axiom-swift-concurrency-ref

Swift Concurrency API Reference

Complete Swift concurrency API reference for copy-paste patterns and syntax lookup.

Complements axiom-swift-concurrency (which covers when and why to use concurrency — progressive journey, decision trees, @concurrent, isolated conformances).

Related skills: axiom-swift-concurrency (progressive journey, decision trees), axiom-synchronization (Mutex, locks), axiom-assume-isolated (assumeIsolated patterns)

Part 1: Actor Patterns

Actor Definition

actor ImageCache {
    private var cache: [URL: UIImage] = [:]

    func image(for url: URL) -> UIImage? {
        cache[url]
    }

    func store(_ image: UIImage, for url: URL) {
        cache[url] = image
    }
}

// Usage — must await across isolation boundary
let cache = ImageCache()
let image = await cache.image(for: url)

All properties and methods on an actor are isolated by default. Callers outside the actor's isolation domain must use await to access them.

Actor Isolation Rules

Every actor's stored properties and methods are isolated to that actor. Access from outside the isolation boundary requires await, which suspends the caller until the actor can process the request.

actor Counter {
    var count = 0              // Isolated — external access requires await
    let name: String           // let constants are implicitly nonisolated

    func increment() {         // Isolated — await required from outside
        count += 1
    }

    nonisolated func identity() -> String {
        name                   // OK: accessing nonisolated let
    }
}

let counter = Counter(name: "main")
await counter.increment()      // Must await across isolation boundary
let id = counter.identity()    // No await needed — nonisolated

nonisolated Keyword

Opt out of isolation for synchronous access to non-mutable state.

actor MyActor {
    let id: UUID               // let constants are implicitly nonisolated

    nonisolated var description: String {
        "Actor \(id)"          // Can only access nonisolated state
    }

    nonisolated func hash(into hasher: inout Hasher) {
        hasher.combine(id)     // Only nonisolated properties
    }
}