swift-concurrency

Swift Concurrency

Overview

Structured concurrency built on async/await. The compiler enforces isolation (actor, @MainActor) and, under Swift 6 strict concurrency, data-race safety via Sendable. Prefer this over GCD/DispatchQueue for new code.

When to use

• Async iOS work: network, disk, or shared-KMP calls
• Updating UI safely from background work (@MainActor)
• Parallel work (async let, TaskGroup)
• Bridging callback/delegate APIs into async (withCheckedContinuation)
• Compiler errors like "call to main actor-isolated … in a synchronous nonisolated context" or Sendable warnings
• Consuming Kotlin suspend/Flow surfaced as Swift async (see kmp-ios-integration)

Quick reference

Example — actor + @MainActor + continuation

actor ImageCache {                         // serialized state, no manual locks
    private var store: [URL: Data] = [:]
    func data(for url: URL) -> Data? { store[url] }
    func insert(_ data: Data, for url: URL) { store[url] = data }
}

@MainActor                                  // all UI state mutates on the main thread
@Observable final class FeedModel {
    var items: [Item] = []

    func load() async throws {
        async let items = api.fetchItems()  // these two run in parallel
        async let promos = api.fetchPromos()
        self.items = try await merge(items, promos)   // already on the main actor
    }
}

// Bridge a delegate/callback API into async/await
func currentLocation() async throws -> CLLocation {
    try await withCheckedThrowingContinuation { continuation in
        locationManager.requestLocation { result in
            continuation.resume(with: result)   // resume EXACTLY once, on every path
        }
    }
}