/swift-concurrency-6-2

name: swift-concurrency-6-2
description: 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 |