Swift concurrency

Swift Concurrency

Fast Path

Before proposing a fix:

1. Analyze 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.
2. Capture the exact diagnostic and offending symbol.
3. Determine the isolation boundary: @MainActor, custom actor, actor instance isolation, or nonisolated.
4. Confirm whether the code is UI-bound or intended to run off the main actor. 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 (// swift-tools-version: is not a reliable proxy)	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_*

If any of these are unknown, ask the developer to confirm them before giving migration-sensitive guidance. Do not guess.

Guardrails:

• Do not recommend @MainActor as a blanket fix. Justify why the code is truly UI-bound.
• Prefer structured concurrency over unstructured tasks. Use Task.detached only with a clear reason.
• If recommending @preconcurrency, @unchecked Sendable, or nonisolated(unsafe), require a documented safety invariant and a follow-up removal plan.
• Optimize for the smallest safe change. Do not refactor unrelated architecture during migration.
• Course references are for deeper learning only. Use them sparingly and only when they clearly help answer the developer's question.

Quick Fix Mode

Use Quick Fix Mode when all of these are true:

• The issue is localized to one file or one type.
• The isolation boundary is clear.
• The fix can be explained in 1-2 behavior-preserving steps.

Skip Quick Fix Mode when any of these are true:

• Build settings or default isolation are unknown.
• The issue crosses module boundaries or changes public API behavior.
• The likely fix depends on unsafe escape hatches.

Common Diagnostics

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
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

When Quick Fixes Fail

1. Gather project settings if not already confirmed.
2. Re-evaluate which isolation boundaries the type crosses.
3. Route to the matching reference file for a deeper fix.
4. If the fix may change behavior, document the invariant and add verification steps.

Smallest Safe Fixes

Prefer changes that preserve behavior while satisfying data-race safety:

• UI-bound state: isolate the type or member to @MainActor.
• Shared mutable state: move it behind an actor, or use @MainActor only if the state is UI-owned.
• Background work: when work must hop off caller isolation, use an async API marked @concurrent; when work can safely inherit caller isolation, use nonisolated without @concurrent. If a task mostly waits or retries before one UI-bound mutation, keep the delay off @MainActor and hop back only for the final update.
• Sendability issues: prefer immutable values and explicit boundaries over @unchecked Sendable.

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 Router

• swift-concurrency-pro is a separate skill that reviews already-written concurrency code for correctness and modern API usage. It is a Skill, not a subagent — invoke it with the Skill tool (skill: swift-concurrency-pro), never as a Task subagent_type. Reach for it when the developer wants a review/audit pass over existing code; use this skill for diagnosing, fixing, and migrating.

These references live in this skill's own references/ directory, alongside this SKILL.md — open the listed path directly (e.g. references/actors.md). This list is exhaustive; do not assume an unlisted filename exists in this skill. Open the smallest reference that matches the question:

• Foundations
  • references/async-await-basics.md — async/await syntax, execution order, async let, URLSession patterns
  • references/tasks.md — Task lifecycle, cancellation, priorities, task groups, structured vs unstructured
  • references/actors.md — Actor isolation, @MainActor, global actors, reentrancy, custom executors, Mutex
  • references/sendable.md — Sendable conformance, value/reference types, @unchecked, region isolation
  • references/threading.md — Execution model, suspension points, Swift 6.2 isolation behavior
• Streams
  • references/async-sequences.md — AsyncSequence, AsyncStream, when to use vs regular async methods
  • references/async-algorithms.md — Debounce, throttle, merge, combineLatest, channels, timers
• Applied topics
  • references/testing.md — Swift Testing first, XCTest fallback, leak checks
  • references/performance.md — Profiling with Instruments, reducing suspension points, execution strategies
  • references/memory-management.md — Retain cycles in tasks, memory safety patterns
  • references/core-data.md — NSManagedObject sendability, custom executors, isolation conflicts
• Migration and tooling
  • references/migration.md — Swift 6 migration strategy, closure-to-async conversion, @preconcurrency, FRP migration
  • references/linting.md — Concurrency-focused lint rules and SwiftLint async_without_await
• Glossary
  • references/glossary.md — Quick definitions of core concurrency terms

Verification Checklist

When changing concurrency code:

1. Re-check build settings before interpreting diagnostics.
2. Build and clear one category of errors before moving on. Do not batch unrelated fixes into the same change.
3. Run tests, especially actor-, lifetime-, and cancellation-sensitive tests.
4. Use Instruments for performance claims instead of guessing.
5. Verify deallocation and cancellation behavior for long-lived tasks.
6. Check Task.isCancelled in long-running operations.
7. Never use semaphores or ad hoc locking in async contexts when actor isolation or Mutex would express ownership more safely.

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