Concurrency in Swift: Async/Await and Beyond

KD Knowledge Diet
3 min readFeb 4, 2024

Concurrency has been a topic of interest and complexity for developers for many years. Writing concurrent code allows programs to perform multiple tasks simultaneously, making better use of resources and providing a more responsive user experience. Swift has introduced modern concurrency features that make asynchronous programming more straightforward and safe. Let’s delve into how Swift handles concurrency, focusing on the async/await pattern and what lies beyond.

Async/Await: Simplifying Asynchronous Code

Before Swift 5.5, concurrency in Swift often involved callbacks and completion handlers, which could lead to nested “callback hell” and make error handling cumbersome. With the introduction of async/await in Swift 5.5, writing asynchronous code has become much more readable and maintainable.

1. Async Functions:
Marking a function with `async` indicates that it performs an asynchronous operation. It can involve waiting for something, like fetching data from the internet, without blocking the current thread.

2. Awaiting Results:
The `await` keyword pauses the function’s execution until the awaited asynchronous task is completed. This lets the program perform other work rather than blocking the thread.

Here’s an example of how async/await can be used in Swift:

func fetchUserData() async -> UserData {
let url = URL(string: "https://api.example.com/user")!
let (data, _) = try await URLSession.shared.data(from: url)
let userData = try JSONDecoder().decode(UserData.self, from: data)
return userData
}
Task {
let userData = await fetchUserData()
print("User data fetched: \(userData)")
}

In this snippet, `fetchUserData` is an asynchronous function. When called with `await` inside a `Task`, it waits for the data to be fetched without blocking the main thread, and then prints the user data.

Beyond Async/Await

While async/await is a significant leap forward, Swift’s concurrency model doesn’t stop there. It includes several more advanced features:

1. Structured Concurrency:
Swift introduces the concept of structured concurrency through the use of `Task` and `TaskGroup`. These constructs allow you to spawn concurrent work and manage it in a way that’s aware of the lifecycle of tasks, handling their cancellation and errors gracefully.

2. Actors:
Actors are a reference type that protects access to their internal state, ensuring that only one piece of code can access that state at a time. This prevents data races and makes state management safer and more predictable.

3. Continuations for Interoperability:
Continuations are a way to bridge between callback-based APIs and async/await. They allow developers to wrap older APIs so they can be used with Swift’s new concurrency model.

4. Async Sequences:
Swift provides the `AsyncSequence` protocol, allowing you to work with a sequence of values asynchronously, using `for await` in a loop, for instance.

Best Practices for Concurrency in Swift

1. Prefer Using Async/Await:
Whenever possible, use async/await to write your asynchronous code. It’s cleaner and reduces the likelihood of mistakes compared to using callbacks.

2. Utilize Structured Concurrency:
Embrace tasks and task groups for managing concurrent operations. They provide a clear structure for your concurrent code and handle the lifecycle of operations automatically.

3. Safeguard Shared State with Actors:
Use actors to manage shared state in a concurrent environment. They are designed to ensure thread safety around the mutable state.

4. Bridge Older APIs Cautiously:
When interfacing with older, callback-based APIs, wrap them with continuations carefully to avoid potential deadlocks and ensure you’re still handling errors correctly.

Conclusion

Swift’s concurrency features, spearheaded by async/await, have been a game-changer for developers, allowing them to write asynchronous code that’s both powerful and easy to read. By leveraging structured concurrency, actors, and async sequences, Swift developers can handle complex concurrent operations safely and efficiently. As the language continues to evolve, it’s clear that Swift is committed to providing a concurrency model that meets the needs of modern app development. Whether you’re fetching data from the web, processing files, or executing complex algorithms, Swift’s concurrency tools can help you write better code.

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KD Knowledge Diet

Software Engineer, Mobile Developer living in Seoul. I hate people using difficult words. Why not using simple words? Keep It Simple Stupid!