swift-concurrency-expert

SKILL.md

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, and whether approachable concurrency (default actor isolation / main-actor-by-default) is enabled.

Identify the current actor context (@MainActor, actor, nonisolated) and whether a default actor isolation mode is enabled.

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:

UI-bound types: annotate the type or relevant members with @MainActor.

Protocol conformance on main actor types: make the conformance isolated (e.g., extension Foo: @MainActor SomeProtocol).

Global/static state: protect with @MainActor or move into an actor.

Background work: move expensive work into a @concurrent async function on a nonisolated type or use an actor to guard mutable state.

Sendable errors: prefer immutable/value types; add Sendable conformance only when correct; avoid @unchecked Sendable unless you can prove thread safety.

3. Verify the fix

Rebuild and confirm all concurrency diagnostics are resolved with no new warnings introduced.

Run the test suite to check for regressions — concurrency changes can introduce subtle runtime issues even when the build is clean.

If the fix surfaces new warnings, treat each one as a fresh triage (return to step 1) and resolve iteratively until the build is clean and tests pass.

Examples

UI-bound type — adding @MainActor

// Before: data-race warning because ViewModel is accessed from the main thread
// but has no actor isolation
class ViewModel: ObservableObject {
    @Published var title: String = ""
    func load() { title = "Loaded" }
}

// After: annotate the whole type so all stored state and methods are
// automatically isolated to the main actor
@MainActor
class ViewModel: ObservableObject {
    @Published var title: String = ""
    func load() { title = "Loaded" }
}

Protocol conformance isolation

// Before: compiler error — SomeProtocol method is nonisolated but the
// conforming type is @MainActor
@MainActor
class Foo: SomeProtocol {
    func protocolMethod() { /* accesses main-actor state */ }
}

// After: scope the conformance to @MainActor so the requirement is
// satisfied inside the correct isolation context
@MainActor
extension Foo: SomeProtocol {
    func protocolMethod() { /* safely accesses main-actor state */ }
}

Background work with @concurrent

// Before: expensive computation blocks the main actor
@MainActor
func processData(_ input: [Int]) -> [Int] {
    input.map { heavyTransform($0) }   // runs on main thread
}

// After: hop off the main actor for the heavy work, then return the result
// The caller awaits the result and stays on its own actor
nonisolated func processData(_ input: [Int]) async -> [Int] {
    await Task.detached(priority: .userInitiated) {
        input.map { heavyTransform($0) }
    }.value
}

// Or, using a @concurrent async function (Swift 6.2+):
@concurrent
func processData(_ input: [Int]) async -> [Int] {
    input.map { heavyTransform($0) }
}

Reference material

See references/swift-6-2-concurrency.md for Swift 6.2 changes, patterns, and examples.

See references/approachable-concurrency.md when the project is opted into approachable concurrency mode.

See references/swiftui-concurrency-tour-wwdc.md for SwiftUI-specific concurrency guidance.

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