swift-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.

When to Use

• When the user asks to review Swift concurrency usage or fix compiler diagnostics.
• When you need guidance on actor isolation, Sendable, @MainActor, or async migration.

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.

Limitations

• Use this skill only when the task clearly matches the scope described above.
• Do not treat the output as a substitute for environment-specific validation, testing, or expert review.
• Stop and ask for clarification if required inputs, permissions, safety boundaries, or success criteria are missing.