Swift Concurrency

Swift Concurrency is a community skill for writing concurrent code in Swift using structured concurrency, covering async/await patterns, task groups, actors, sendable types, and concurrency safety for iOS and server-side Swift applications.

What Is This?

Overview

Swift Concurrency provides patterns for managing asynchronous operations using Swift's built-in concurrency model. It covers async/await syntax for writing sequential-looking asynchronous code that suspends without blocking threads, task groups for running multiple operations in parallel and collecting their results, actors for protecting mutable state from data races by serializing access to shared resources, sendable types and protocols that ensure data crossing concurrency boundaries is safe, and structured concurrency with task hierarchies that manage cancellation and lifetime automatically. The skill enables developers to write safe, performant concurrent code that the compiler verifies for data race freedom.

Who Should Use This

This skill serves iOS developers adopting modern concurrency patterns to replace completion handlers, teams building server-side Swift applications with concurrent request handling, and engineers migrating from GCD or Combine to structured concurrency.

Why Use It?

Problems It Solves

Completion handler pyramids create deeply nested callback code that is difficult to follow and debug. Grand Central Dispatch queues do not prevent data races at compile time. Error handling across asynchronous boundaries requires manual propagation through callbacks. Cancellation of in-flight operations needs manual tracking without structured task hierarchies.

Core Highlights

Async/await transforms callback chains into linear sequential code. Actor isolation serializes access to mutable state with compiler enforcement. Task groups enable parallel execution with automatic child task management. Sendable checking prevents unsafe data sharing at compile time.

How to Use It?

Basic Usage

// Async/await with actors
actor ImageCache {
    private var cache:
        [URL: Data] = [:]

    func image(
        for url: URL
    ) async throws -> Data {
        if let cached =
            cache[url] {
            return cached
        }

        let (data, _) =
            try await URLSession
                .shared.data(
                    from: url)
        cache[url] = data
        return data
    }

    func clear() {
        cache.removeAll()
    }
}

// Usage
let cache = ImageCache()
let data =
    try await cache.image(
        for: imageURL)

Real-World Examples

// Task group for parallel fetch
func fetchAllProfiles(
    ids: [String]
) async throws -> [Profile] {
    try await withThrowingTaskGroup(
        of: Profile.self
    ) { group in
        for id in ids {
            group.addTask {
                try await
                    fetchProfile(
                        id: id)
            }
        }

        var profiles:
            [Profile] = []
        for try await profile
            in group {
            profiles.append(
                profile)
        }
        return profiles
    }
}

func fetchProfile(
    id: String
) async throws -> Profile {
    let url = URL(
        string: "https://api"
        + ".example.com/users/"
        + id)!
    let (data, _) =
        try await URLSession
            .shared.data(
                from: url)
    return try JSONDecoder()
        .decode(
            Profile.self,
            from: data)
}

Advanced Tips

Use @MainActor for UI-updating code to guarantee main thread execution without manual dispatching. Mark types as Sendable or use @unchecked Sendable only when you can guarantee thread safety. Use AsyncStream to bridge callback-based APIs into the async/await world.

When to Use It?

Use Cases

Build an image loading pipeline that caches results using an actor and fetches in parallel with task groups. Create a network layer that replaces completion handler APIs with async/await methods. Implement background data synchronization with structured cancellation support.

Related Topics

Swift actors, async/await, task groups, sendable types, and structured concurrency.

Important Notes

Requirements

Swift 5.5 or later with concurrency support enabled. Xcode 13 or later for iOS development. Minimum deployment target of iOS 13 with back-deployment or iOS 15 for full support.

Usage Recommendations

Do: prefer structured concurrency with task groups over unstructured Task creation for automatic cancellation. Use actors for shared mutable state rather than manual locking. Check Task.isCancelled in long-running operations to support cooperative cancellation. Use continuations to bridge delegate-based APIs into async/await patterns safely.

Don't: use @unchecked Sendable to silence compiler warnings without verifying actual thread safety. Create detached tasks when structured tasks would properly propagate cancellation. Block an actor with synchronous long-running work that should be offloaded.

Limitations

Actor reentrancy means state may change between suspension points within the same actor. Back-deployment to older iOS versions has limited concurrency feature support. Debugging concurrent code requires understanding of task hierarchies and suspension points. Global actors have performance implications when many tasks contend for the same isolation context.