ECC/skills/swift-concurrency-6-2/SKILL.md

name, description

name	description
swift-concurrency-6-2	Swift 6.2 Approachable Concurrency — single-threaded by default, @concurrent for explicit background offloading, isolated conformances for main actor types. Use when adopting Swift 6.2 concurrency — offloading with @concurrent or resolving main-actor isolation.

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:

1. Mark the containing type as nonisolated
2. Add @concurrent to the function
3. Add async if not already asynchronous
4. 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

1. Enable in Xcode: Swift Compiler > Concurrency section in Build Settings
2. Enable in SPM: Use SwiftSettings API in package manifest
3. Use migration tooling: Automatic code changes via swift.org/migration
4. Start with MainActor defaults: Enable inference mode for app targets
5. Add @concurrent where needed: Profile first, then offload hot paths
6. 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