菜单
Diagnose Swift Concurrency issues, refactor callback-based code to async/await, and guide Swift 6 migration when working with tasks, actors, @MainActor, Sendable, data races, thread safety, or concurrency-related compiler and linter warnings.
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| name | swift-concurrency |
| description | Diagnose Swift Concurrency issues, refactor callback-based code to async/await, and guide Swift 6 migration when working with tasks, actors, @MainActor, Sendable, data races, thread safety, or concurrency-related compiler and linter warnings. |
Before proposing a fix:
Package.swift or .pbxproj to determine Swift language mode, strict concurrency level, default isolation, and upcoming features. Do this always, not only for migration work.@MainActor, custom actor, actor instance isolation, or nonisolated.await): start on @MainActor only when that prefix truly needs main-actor access; otherwise use Task { @concurrent in ... } and hop back with MainActor.run only after the suspension. A trivial non-main line (for example, print) followed by main-actor work in the same prefix is not a reason to use @concurrent. For delayed retries, timers, and backoff tasks, separate the waiting from the UI mutation. The sleep often belongs off the main actor even when the final state update belongs on it.Project settings that change concurrency behavior:
| Setting | SwiftPM (Package.swift) | Xcode (.pbxproj) |
|---|---|---|
| Language mode | swiftLanguageVersions or -swift-version ( is not a reliable proxy) |
// swift-tools-version:| Swift Language Version |
| Strict concurrency | .enableExperimentalFeature("StrictConcurrency=targeted") | SWIFT_STRICT_CONCURRENCY |
| Default isolation | .defaultIsolation(MainActor.self) | SWIFT_DEFAULT_ACTOR_ISOLATION |
| Upcoming features | .enableUpcomingFeature("NonisolatedNonsendingByDefault") | SWIFT_UPCOMING_FEATURE_* |
| Approachable Concurrency | N/A (use individual upcoming features) | SWIFT_APPROACHABLE_CONCURRENCY |
Xcode 26 note: New projects created in Xcode 26 will often start with
SWIFT_DEFAULT_ACTOR_ISOLATION = MainActorandSWIFT_APPROACHABLE_CONCURRENCY = YESenabled by default. Treat these as likely defaults for newly created projects, not as confirmed settings.
If any of these are unknown, ask the developer to confirm them before giving migration-sensitive guidance. Do not guess, even for new Xcode 26 projects.
Guardrails:
@MainActor as a blanket fix. Justify why the code is truly UI-bound.Task.detached only with a clear reason.@preconcurrency, @unchecked Sendable, or nonisolated(unsafe), require a documented safety invariant and a follow-up removal plan.Use Quick Fix Mode when all of these are true:
Skip Quick Fix Mode when any of these are true:
| Diagnostic | First check | Smallest safe fix | Escalate to |
|---|---|---|---|
Main actor-isolated ... cannot be used from a nonisolated context | Is this truly UI-bound? | Isolate the caller to @MainActor or use await MainActor.run { ... } only when main-actor ownership is correct. | references/actors.md, references/threading.md |
Actor-isolated type does not conform to protocol | Must the requirement run on the actor? | Prefer isolated conformance (e.g., extension Foo: @MainActor SomeProtocol); use nonisolated only for truly nonisolated requirements. | references/actors.md |
Sending value of non-Sendable type ... risks causing data races | What isolation boundary is being crossed? | Keep access inside one actor, or convert the transferred value to an immutable/value type. | references/sendable.md, references/threading.md |
SwiftLint async_without_await | Is async actually required by protocol, override, or @concurrent? | Remove async, or use a narrow suppression with rationale. Never add fake awaits. | references/linting.md |
wait(...) is unavailable from asynchronous contexts | Is this legacy XCTest async waiting? | Replace with await fulfillment(of:) or Swift Testing equivalents. | references/testing.md |
| Core Data concurrency warnings | Are NSManagedObject instances crossing contexts or actors? | Pass NSManagedObjectID or map to a Sendable value type. | references/core-data.md |
@Observable isolation or Sendable errors | Is the @Observable class annotated with the correct actor? | Add @MainActor for UI state; pass Sendable snapshots across boundaries. | references/observation.md |
Thread.current unavailable from asynchronous contexts | Are you debugging by thread instead of isolation? | Reason in terms of isolation and use Instruments/debugger instead. | references/threading.md |
| SwiftLint concurrency-related warnings | Which specific lint rule triggered? | Use references/linting.md for rule intent and preferred fixes; avoid dummy awaits. | references/linting.md |
... cannot satisfy conformance requirement for a 'Sendable' type parameter (SendableMetatype) | Does the conformance carry global-actor isolation? | Remove actor isolation from the conformance, or avoid passing the metatype across isolation boundaries. See SendableMetatype section in references/actors.md. | references/actors.md |
Prefer changes that preserve behavior while satisfying data-race safety:
@MainActor.actor, or use @MainActor only if the state is UI-owned.async API marked @concurrent; when work can safely inherit caller isolation, use nonisolated without @concurrent. When spawning a Task, match entry isolation to its synchronous prefix. If nothing before the first await needs the main actor, use Task { @concurrent in ... } and hop back via await MainActor.run { ... } for the UI update. If the prefix mixes a trivial non-main statement with main-actor work, keep the inherited @MainActor start—splitting the cheap line off-main is not worth an extra hop.@unchecked Sendable.| Need | Tool | Key Guidance |
|---|---|---|
| Single async operation | async/await | Default choice for sequential async work |
| Fixed parallel operations | async let | Known count at compile time; auto-cancelled on throw |
| Dynamic parallel operations | withTaskGroup | Unknown count; structured — cancels children on scope exit |
| Sync → async bridge | Task { } | Inherits actor context; use Task.detached only with documented reason |
| Shared mutable state | actor | Prefer over locks/queues; keep isolated sections small |
| UI-bound state | @MainActor | Only for truly UI-related code; justify isolation |
Network request with UI update
Task { @concurrent in
let data = try await fetchData()
await MainActor.run { self.updateUI(with: data) }
}
Processing array items in parallel
await withTaskGroup(of: ProcessedItem.self) { group in
for item in items {
group.addTask { await process(item) }
}
for await result in group {
results.append(result)
}
}
Match a Task's entry isolation to its synchronous prefix (everything from { to the first await).
@MainActor, keep the inherited @MainActor start.@MainActor, prefer Task { @concurrent in ... } and hop back only for UI-owned mutation.// ❌ Synchronous prefix is empty; first work hops away
Task {
await hopToOtherIsolationDomain()
}
// ❌ Synchronous prefix is only `print` (trivial, non-main); first await hops away
Task {
print("Also not main-thread-bound")
await hopToOtherIsolationDomain()
}
// ✅ Start off the main actor, hop back only for UI work
Task { @concurrent in
await hopToOtherIsolationDomain()
await MainActor.run { updateUI() }
}
// ✅ Synchronous prefix DOES contain main-actor work — keep inheritance
Task {
print("debug") // trivial, non-main — rides along
self.isLoading = true // needs @MainActor, before any await
await fetchData()
}
Key changes in Swift 6:
Apply this cycle for each migration change:
swift build or Xcode build to surface new diagnosticsswift test or Cmd+U)If a fix introduces new warnings, resolve them before continuing. Never batch multiple unrelated fixes — keep commits small and reviewable.
For detailed migration steps, see references/migration.md.
Open the smallest reference that matches the question:
references/async-await-basics.md — async/await syntax, execution order, async let, URLSession patternsreferences/tasks.md — Task lifecycle, cancellation, priorities, task groups, structured vs unstructuredreferences/actors.md — Actor isolation, @MainActor, global actors, reentrancy, custom executors, Mutexreferences/sendable.md — Sendable conformance, value/reference types, @unchecked, region isolationreferences/threading.md — Execution model, suspension points, Swift 6.2 isolation behaviorreferences/async-sequences.md — AsyncSequence, AsyncStream, when to use vs regular async methodsreferences/async-algorithms.md — Debounce, throttle, merge, combineLatest, channels, timersreferences/testing.md — Swift Testing first, XCTest fallback, leak checksreferences/performance.md — Profiling with Instruments, reducing suspension points, execution strategiesreferences/memory-management.md — Retain cycles in tasks, memory safety patternsreferences/core-data.md — NSManagedObject sendability, custom executors, isolation conflictsreferences/observation.md — @Observable with @MainActor, cross-isolation access, Sendable constraintsreferences/migration.md — Swift 6 migration strategy, closure-to-async conversion, @preconcurrency, FRP migrationreferences/linting.md — Concurrency-focused lint rules and SwiftLint async_without_awaitreferences/glossary.md — Quick definitions of core concurrency termsWhen changing concurrency code:
Task.isCancelled in long-running operations.Mutex would express ownership more safely.Note: This skill is based on the comprehensive Swift Concurrency Course by Antoine van der Lee.
基于 SOC 职业分类
