Swift Concurrency Developer

Swift Concurrency Developer (Smart Router)

Purpose

Expert guidance on Swift's concurrency system using the "Office Building" mental model from Fucking Approachable Swift Concurrency, combined with comprehensive reference material from Swift Concurrency Course.

When Auto-Activated

• Working with actors, isolation, Sendable, TaskGroups
• Keywords: actor, isolation, Sendable, TaskGroup, nonisolated, async let
• Fixing concurrency warnings or data race issues

Agent Behavior Contract (Follow These Rules)

1. Analyze the project/package file to find out which Swift language mode (Swift 5.x vs Swift 6) and which Xcode/Swift toolchain is used when advice depends on it.
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 add verification steps.
7. Course references are for deeper learning only. Use them sparingly and only when they clearly help answer the developer's question.

Project Settings Discovery

Always confirm these before interpreting diagnostics or giving migration-sensitive guidance. Do not guess — if any are unknown, ask the developer.

Tools: Read on Package.swift, Grep on .pbxproj.

Core Mental Model: The Office Building

Think of your app as an office building where isolation domains are private offices with locks:

Key insight: You can't barge into someone's office. You knock (await) and wait.

Quick Decision Tree

When a developer needs concurrency guidance:

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".
• "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
• "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)
• 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)

Quick Patterns

Async/Await

func fetchUser(id: Int) async throws -> User {
    let (data, _) = try await URLSession.shared.data(from: url)
    return try JSONDecoder().decode(User.self, from: data)
}

Parallel Work with async let

async let avatar = fetchImage("avatar.jpg")
async let banner = fetchImage("banner.jpg")
return Profile(avatar: try await avatar, banner: try await banner)

Tasks

// SwiftUI - cancels when view disappears
.task { avatar = await downloadAvatar() }

// Manual task (inherits actor context)
Task { await saveProfile() }

Task Entry Isolation (Swift 6.2)

Match a Task's entry isolation to its synchronous prefix — everything from { to the first await. Whatever runs in that prefix executes on the inherited actor.

• If the prefix needs @MainActor (touches UI state, mutates self.isLoading, etc.), keep the inherited start.
• If the prefix has nothing main-actor-bound, prefer Task { @concurrent in ... } and hop back via MainActor.run { ... } only for the UI mutation.
• A trivial non-main statement (e.g. print) followed by main-actor work is not a reason to switch to @concurrent — the cheap line rides along.
• For delayed retries, timers, and backoff: separate the waiting from the UI mutation. The sleep usually belongs off-main even when the final state update belongs on-main.

// ❌ Called from @MainActor; fetchData() is nonisolated, so the task starts on main then hops away
// (whether the hop happens depends on fetchData()'s declared isolation — nonisolated/@concurrent hop, @MainActor does not)
Task {
    await fetchData()  // nonisolated async
}

// ✅ Start off the main actor, hop back only for UI work
Task { @concurrent in
    let data = try await fetchData()
    await MainActor.run { self.items = data }
}

// ✅ Prefix DOES need main actor — keep inheritance
Task {
    self.isLoading = true       // needs @MainActor, before any await
    await fetchData()
    self.isLoading = false
}

TaskGroup for Dynamic Parallel Work

try await withThrowingTaskGroup(of: Void.self) { group in
    group.addTask { avatar = try await downloadAvatar() }
    group.addTask { bio = try await fetchBio() }
    try await group.waitForAll()
}

Actors

actor BankAccount {
    var balance: Double = 0
    func deposit(_ amount: Double) { balance += amount }

    // No await needed - can access directly inside actor
    nonisolated func bankName() -> String { "Acme Bank" }
}

await account.deposit(100)  // Must await from outside
let name = account.bankName()  // No await needed

Sendable Types

// Automatically Sendable - value type
struct User: Sendable {
    let id: Int
    let name: String
}

// Thread-safe class with internal synchronization
final class ThreadSafeCache: @unchecked Sendable {
    private let lock = NSLock()
    private var storage: [String: Data] = [:]
}

Common Mistakes

1. Thinking async = background

// WRONG: Still blocks main thread!
@MainActor func slowFunction() async {
    let result = expensiveCalculation()  // Synchronous = blocking
}

// CORRECT: Use detached task for CPU-heavy work
Task.detached(priority: .userInitiated) {
    let result = expensiveCalculation()
    await MainActor.run { updateUI(result) }
}

Production impact: Apps get rejected for "became unresponsive." See references/production-pitfalls.md section 2.

2. Creating too many actors

Most things can live on MainActor. Only create actors when you have shared mutable state that can't be on MainActor.

3. Using MainActor.run unnecessarily

// WRONG
await MainActor.run { self.data = data }

// CORRECT - annotate the function
@MainActor func loadData() async { self.data = await fetchData() }

4. Blocking the cooperative thread pool (violates runtime contract)

Never use DispatchSemaphore, DispatchGroup.wait(), or condition variables in async code.

Why: These primitives hide dependencies from the runtime. The cooperative thread pool has a contract that threads will always make forward progress. Blocking primitives violate this contract and can cause deadlock.

// ❌ DANGEROUS: Can deadlock the cooperative pool
let semaphore = DispatchSemaphore(value: 0)
Task {
    await doWork()
    semaphore.signal()
}
semaphore.wait()  // Thread blocked, runtime unaware

// ✅ Use async/await instead
let result = await doWork()

Debug tip: Set LIBDISPATCH_COOPERATIVE_POOL_STRICT=1 to catch blocking calls during development.

5. Creating unnecessary Tasks

// WRONG - unstructured
Task { await fetchUsers() }
Task { await fetchPosts() }

// CORRECT - structured concurrency
async let users = fetchUsers()
async let posts = fetchPosts()
await (users, posts)

6. Making everything Sendable

Not everything needs to cross boundaries. Ask if data actually moves between isolation domains.

7. Not batching MainActor hops

The main thread is separate from the cooperative thread pool. Each hop to/from MainActor requires a full context switch.

// ❌ Multiple context switches
for item in items {
    let processed = await processItem(item)
    await MainActor.run { displayItem(processed) }  // Context switch per item
}

// ✅ Single context switch
let processed = await processAllItems(items)
await MainActor.run {
    for item in processed { displayItem(item) }
}

8. Async for loops silently losing data

Using try? or empty catch {} in async loops swallows failures. Users lose data with zero indication. Acceptable for fire-and-forget (cache warming, analytics), dangerous for uploads/sync/migration. See references/production-pitfalls.md section 1.

9. Ignoring Task cancellation in long-running loops

for await under .task modifier is safe (structured concurrency propagates cancellation). But for await or while loops in stored Task { } properties need explicit Task.isCancelled checks. See references/production-pitfalls.md section 3.

10. Manual migration pitfalls (@preconcurrency + DispatchQueue mixing)

@preconcurrency and nonisolated(unsafe) hide real data races. Mixing DispatchQueue with async/await creates confusing execution contexts. Always document safety invariants and plan removal. See references/production-pitfalls.md section 4.

11. Task inside onAppear instead of .task modifier

Task { } in .onAppear is unstructured: not cancelled on disappear, fires on every re-appear. Use .task { } for async work tied to view lifecycle, .onAppear for sync-only setup. See references/production-pitfalls.md section 5.

Quick Reference

When the Compiler Complains

Trace the isolation: Where did it come from? Where is code trying to run? What data crosses a boundary?

The answer is usually obvious once you ask the right question.

Reference Files

Load these files as needed for specific topics:

Foundational Concepts

• 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
• glossary.md - Quick definitions of core concurrency terms

Isolation & Safety

• actors.md - Actor isolation, @MainActor, global actors, reentrancy, custom executors, Mutex
• sendable.md - Sendable conformance, value/reference types, @unchecked, region isolation
• memory-management.md - Retain cycles in tasks, memory safety patterns

Advanced Patterns

• async-sequences.md - AsyncSequence, AsyncStream, when to use vs regular async methods
• async-algorithms.md - Swift Async Algorithms package, combining sequences
• task-local-values.md - Task-local context propagation, tracing, logging patterns
• core-data.md - NSManagedObject sendability, custom executors, isolation conflicts

Quality & Migration

• 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
• linting.md - Concurrency-focused lint rules and SwiftLint async_without_await
• production-pitfalls.md - Silent data loss, cancellation gaps, migration bridges, .task vs onAppear

Project-Specific References (Anytype)

• approachable-concurrency.md - Approachable concurrency quick guide
• swift-6-2-concurrency.md - Swift 6.2 concurrency updates (future reference)
• swiftui-concurrency-tour.md - SwiftUI-specific concurrency patterns

Best Practices Summary

1. Prefer structured concurrency - Use task groups over unstructured tasks when possible
2. Minimize suspension points - Keep actor-isolated sections small to reduce context switches
3. Use @MainActor judiciously - Only for truly UI-related code
4. Make types Sendable - Enable safe concurrent access by conforming to Sendable
5. Handle cancellation - Check Task.isCancelled in long-running operations
6. Avoid blocking - Never use semaphores or locks in async contexts (violates runtime contract)
7. Test concurrent code - Use proper async test methods and consider timing issues
8. Batch MainActor hops - Group UI updates to minimize context switches to/from main thread
9. Understand the runtime contract - Threads must always make forward progress; use safe primitives
10. Use LIBDISPATCH_COOPERATIVE_POOL_STRICT=1 - Debug environment variable to catch blocking calls

Verification Checklist (When You Change Concurrency Code)

• Confirm build settings (default isolation, strict concurrency, upcoming features) before interpreting diagnostics.
• After refactors:
  • Run tests, especially concurrency-sensitive ones (see references/testing.md).
  • If performance-related, verify with Instruments (see references/performance.md).
  • If lifetime-related, verify deinit/cancellation behavior (see references/memory-management.md).

Migration Validation Loop

For Swift 6 / strict-concurrency migration, apply this cycle for each change:

1. Build — surface new diagnostics
2. Fix — address one category at a time (e.g., all Sendable issues first, then all isolation issues)
3. Rebuild — confirm the fix compiles cleanly before moving on
4. Test — run the suite to catch regressions
5. Only then proceed to the next file/module

Never batch unrelated fixes into one change. If a fix introduces new warnings, resolve them before continuing. Keep commits small and reviewable. See references/migration.md for detailed migration steps.

Further Reading

• Fucking Approachable Swift Concurrency - Office Building mental model
• Swift Concurrency Course - Comprehensive reference (Antoine van der Lee)
• WWDC21: Swift Concurrency: Behind the Scenes - Runtime internals, threading model, runtime contract
• WWDC23: Beyond the basics of structured concurrency - Task groups, cancellation handlers, task locals
• Matt Massicotte's Blog - Deep dives on concurrency
• Swift Concurrency Documentation

Related Skills & Docs

• ios-dev-guidelines → IOS_DEVELOPMENT_GUIDE.md - General Swift/iOS patterns, MVVM, Coordinators
• tests-developer → Testing async code with Swift Testing framework
• swiftui-performance-developer → Performance optimization in SwiftUI views

Navigation: This skill provides concurrency mental models. For general Swift/iOS patterns, see ios-dev-guidelines.

Attribution: Office Building mental model from Dimillian/Skills. Reference files from AvdLee/Swift-Concurrency-Agent-Skill.