Concurrency (async/await)

Code

import Foundation

// async function
func fetch(_ url: String) async throws -> String {
    try await Task.sleep(nanoseconds: 100_000_000)
    return "data from \(url)"
}

// Parallel fetch with async let
func loadAll() async throws -> [String] {
    async let a = fetch("https://a.example.com")
    async let b = fetch("https://b.example.com")
    async let c = fetch("https://c.example.com")
    return try await [a, b, c]
}

// Task groups for fan-out work
func sum(_ n: Int) async -> Int {
    await withTaskGroup(of: Int.self) { group in
        for i in 0..<n {
            group.addTask { i * i }
        }
        var total = 0
        for await v in group { total += v }
        return total
    }
}

// Cancellation
func loop() async {
    let task = Task {
        for i in 0..<100 {
            if Task.isCancelled { print("cancelled"); return }
            try? await Task.sleep(nanoseconds: 10_000_000)
        }
    }
    task.cancel()
}

// Top-level await (in async context)
Task {
    let results = try await loadAll()
    print(results)
    print(await sum(5))
    await loop()
}

Explanation

async/await turns asynchronous code into straight-line reads, suspending at await points without blocking threads. async let runs several awaitables concurrently, and Task groups fan out a dynamic number of child tasks. Tasks support structured cancellation that propagates to children when a parent is cancelled.

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