Modern Concurrency: Async / Await & Actors

Structured concurrency, Task groups, and eliminating data races with actors

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Swift 5.5 introduced structured concurrency with async/await and actor types that provide compiler-enforced data race safety.

Async/Await and Actors

async: Marks functions that perform asynchronous non-blocking work.

await: Pauses execution until the asynchronous task yields a result.

actor: A reference type like a class, but with thread-safe isolation. Only one task can access an actor's mutable state at any given moment, making data races physically impossible!

@MainActor: Guarantees code runs strictly on the main thread for UI updates.

Thread-Safe Bank Account using Swift Actor

Try It Yourself »

// Actors protect internal mutable state from concurrent race conditions
actor BankAccount {
    var balance: Double = 1000.0

    func withdraw(amount: Double) -> Boolean {
        guard balance >= amount else { return false }
        balance -= amount
        return true
    }
}

// Accessing actor methods requires await!
let account = BankAccount()
Task {
    let success = await account.withdraw(amount: 250.0)
    print("Withdrawal success: \(success)")
}

Actors serialize access to their internal state automatically.

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Test Yourself With Exercise

+15 XP

What unique safety feature does an `actor` provide over a standard `class` in Swift?

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