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Master Swift concurrency - async/await, actors, structured concurrency, Sendable, TaskGroups
Master iOS/macOS app architecture - MVVM, Clean Architecture, Coordinator, DI, Repository
Master Combine framework for reactive programming - publishers, subscribers, operators, schedulers
Master UIKit for iOS app development - views, view controllers, Auto Layout, table/collection views
Persist data with Core Data - models, contexts, fetch requests, migrations, SwiftData
Master Swift programming fundamentals - syntax, types, optionals, protocols, error handling
Build iOS applications - project setup, app lifecycle, Info.plist, capabilities
| name | swift-concurrency |
| description | Master Swift concurrency - async/await, actors, structured concurrency, Sendable, TaskGroups |
| version | 2.0.0 |
| sasmp_version | 1.3.0 |
| bonded_agent | 07-swift-performance |
| bond_type | SECONDARY_BOND |
Modern Swift concurrency patterns using async/await, actors, and structured concurrency.
parameters:
strict_concurrency:
type: string
enum: [minimal, targeted, complete]
default: complete
description: Concurrency checking level
actor_isolation:
type: boolean
default: true
use_main_actor:
type: boolean
default: true
description: MainActor for UI code
| Concept | Purpose |
|---|---|
| async/await | Sequential async code |
| Actor | Data isolation |
| Task | Unit of async work |
| Sendable | Thread-safe types |
| MainActor | Main thread isolation |
| Type | Lifetime | Cancellation |
|---|---|---|
| Task {} | Independent | Manual |
| Task.detached {} | No context inherited | Manual |
| async let | Structured | Automatic |
| TaskGroup | Structured, multiple | Automatic |
| Annotation | Meaning |
|---|---|
actor | Type is an actor |
@MainActor | Runs on main thread |
nonisolated | Opt out of isolation |
isolated | Parameter isolation |
// Sequential async operations
func fetchUserProfile(userId: String) async throws -> UserProfile {
let user = try await api.fetchUser(userId)
let posts = try await api.fetchPosts(userId: userId)
let followers = try await api.fetchFollowers(userId: userId)
return UserProfile(user: user, posts: posts, followers: followers)
}
// Concurrent with async let
func fetchUserProfileConcurrently(userId: String) async throws -> UserProfile {
async let user = api.fetchUser(userId)
async let posts = api.fetchPosts(userId: userId)
async let followers = api.fetchFollowers(userId: userId)
// All three run concurrently, await collects results
return try await UserProfile(user: user, posts: posts, followers: followers)
}
actor ImageCache {
private var cache: [URL: UIImage] = [:]
private var inProgress: [URL: Task<UIImage, Error>] = [:]
func image(for url: URL) async throws -> UIImage {
// Return cached
if let cached = cache[url] {
return cached
}
// Return in-progress task (avoid duplicate downloads)
if let existing = inProgress[url] {
return try await existing.value
}
// Start new download
let task = Task {
let (data, _) = try await URLSession.shared.data(from: url)
guard let image = UIImage(data: data) else {
throw ImageError.invalidData
}
return image
}
inProgress[url] = task
do {
let image = try await task.value
cache[url] = image
inProgress[url] = nil
return image
} catch {
inProgress[url] = nil
throw error
}
}
func clearCache() {
cache.removeAll()
}
// Nonisolated for synchronous read
nonisolated var cacheDescription: String {
"ImageCache instance"
}
}
func fetchAllProducts(ids: [String]) async throws -> [Product] {
try await withThrowingTaskGroup(of: Product.self) { group in
for id in ids {
group.addTask {
try await self.api.fetchProduct(id: id)
}
}
var products: [Product] = []
for try await product in group {
products.append(product)
}
return products
}
}
// With concurrency limit
func fetchWithLimit(ids: [String], maxConcurrent: Int = 4) async throws -> [Product] {
try await withThrowingTaskGroup(of: Product.self) { group in
var iterator = ids.makeIterator()
var products: [Product] = []
// Start initial batch
for _ in 0..<min(maxConcurrent, ids.count) {
if let id = iterator.next() {
group.addTask { try await self.api.fetchProduct(id: id) }
}
}
// As each completes, add another
for try await product in group {
products.append(product)
if let id = iterator.next() {
group.addTask { try await self.api.fetchProduct(id: id) }
}
}
return products
}
}
@MainActor
final class ProductListViewModel: ObservableObject {
@Published private(set) var products: [Product] = []
@Published private(set) var isLoading = false
@Published private(set) var error: Error?
private let repository: ProductRepository
init(repository: ProductRepository) {
self.repository = repository
}
func loadProducts() async {
isLoading = true
error = nil
do {
products = try await repository.fetchProducts()
} catch {
self.error = error
}
isLoading = false
}
// Nonisolated for non-UI work
nonisolated func precomputeHash(for product: Product) -> Int {
product.hashValue
}
}
// Value types are Sendable automatically if properties are
struct Product: Sendable {
let id: String
let name: String
let price: Decimal
}
// Classes need explicit conformance
final class ProductCache: @unchecked Sendable {
private let lock = NSLock()
private var cache: [String: Product] = [:]
func get(_ id: String) -> Product? {
lock.lock()
defer { lock.unlock() }
return cache[id]
}
func set(_ product: Product) {
lock.lock()
defer { lock.unlock() }
cache[product.id] = product
}
}
// Sendable closure
func process(_ items: [Item], transform: @Sendable (Item) -> Result) async -> [Result] {
await withTaskGroup(of: Result.self) { group in
for item in items {
group.addTask {
transform(item)
}
}
var results: [Result] = []
for await result in group {
results.append(result)
}
return results
}
}
func downloadLargeFile(url: URL) async throws -> Data {
var data = Data()
let (stream, response) = try await URLSession.shared.bytes(from: url)
let expectedLength = response.expectedContentLength
for try await byte in stream {
// Check for cancellation periodically
try Task.checkCancellation()
data.append(byte)
// Report progress (would need actor for thread safety)
let progress = Double(data.count) / Double(expectedLength)
await reportProgress(progress)
}
return data
}
// Usage with timeout
func downloadWithTimeout(url: URL, timeout: Duration) async throws -> Data {
try await withThrowingTaskGroup(of: Data.self) { group in
group.addTask {
try await self.downloadLargeFile(url: url)
}
group.addTask {
try await Task.sleep(for: timeout)
throw DownloadError.timeout
}
// First to complete wins, other is cancelled
let result = try await group.next()!
group.cancelAll()
return result
}
}
| Issue | Cause | Solution |
|---|---|---|
| "Actor-isolated property cannot be accessed" | Cross-actor access | Use await or nonisolated |
| "Capture of non-sendable type" | Non-Sendable in closure | Make type Sendable or use actor |
| "Reference to captured var in concurrently-executing code" | Mutable capture | Use let or actor |
| Task hangs | Missing await | Add await to all async calls |
| Deadlock | Actor calling itself | Use nonisolated for pure functions |
// Print current task priority
print("Priority: \(Task.currentPriority)")
// Check if cancelled
if Task.isCancelled {
return
}
// Add task-local values for debugging
enum RequestID: TaskLocalKey {
static var defaultValue: String? { nil }
}
extension Task where Success == Never, Failure == Never {
static var requestID: String? {
get { self[RequestID.self] }
set { self[RequestID.self] = newValue }
}
}
validation:
- rule: strict_concurrency
severity: error
check: Build with -strict-concurrency=complete
- rule: sendable_conformance
severity: warning
check: Types crossing actor boundaries must be Sendable
- rule: main_actor_ui
severity: error
check: UI updates must be on MainActor
Skill("swift-concurrency")
swift-fundamentals - Language basicsswift-combine - Reactive alternativeswift-testing - Testing async code