Swift Concurrency

Swift Concurrency

This skill covers two jobs: fixing concurrency problems in code you're changing, and reviewing concurrency code you didn't write for correctness. Use Quick Fix Mode / When Quick Fixes Fail for the former, Review Mode for the latter. Both share the same reference files below.

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. When spawning unstructured tasks, inspect the synchronous prefix (everything before the first 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:

Xcode 26 note: New projects created in Xcode 26 will often start with SWIFT_DEFAULT_ACTOR_ISOLATION = MainActor and SWIFT_APPROACHABLE_CONCURRENCY = YES enabled 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. If code spans multiple targets or packages, compare their concurrency build settings before assuming behavior should match across them — a fix that's correct in one module can be wrong in a neighbor with a different default isolation.

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.
• Prefer async/await over a closure-based variant whenever an API offers both.
• GCD (DispatchQueue, locks, serial queues) is still a legitimate tool in low-level libraries, framework interop, and performance-critical synchronous sections — don't flag it just because it's not async/await. See references/migration.md for when to actually convert it.
• If recommending @preconcurrency, @unchecked Sendable, or nonisolated(unsafe), require a documented safety invariant and a follow-up removal plan. @unchecked Sendable's only broadly legitimate use is a type with its own internal locking that's provably thread-safe — check whether Swift 6 region-based isolation already makes it unnecessary before reaching for it.
• Do not introduce third-party concurrency frameworks without asking first.
• 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.

For open-ended "review this code" requests rather than a specific fix, use Review Mode below instead.

Common Diagnostics

For compiler-error messages not in this table, or for an ordered "try these fixes in order" walkthrough, see references/diagnostics.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. 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.
• Sendability issues: prefer immutable values and explicit boundaries over @unchecked Sendable.

Concurrency Tool Selection

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)
    }
}

Task entry isolation

Match a Task's entry isolation to its synchronous prefix (everything from { to the first await).

• If anything in that prefix needs @MainActor, keep the inherited @MainActor start.
• If nothing in that prefix needs @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()
}

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.

Review Mode

Use Review Mode when asked to review, audit, or critique existing concurrency code rather than fix one named diagnostic. Report only genuine problems — do not nitpick or invent issues. If doing a partial review, load only the reference files relevant to what changed.

Review order (skip steps that don't apply):

1. Scan for known-dangerous patterns to prioritize what to inspect — references/hotspots.md.
2. Check whether recent Swift 6.2 behavior changes the right answer (default isolation, @concurrent, isolated conformances) — references/actors.md, references/threading.md.
3. Validate actor usage for reentrancy, isolation, and global/static state — references/actors.md.
4. Confirm structured concurrency (task groups) is preferred over unstructured (Task {}, Task.detached) where appropriate — references/tasks.md.
5. Verify cancellation is propagated and checked correctly — references/tasks.md.
6. Validate async stream, sequence, and continuation usage — references/async-sequences.md.
7. Check bridging code between sync and async worlds (continuations, @unchecked Sendable) — references/sendable.md, references/migration.md.
8. Review any legacy-concurrency migrations (GCD, delegates, Combine) — references/migration.md.
9. Cross-check against common failure modes — references/bug-patterns.md.
10. If the project has strict-concurrency errors, map diagnostics to fixes — references/diagnostics.md.
11. If reviewing tests, check async test patterns — references/testing.md.

Review-specific guardrails, in addition to the Fast Path guardrails above:

• Target Swift 6.2+ with strict concurrency checking unless the project's settings say otherwise (see Fast Path step 1).
• If an API offers both async/await and closure-based variants, flag call sites still using the closure form.
• Don't flag GCD, locks, or Task.detached on sight — check the context first (see Fast Path guardrails).

Review Output Format

Organize findings by file. For each issue: state the file and line(s), name the rule being violated, and show a brief before/after fix. Skip files with no issues. End with a prioritized summary — highest-impact fix first.

### DataLoader.swift

**Line 18: Actor reentrancy — state may have changed across the `await`.**

// Before
actor Cache {
    var items: [String: Data] = [:]
    func fetch(_ key: String) async throws -> Data {
        if items[key] == nil { items[key] = try await download(key) }
        return items[key]!
    }
}

// After
actor Cache {
    var items: [String: Data] = [:]
    func fetch(_ key: String) async throws -> Data {
        if let existing = items[key] { return existing }
        let data = try await download(key)
        items[key] = data
        return data
    }
}

### Summary
1. **Correctness (high):** Actor reentrancy bug on line 18 may cause duplicate downloads and a force-unwrap crash.

Reference Router

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, withTaskCancellationHandler, priorities, task groups, structured vs unstructured, Task.immediate, task naming
  • references/actors.md — Actor isolation, @MainActor, global actors, reentrancy (incl. in-flight-task dedup), 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 (incl. makeStream(of:)), 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, race detection with Thread Sanitizer
  • 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/delegate-to-async conversion, continuation resume rules, @preconcurrency, GCD and FRP migration
  • references/linting.md — Concurrency-focused lint rules and SwiftLint async_without_await
• Review
  • references/hotspots.md — Grep targets for code review: known-dangerous patterns and what to check for each
  • references/bug-patterns.md — Common concurrency failure modes and their fixes, for fast review scanning
  • references/diagnostics.md — Strict-concurrency compiler errors mapped to ordered, "try this first" fixes
• 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.

Note: This skill is based on the comprehensive Swift Concurrency Course by Antoine van der Lee, and incorporates review methodology from the Swift Concurrency Agent Skill by Paul Hudson.