Swift 6.2 Approachable Concurrency

Patterns for adopting Swift 6.2's concurrency model where code runs single-threaded by default and concurrency is introduced explicitly. Eliminates common data-race errors without sacrificing performance.

When to Activate

• Migrating Swift 5.x or 6.0/6.1 projects to Swift 6.2

• Resolving data-race safety compiler errors

• Designing MainActor-based app architecture

• Offloading CPU-intensive work to background threads

• Implementing protocol conformances on MainActor-isolated types

• Enabling Approachable Concurrency build settings in Xcode 26

Core Problem: Implicit Background Offloading

In Swift 6.1 and earlier, async functions could be implicitly offloaded to background threads, causing data-race errors even in seemingly safe code:

// Swift 6.1: ERROR

@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 fixes this: async functions stay on the calling actor by default.

// Swift 6.2: OK — async stays on MainActor, no data race

@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)

    }

}

Core Pattern — Isolated Conformances

MainActor types can now conform to non-isolated protocols safely:

protocol Exportable {

    func export()

}

// Swift 6.1: ERROR — crosses into main actor-isolated code

// Swift 6.2: OK with isolated conformance

extension StickerModel: @MainActor Exportable {

    func export() {

        photoProcessor.exportAsPNG()

    }

}

The compiler ensures the conformance is only used on the main actor:

// OK — ImageExporter is also @MainActor

@MainActor

struct ImageExporter {

    var items: [any Exportable]

    mutating func add(_ item: StickerModel) {

        items.append(item)  // Safe: same actor isolation

    }

}

// ERROR — nonisolated context can't use MainActor conformance

nonisolated struct ImageExporter {

    var items: [any Exportable]

    mutating func add(_ item: StickerModel) {

        items.append(item)  // Error: Main actor-isolated conformance cannot be used here

    }

}

Core Pattern — Global and Static Variables

Protect global/static state with MainActor:

// Swift 6.1: ERROR — non-Sendable type may have shared mutable state

final class StickerLibrary {

    static let shared: StickerLibrary = .init()  // Error

}

// Fix: Annotate with @MainActor

@MainActor

final class StickerLibrary {

    static let shared: StickerLibrary = .init()  // OK

}

MainActor Default Inference Mode

Swift 6.2 introduces a mode where MainActor is inferred by default — no manual annotations needed:

// With MainActor default inference enabled:

final class StickerLibrary {

    static let shared: StickerLibrary = .init()  // Implicitly @MainActor

}

final class StickerModel {

    let photoProcessor: PhotoProcessor

    var selection: [PhotosPickerItem]  // Implicitly @MainActor

}

extension StickerModel: Exportable {  // Implicitly @MainActor conformance

    func export() {

        photoProcessor.exportAsPNG()

    }

}

This mode is opt-in and recommended for apps, scripts, and other executable targets.

Core Pattern — @concurrent for Background Work

When you need actual parallelism, explicitly offload with @concurrent:

Important: This example requires Approachable Concurrency build settings — SE-0466 (MainActor default isolation) and SE-0461 (NonisolatedNonsendingByDefault). With these enabled, extractSticker stays on the caller's actor, making mutable state access safe. Without these settings, this code has a data race — the compiler will flag it.

nonisolated final class PhotoProcessor {

    private 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

    }

    // Offload expensive work to concurrent thread pool

    @concurrent

    static func extractSubject(from data: Data) async -> Sticker { /* ... */ }

}

// Callers must await

let processor = PhotoProcessor()

processedPhotos[item.id] = await processor.extractSticker(data: data, with: item.id)

To use @concurrent:

• Mark the containing type as nonisolated

• Add @concurrent to the function

• Add async if not already asynchronous

• Add await at call sites

Key Design Decisions

Decision

Rationale

Single-threaded by default

Most natural code is data-race free; concurrency is opt-in

Async stays on calling actor

Eliminates implicit offloading that caused data-race errors

Isolated conformances

MainActor types can conform to protocols without unsafe workarounds

@concurrent explicit opt-in

Background execution is a deliberate performance choice, not accidental

MainActor default inference

Reduces boilerplate @MainActor annotations for app targets

Opt-in adoption

Non-breaking migration path — enable features incrementally

Migration Steps

• Enable in Xcode: Swift Compiler > Concurrency section in Build Settings

• Enable in SPM: Use SwiftSettings API in package manifest

• Use migration tooling: Automatic code changes via swift.org/migration

• Start with MainActor defaults: Enable inference mode for app targets

• **Add @concurrent where needed**: Profile first, then offload hot paths

• Test thoroughly: Data-race issues become compile-time errors

Best Practices

• Start on MainActor — write single-threaded code first, optimize later

• **Use @concurrent only for CPU-intensive work** — image processing, compression, complex computation

• Enable MainActor inference mode for app targets that are mostly single-threaded

• Profile before offloading — use Instruments to find actual bottlenecks

• Protect globals with MainActor — global/static mutable state needs actor isolation

• Use isolated conformances instead of nonisolated workarounds or @Sendable wrappers

• Migrate incrementally — enable features one at a time in build settings

Anti-Patterns to Avoid

• Applying @concurrent to every async function (most don't need background execution)

• Using nonisolated to suppress compiler errors without understanding isolation

• Keeping legacy DispatchQueue patterns when actors provide the same safety

• Skipping model.availability checks in concurrency-related Foundation Models code

• Fighting the compiler — if it reports a data race, the code has a real concurrency issue

• Assuming all async code runs in the background (Swift 6.2 default: stays on calling actor)

When to Use

• All new Swift 6.2+ projects (Approachable Concurrency is the recommended default)

• Migrating existing apps from Swift 5.x or 6.0/6.1 concurrency

• Resolving data-race safety compiler errors during Xcode 26 adoption

• Building MainActor-centric app architectures (most UI apps)

• Performance optimization — offloading specific heavy computations to background