOS 27+ beta).

Use async defer when cleanup itself must call async APIs. The defer body inherits surrounding isolation and is implicitly awaited at scope exit, but it does not hide cancellation from cleanup code.

Use withTaskCancellationShield only for short cleanup or rollback that must finish after cancellation. Do not wrap normal user-cancelable work in a shield.

Isolated Conformances

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

protocol Exportable {
    func export()
}

extension StickerModel: @MainActor Exportable {
    func export() { photoProcessor.exportAsPNG() }
}

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

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

// But in a nonisolated context:
nonisolated struct GenericExporter {
    var items: [any Exportable]

    mutating func add(_ item: StickerModel) {
        // Error: Main actor-isolated conformance of 'StickerModel' to
        // 'Exportable' cannot be used in nonisolated context
        items.append(item)
    }
}

SE-0481: weak let

Immutable weak references (weak let) enable Sendable conformance for types that hold weak references, since immutability guarantees thread safety. SE-0481 is implemented in Swift 6.3.

SE-0475: Transactional Observation (Observations)

Observations { } provides transactional observation of @Observable types via AsyncSequence.

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

Global and Static State

Global and static variables are prone to data races. The most common protection is @MainActor:

@MainActor
final class StickerLibrary {
    static let shared = StickerLibrary()  // protected by MainActor
}

With default MainActor isolation (SE-0466), this annotation is implicit.

Migration and Build Settings

All approachable concurrency features are opt-in via:

• Xcode 26: Swift Compiler > Concurrency section in build settings.
• SwiftPM: swiftSettings in Package.swift using the SwiftSetting API.

For Swift 6 language mode, strict concurrency checking is complete and data-race diagnostics are errors. Use Targeted or Minimal only as Swift 5 migration settings while preparing code for Swift 6.

Swift 6.2 includes migration tooling to help make necessary code changes automatically. See swift.org/migration for details.

Summary

The Swift 6.2 concurrency progression:

1. Start with code that runs on the main actor by default (no data race risk).
2. Async functions run wherever they are called from (still no data race risk).
3. When you need performance, offload specific code with @concurrent.