/swift-concurrency

Sendable

Use this when:

• A value or reference type must cross an isolation boundary safely.
• You are resolving "non-Sendable type" compiler diagnostics.
• You need to decide between value types, @unchecked Sendable, actors, or region-based isolation.

Skip this file if:

• The issue is about which actor should own the state. Use actors.md.
• The issue is about how async functions execute. Use threading.md.

Jump to:

• Isolation Domains
• Value Types (Structs, Enums)
• Reference Types (Classes)
• Functions and Closures (@Sendable)
• @unchecked Sendable
• Region-Based Isolation / sending
• Global Variables
• Decision Tree

What is Sendable?

Sendable indicates a type is safe to share across isolation domains (actors, tasks, threads). The compiler verifies thread-safety at compile time.

public protocol Sendable {}

Empty protocol, but triggers compiler verification of thread-safety.

Course Deep Dive: This topic is covered in detail in Lesson 4.1: Explaining the concept of Sendable in Swift

Isolation Domains

Three types of isolation in Swift Concurrency:

1. Nonisolated (default)

No concurrency restrictions, but can't modify isolated state:

func computeValue(a: Int, b: Int) -> Int {
    return a + b
}

2. Actor-isolated

Dedicated isolation domain with serialized access:

actor Library {
    var books: [String] = []
    
    func addBook(_ title: String) {
        books.append(title)
    }
}

// External access requires await
await library.addBook("Swift Concurrency")

3. Global actor-isolated

Shared isolation domain across types:

@MainActor
func updateUI() {
    // Runs on main thread
}

Data Races vs Race Conditions

Data Race

Multiple threads access shared mutable state, at least one writes, without synchronization:

// ⚠️ Data race
var counter = 0
DispatchQueue.global().async { counter += 1 }
DispatchQueue.global().async { counter += 1 }

Detection: Enable Thread Sanitizer in scheme settings.

Prevention: Use actors or Sendable types:

actor Counter {
    private var value = 0
    
    func increment() {
        value += 1
    }
}

Race Condition

Timing-dependent behavior leading to unpredictable results:

let counter = Counter()

for _ in 1...10 {
    Task { await counter.increment() }
}

// May print inconsistent values
print(await counter.getValue())

Key difference: Swift Concurrency prevents data races but not race conditions. You must still ensure proper sequencing.

Course Deep Dive: This topic is covered in detail in Lesson 4.2: Understanding Data Races vs. Race Conditions: Key Differences Explained

Value Types (Structs, Enums)

Implicit conformance

Non-public structs/enums with Sendable members:

// Implicitly Sendable
struct Person {
    var name: String
}

Explicit conformance required

Public types need explicit declaration:

public struct Person: Sendable {
    var name: String
}

Why: Compiler can't verify internal details of public types across modules.

Frozen types

Public frozen types can be implicitly Sendable:

@frozen
public struct Point: Sendable {
    public var x: Double
    public var y: Double
}

All members must be Sendable

public struct Person: Sendable {
    var name: String
    var hometown: Location // Must also be Sendable
}

public struct Location: Sendable {
    var name: String
}

Course Deep Dive: This topic is covered in detail in Lesson 4.3: Conforming your code to the Sendable protocol

Copy-on-write makes mutability safe

public struct Person: Sendable {
    var name: String // Mutable but safe due to COW
}

Each mutation creates a copy, preventing concurrent access to same instance.

Course Deep Dive: This topic is covered in detail in Lesson 4.4: Sendable and Value Types

Reference Types (Classes)

Requirements for Sendable classes

Must be:

1. final (no inheritance)
2. Immutable stored properties only
3. All properties Sendable
4. No superclass or NSObject only

final class User: Sendable {
    let name: String
    let id: Int
    
    init(name: String, id: Int) {
        self.name = name
        self.id = id
    }
}

Why non-final classes can't be Sendable

Child classes could introduce unsafe mutability:

// Can't be Sendable
class Purchaser {
    func purchase() { }
}

// Could introduce data races
class GamePurchaser: Purchaser {
    var credits: Int = 0 // Mutable!
}

Actor isolation makes classes Sendable

@MainActor
class ViewModel {
    var data: [Item] = [] // Safe due to actor isolation
}
// Implicitly Sendable

Composition over inheritance

final class Purchaser: Sendable {
    func purchase() { }
}

final class GamePurchaser {
    let purchaser: Purchaser = Purchaser()
    // Handle credits separately
}

Course Deep Dive: This topic is covered in detail in Lesson 4.5: Sendable and Reference Types

Functions and Closures (@Sendable)

Mark functions/closures that cross isolation domains:

actor ContactsStore {
    func removeAll(_ shouldRemove: @Sendable (Contact) -> Bool) async {
        contacts.removeAll { shouldRemove($0) }
    }
}

Captured values must be Sendable

let query = "search"

// ✅ Immutable capture
store.filter { contact in
    contact.name.contains(query)
}

var query = "search"

// ❌ Mutable capture
store.filter { contact in
    contact.name.contains(query) // Error
}

Capture lists for mutable values

var query = "search"

// ✅ Capture immutable snapshot
store.filter { [query] contact in
    contact.name.contains(query)
}

Course Deep Dive: This topic is covered in detail in Lesson 4.6: Using @Sendable with closures

@unchecked Sendable

Use as last resort. Tells compiler to skip verification—you guarantee thread-safety.

When to use

Manual locking mechanisms the compiler can't verify:

final class Cache: @unchecked Sendable {
    private let lock = NSLock()
    private var items: [String: Data] = [:]
    
    func get(_ key: String) -> Data? {
        lock.lock()
        defer { lock.unlock() }
        return items[key]
    }
    
    func set(_ key: String, value: Data) {
        lock.lock()
        defer { lock.unlock() }
        items[key] = value
    }
}

Risks

• No compile-time safety
• Easy to introduce data races
• Must manually ensure all access uses lock

final class Cache: @unchecked Sendable {
    private let lock = NSLock()
    private var items: [String: Data] = [:]
    
    // ⚠️ Forgot lock - data race!
    var count: Int {
        items.count
    }
}

Better: Use actor instead:

actor Cache {
    private var items: [String: Data] = [:]
    
    var count: Int { items.count }
    
    func get(_ key: String) -> Data? {
        items[key]
    }
    
    func set(_ key: String, value: Data) {
        items[key] = value
    }
}

Course Deep Dive: This topic is covered in detail in Lesson 4.7: Using @unchecked Sendable

Region-Based Isolation

Compiler allows non-Sendable types in same scope:

class Article {
    var title: String
    init(title: String) { self.title = title }
}

func check() {
    let article = Article(title: "Swift")
    
    Task {
        print(article.title) // ✅ OK - same region
    }
}

Why: No mutation after transfer, so no data race risk.

Breaks when accessed after transfer

func check() {
    let article = Article(title: "Swift")
    
    Task {
        print(article.title)
    }
    
    print(article.title) // ❌ Error - accessed after transfer
}

The sending Keyword

Enforces ownership transfer for non-Sendable types:

Parameter values

actor Logger {
    func log(article: Article) {
        print(article.title)
    }
}

func printTitle(article: sending Article) async {
    let logger = Logger()
    await logger.log(article: article)
}

// Usage
let article = Article(title: "Swift")
await printTitle(article: article)
// article no longer accessible here

Return values

@SomeActor
func createArticle(title: String) -> sending Article {
    return Article(title: title)
}

Transfers ownership to caller's region.

Course Deep Dive: This topic is covered in detail in Lesson 4.8: Understanding region-based isolation and the sending keyword

Global Variables

Must be concurrency-safe since accessible from any context.

Problem

class ImageCache {
    static var shared = ImageCache() // ⚠️ Not concurrency-safe
}

Solution 1: Actor isolation

@MainActor
class ImageCache {
    static var shared = ImageCache()
}

Solution 2: Immutable + Sendable

final class ImageCache: Sendable {
    static let shared = ImageCache()
}

Solution 3: nonisolated(unsafe)

Last resort - you guarantee safety:

struct APIProvider: Sendable {
    nonisolated(unsafe) static private(set) var shared: APIProvider!
    
    static func configure(apiURL: URL) {
        shared = APIProvider(apiURL: apiURL)
    }
}

Use private(set) to limit mutation points.

Course Deep Dive: This topic is covered in detail in Lesson 4.9: Concurrency-safe global variables

Custom Locks + Sendable

Legacy code with locks

final class BankAccount: @unchecked Sendable {
    private var balance: Int = 0
    private let lock = NSLock()
    
    func deposit(amount: Int) {
        lock.lock()
        balance += amount
        lock.unlock()
    }
    
    func getBalance() -> Int {
        lock.lock()
        defer { lock.unlock() }
        return balance
    }
}

Migration strategy

New code: Use actors

Existing code:

1. If isolated and small scope → migrate to actor
2. If widely used → use @unchecked Sendable, file migration ticket

// Better: Migrate to actor
actor BankAccount {
    private var balance: Int = 0
    
    func deposit(amount: Int) {
        balance += amount
    }
    
    func getBalance() -> Int {
        balance
    }
}

Course Deep Dive: This topic is covered in detail in Lesson 4.10: Combining Sendable with custom Locks

Decision Tree

Need to share type across isolation domains?
├─ Value type (struct/enum)?
│  ├─ Public? → Add explicit Sendable
│  └─ Internal? → Implicit Sendable (if members Sendable)
│
├─ Reference type (class)?
│  ├─ Can be final + immutable? → Sendable
│  ├─ Needs mutation?
│  │  ├─ Can use actor? → Use actor (automatic Sendable)
│  │  ├─ Main thread only? → @MainActor
│  │  └─ Has custom lock? → @unchecked Sendable (temporary)
│  └─ Can be struct instead? → Refactor to struct
│
└─ Function/closure? → @Sendable attribute

Common Patterns

Restructure to avoid non-Sendable dependencies

// Instead of storing non-Sendable type
public struct Person: Sendable {
    var hometown: String // Just the name
    
    init(hometown: Location) {
        self.hometown = hometown.name
    }
}

Prefer actors for mutable state

// Instead of @unchecked Sendable with locks
actor Cache {
    private var items: [String: Data] = [:]
    
    func get(_ key: String) -> Data? {
        items[key]
    }
}

Use @MainActor for UI-bound types

@MainActor
class ViewModel: ObservableObject {
    @Published var items: [Item] = []
}

Best Practices

1. Prefer value types - structs/enums are easier to make Sendable
2. Use actors for mutable state - automatic thread-safety
3. Avoid @unchecked Sendable - use only for proven thread-safe code
4. Mark public types explicitly - don't rely on implicit conformance
5. Ensure all members Sendable - one non-Sendable breaks the chain
6. Use @MainActor for UI types - simple isolation for view models
7. Capture immutably - use capture lists for mutable variables
8. Test with Thread Sanitizer - catches runtime data races
9. File migration tickets - track @unchecked Sendable usage

Further Learning

For migration strategies, real-world examples, and actor patterns, see Swift Concurrency Course.