concurrency-expert

Swift Concurrency Expert

Overview

Review and fix Swift Concurrency issues in Swift 6.2+ codebases by applying actor isolation, Sendable safety, and modern concurrency patterns with minimal behavior changes.

Workflow

1. Triage the issue

• Capture the exact compiler diagnostics and the offending symbol(s)
• Check project concurrency settings: Swift language version (6.2+), strict concurrency level
• Check if approachable concurrency (default actor isolation / main-actor-by-default) is enabled
• Identify the current actor context (@MainActor, actor, nonisolated)
• Confirm whether the code is UI-bound or intended to run off the main actor

2. Apply the smallest safe fix

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

Common fixes:

Issue	Fix
UI-bound types	Annotate the type or members with @MainActor
Protocol conformance on main actor types	Make conformance isolated: extension Foo: @MainActor SomeProtocol
Global/static state	Protect with @MainActor or move into an actor
Background work	Use @concurrent async function on a nonisolated type
Sendable errors	Prefer immutable/value types; add Sendable only when correct

Swift 6.2 Key Changes

Default Actor Isolation

Swift 6.2 stays single-threaded by default until you choose to introduce concurrency:

// In Swift 6.2 with approachable concurrency enabled,
// this no longer produces a data race 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
        }
        // Safe - runs on main actor by default
        return await photoProcessor.extractSticker(data: data, with: item.itemIdentifier)
    }
}

Isolated Conformances

Conformances that need main actor state are now supported:

protocol Exportable {
    func export()
}

// Isolated conformance - safe because compiler ensures
// it's only used on the main actor
extension StickerModel: @MainActor Exportable {
    func export() {
        photoProcessor.exportAsPNG()
    }
}

Protecting Global State

// Protect with @MainActor
@MainActor
final class StickerLibrary {
    static let shared: StickerLibrary = .init()
}

// Or enable main-actor-by-default mode for the whole project

Offloading Work to Background

Use @concurrent to explicitly run code on the concurrent thread pool:

nonisolated struct PhotoProcessor {
    @concurrent
    func process(data: Data) async -> ProcessedPhoto? {
        // Runs on background thread
    }
}

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

Migration Checklist

1. Check Swift version in build settings (needs 6.2+)
2. Enable approachable concurrency features in build settings
3. Run Swift migration tooling: swift.org/migration
4. Fix remaining compiler errors using patterns above
5. Test thoroughly - concurrency bugs may surface at runtime

Common Patterns

UI-Bound Class

@MainActor
final class ViewModel {
    var items: [Item] = []

    func load() async {
        items = try await service.fetchItems()
    }
}

Background Processing

nonisolated struct ImageProcessor {
    @concurrent
    static func resize(_ image: UIImage, to size: CGSize) async -> UIImage {
        // Heavy work runs off main actor
    }
}

Actor for Shared Mutable State

actor Cache {
    private var storage: [String: Data] = [:]

    func get(_ key: String) -> Data? {
        storage[key]
    }

    func set(_ key: String, value: Data) {
        storage[key] = value
    }
}

Build Settings

Enable in Xcode under Swift Compiler - Concurrency:

• SWIFT_STRICT_CONCURRENCY = complete
• Approachable concurrency features (Swift 6.2+)

Or in Package.swift:

swiftSettings: [
    .enableExperimentalFeature("StrictConcurrency")
]