swift-concurrency-expert

name swift-concurrency-expert description Swift Concurrency review and remediation for Swift 6.2+. Use when asked to review Swift Concurrency usage, improve concurrency compliance, or fix Swift concurrency compiler errors in a feature or file. Concrete actions include adding Sendable conformance, applying @MainActor annotations, resolving actor isolation warnings, fixing data race diagnostics, and migrating completion handlers to async/await. 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.