swift-concurrency

name

swift-concurrency

description

Diagnose data races, convert callback-based code to async/await, implement actor isolation patterns, resolve Sendable conformance issues, and guide Swift 6 migration. Use when developers mention: (1) Swift Concurrency, async/await, actors, or tasks, (2) "use Swift Concurrency" or "modern concurrency patterns", (3) migrating to Swift 6, (4) data races or thread safety issues, (5) refactoring closures to async/await, (6) @MainActor, Sendable, or actor isolation, (7) concurrent code architecture or performance optimization, (8) concurrency-related linter warnings (SwiftLint or similar; e.g. async_without_await, Sendable/actor isolation/MainActor lint).

Swift Concurrency

Agent Rules

1. Analyze Package.swift or .pbxproj to determine Swift language mode (5.x vs 6) and toolchain before giving advice.
2. Before proposing fixes, identify the isolation boundary: @MainActor, custom actor, actor instance isolation, or nonisolated.
3. Do not recommend @MainActor as a blanket fix. Justify why main-actor isolation is correct for the code.
4. Prefer structured concurrency (child tasks, task groups) over unstructured tasks. Use Task.detached only with a clear reason.
5. If recommending @preconcurrency, @unchecked Sendable, or nonisolated(unsafe), require:
   • a documented safety invariant
   • a follow-up ticket to remove or migrate it
6. For migration work, optimize for minimal blast radius (small, reviewable changes) and follow the validation loop: Build → Fix errors → Rebuild → Only proceed when clean.
7. Course references are for deeper learning only. Use them sparingly and only when they clearly help answer the developer's question.

Triage Checklist (Before Advising)

• Capture the exact compiler diagnostics and the offending symbol(s).
• Identify the current isolation boundary and module defaults (@MainActor, custom actor, default isolation).
• Confirm whether the code is UI-bound or intended to run off the main actor.

Quick Fix Mode (Use When)

Use Quick Fix Mode when:

• The errors are localized (single file or one type) and the isolation boundary is clear.
• The fix does not require API redesign or multi-module changes.
• You can explain the fix in 1–2 steps without changing behavior.

Skip Quick Fix Mode when:

• Default isolation or strict concurrency settings are unknown and likely affect behavior.
• The error crosses module boundaries or involves public API changes.
• The fix would require @unchecked Sendable, @preconcurrency, or nonisolated(unsafe) without a clear invariant.

Project Settings Intake (Evaluate Before Advising)

Concurrency behavior depends on build settings. Before advising, determine these via Read on Package.swift or Grep in .pbxproj files:

Setting	SwiftPM (Package.swift)	Xcode (.pbxproj)
Default isolation	.defaultIsolation(MainActor.self)	SWIFT_DEFAULT_ACTOR_ISOLATION
Strict concurrency	.enableExperimentalFeature("StrictConcurrency=targeted")	SWIFT_STRICT_CONCURRENCY
Upcoming features	.enableUpcomingFeature("NonisolatedNonsendingByDefault")	SWIFT_UPCOMING_FEATURE_*
Language mode	// swift-tools-version: at top	Swift Language Version build setting

If any of these are unknown, ask the developer to confirm them before giving migration-sensitive guidance.

Smallest Safe Fixes (Quick Wins)

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

• UI-bound types: isolate the type or specific members to @MainActor (justify why UI-bound).
• Global/static mutable state: move into an actor or isolate to @MainActor if UI-only.
• Background work: for work that should always hop off the caller’s isolation, move expensive work into an async function marked @concurrent; for work that doesn’t touch isolated state but can inherit the caller’s isolation (for example with NonisolatedNonsendingByDefault), use nonisolated without @concurrent, or use an actor to guard mutable state.
• Sendable errors: prefer immutable/value types; avoid @unchecked Sendable unless you can prove and document thread safety.

Quick Fix Playbook (Common Diagnostics -> Minimal Fix)

• "Main actor-isolated ... cannot be used from a nonisolated context"
  • Quick fix: if UI-bound, make the caller @MainActor or hop with await MainActor.run { ... }.
  • Escalate if this is non-UI code or causes reentrancy; use references/actors.md.
• "Actor-isolated type does not conform to protocol"
  • Quick fix: add isolated conformance (e.g., extension Foo: @MainActor SomeProtocol).
  • Escalate if the protocol requirements must be nonisolated; use references/actors.md.
• "Sending value of non-Sendable type ... risks causing data races"
  • Quick fix: confine access inside an actor or convert to a value type with immutable (let) state.
  • Escalate before @unchecked Sendable; use references/sendable.md and references/threading.md.
• SwiftLint async_without_await
  • Quick fix: remove async if not required; if required by protocol/override/@concurrent, use narrow suppression with rationale. See references/linting.md.
• "wait(...) is unavailable from asynchronous contexts" (XCTest)
  • Quick fix: use await fulfillment(of:) or Swift Testing equivalents. See references/testing.md.

Escalation Path (When Quick Fixes Aren't Enough)

1. Gather project settings (default isolation, strict concurrency level, upcoming features).
2. Re-evaluate isolation boundaries and which types cross them.
3. Use the decision tree + references for the deeper fix.
4. If behavior changes are possible, document the invariant and add tests/verification steps.

Quick Decision Tree

When a developer needs concurrency guidance, follow this decision tree:

1. Starting fresh with async code?

   • Read references/async-await-basics.md for foundational patterns
   • For parallel operations → references/tasks.md (async let, task groups)

2. Protecting shared mutable state?

   • Need to protect class-based state → references/actors.md (actors, @MainActor)
   • Need thread-safe value passing → references/sendable.md (Sendable conformance)

3. Managing async operations?

   • Structured async work → references/tasks.md (Task, child tasks, cancellation)
   • Streaming data → references/async-sequences.md (AsyncSequence, AsyncStream)

4. Working with legacy frameworks?

   • Core Data integration → references/core-data.md
   • General migration → references/migration.md

5. Performance or debugging issues?

   • Slow async code → references/performance.md (profiling, suspension points)
   • Testing concerns → references/testing.md (XCTest, Swift Testing)

6. Understanding threading behavior?

   • Read references/threading.md for thread/task relationship and isolation

7. Memory issues with tasks?

   • Read references/memory-management.md for retain cycle prevention

Triage-First Playbook (Common Errors -> Next Best Move)

• SwiftLint concurrency-related warnings
  • Use references/linting.md for rule intent and preferred fixes; avoid dummy awaits as “fixes”.
• SwiftLint async_without_await warning
  • Remove async if not required; if required by protocol/override/@concurrent, prefer narrow suppression over adding fake awaits. See references/linting.md.
• "Sending value of non-Sendable type ... risks causing data races"
  • First: identify where the value crosses an isolation boundary
  • Then: use references/sendable.md and references/threading.md (especially Swift 6.2 behavior changes)
• "Main actor-isolated ... cannot be used from a nonisolated context"
  • First: decide if it truly belongs on @MainActor
  • Then: use references/actors.md (global actors, nonisolated, isolated parameters) and references/threading.md (default isolation)
• "Class property 'current' is unavailable from asynchronous contexts" (Thread APIs)
  • Use references/threading.md to avoid thread-centric debugging and rely on isolation + Instruments
• "Actor-isolated type does not conform to protocol" (protocol conformance errors)
  • First: determine whether the protocol requirements must execute on the actor (for example, UI work on @MainActor) or can safely be nonisolated.
  • Then: follow the Quick Fix Playbook entry for actor-isolated protocol conformance and references/actors.md for implementation patterns (isolated conformances, nonisolated requirements, and escalation steps).
• XCTest async errors like "wait(...) is unavailable from asynchronous contexts"
  • Use references/testing.md (await fulfillment(of:) and Swift Testing patterns)
• Core Data concurrency warnings/errors
  • Use references/core-data.md (DAO/NSManagedObjectID, default isolation conflicts)

Core Patterns Reference

Concurrency Tool Selection

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

Common Scenarios

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)
    }
}

Swift 6 Migration Quick Guide

Key changes in Swift 6:

• Strict concurrency checking enabled by default
• Complete data-race safety at compile time
• Sendable requirements enforced on boundaries
• Isolation checking for all async boundaries

Migration Validation Loop

Apply this cycle for each migration change:

1. Build — Run swift build or Xcode build to surface new diagnostics
2. Fix — Address one category of error at a time (e.g., all Sendable issues first)
3. Rebuild — Confirm the fix compiles cleanly before moving on
4. Test — Run the test suite to catch regressions (swift test or Cmd+U)
5. Only proceed to the next file/module when all diagnostics are resolved

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.

Reference Files

Load these files as needed for specific topics:

• async-await-basics.md - async/await syntax, execution order, async let, URLSession patterns
• tasks.md - Task lifecycle, cancellation, priorities, task groups, structured vs unstructured
• threading.md - Thread/task relationship, suspension points, isolation domains, nonisolated
• memory-management.md - Retain cycles in tasks, memory safety patterns
• actors.md - Actor isolation, @MainActor, global actors, reentrancy, custom executors, Mutex
• sendable.md - Sendable conformance, value/reference types, @unchecked, region isolation
• linting.md - Concurrency-focused lint rules and SwiftLint async_without_await
• async-sequences.md - AsyncSequence, AsyncStream, when to use vs regular async methods
• core-data.md - NSManagedObject sendability, custom executors, isolation conflicts
• performance.md - Profiling with Instruments, reducing suspension points, execution strategies
• testing.md - XCTest async patterns, Swift Testing, concurrency testing utilities
• migration.md - Swift 6 migration strategy, closure-to-async conversion, @preconcurrency, FRP migration

Verification Checklist (When You Change Concurrency Code)

1. Confirm build settings (default isolation, strict concurrency, upcoming features) before interpreting diagnostics.
2. Build — Verify the project compiles without new warnings or errors.
3. Test — Run tests, especially concurrency-sensitive ones (see references/testing.md).
4. Performance — If performance-related, verify with Instruments (see references/performance.md).
5. Lifetime — If lifetime-related, verify deinit/cancellation behavior (see references/memory-management.md).
6. Check Task.isCancelled in long-running operations.
7. Never use semaphores or locks in async contexts — use actors or Mutex instead.

Glossary

See references/glossary.md for quick definitions of core concurrency terms used across this skill.

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Note: This skill is based on the comprehensive Swift Concurrency Course by Antoine van der Lee.