ax-concurrency

Concurrency

Quick Patterns

Progressive Journey (Start Simple)

Single-Threaded → Asynchronous → Concurrent → Actors
     |                |             |           |
   Start here    Hide latency   Background   Move data
                 (network)      CPU work     off main

Advance only when profiling proves it's needed.

1. Stay single-threaded if UI is responsive and operations are fast (<16ms)
2. Add async/await when high-latency operations (network, file I/O) block UI
3. Add @concurrent when CPU-intensive work freezes UI (image processing, parsing)
4. Add actors when too much main actor code causes contention

Main Actor Mode (Xcode 26+)

Build Settings → Swift Compiler — Language
→ "Default Actor Isolation" = Main Actor
→ "Approachable Concurrency" = Yes

Build Settings → Swift Compiler — Concurrency
→ "Strict Concurrency Checking" = Complete

All code protected by @MainActor by default unless explicitly marked otherwise.

Async/Await (Hide Latency)

// Suspends without blocking main thread
func fetchAndDisplay(url: URL) async throws {
    let (data, _) = try await URLSession.shared.data(from: url)  // Suspends here
    let image = decodeImage(data)  // Resumes on main thread
    view.displayImage(image)
}

// Create task for user action
func onTapEvent() {
    Task {
        do {
            try await fetchAndDisplay(url: url)
        } catch {
            displayError(error)
        }
    }
}

@concurrent (Background CPU Work, Swift 6.2+)

// Always runs on background thread pool
@concurrent
func decodeImage(_ data: Data) async -> UIImage {
    // Image processing, parsing, heavy computation
    return UIImage(data: data)!
}

// Usage - automatically offloads
let image = await decodeImage(data)

Use @concurrent when work should always run on background (image processing, parsing). Use nonisolated when caller decides where work runs (library APIs).

@MainActor for UI Code