Understanding “async let” in Swift — Unlocking Concurrency with Ease

Swift’s async let simplifies concurrency by enabling multiple asynchronous tasks to run in parallel. It allows for cleaner, more efficient code that enhances performance without the complexity of managing threads manually. In this post, we’ll explore how async let works, its benefits, and when to use it with practical examples.

What is `async let` ?

async let lets you run independent asynchronous tasks concurrently and wait for their results only when needed, making your app more efficient by reducing wait times.

How does `async let` work?

Let’s look at a simple example. Imagine you want to fetch data from two APIs concurrently. Without async let, you would typically write:

func fetchData() async throws {
    let result1 = try await fetchFromAPI1()
    let result2 = try await fetchFromAPI2()
    
    // Do something with result1 and result2
}

In this case, fetchFromAPI2() will only start after fetchFromAPI1() finishes. This is sequential execution, even though both functions are asynchronous.

Now, using async let, we can run both tasks concurrently:

func fetchData() async throws {
    async let result1 = fetchFromAPI1()
    async let result2 = fetchFromAPI2()
    
    // Await the results
    let finalResult1 = try await result1
    let finalResult2 = try await result2
    
    // Do something with finalResult1 and finalResult2
}

With async let, fetchFromAPI1() and fetchFromAPI2() will start running in parallel, significantly reducing the overall execution time.

Benefits of Using async let

Parallelism and Speed: The most obvious benefit is that it allows you to run multiple tasks concurrently. This can lead to significant performance improvements when dealing with tasks like network requests, image processing, or database queries.

Clean Syntax: The async let syntax is straightforward. It makes your code more readable and easier to understand. Instead of dealing with callback hell or complex queue management, you can write linear-looking code that performs asynchronous tasks in parallel.

Built-in Task Management: Swift’s concurrency model takes care of task management, ensuring the execution of tasks in a safe and structured manner. async let tasks are scoped to the current function or block, and their results must be awaited before exiting the scope, preventing potential memory leaks or unfinished tasks.

Automatic Propagation of Errors: Any errors that occur in the tasks are automatically thrown when awaiting the results, making error handling more intuitive.

Where to Use `async let`

1. Network Calls: If your app relies on fetching data from multiple APIs, async let allows you to perform these calls concurrently and handle the results when they are all ready.
2. File I/O Operations: Reading or writing to multiple files can be done concurrently using async let, saving time when dealing with large amounts of data.
3. Image Processing: You can use async let to apply multiple image filters concurrently, allowing the user to see processed images faster.
4. Database Queries: Querying multiple tables or datasets concurrently can significantly reduce the time it takes to gather data from a database.

Caution: When Not to Use `async let`

While async let is powerful, it’s not suitable for every situation:

• Tightly Coupled Tasks: If one task depends on the result of another, you should not use async let. In such cases, tasks need to run sequentially.
• UI Updates: Avoid performing UI updates in parallel unless each update is independent. UI changes typically need to be done on the main thread, so parallelism might introduce bugs if not handled carefully.

async let is a simple yet powerful tool that unlocks parallelism in Swift’s structured concurrency model. It shines when you need to run independent tasks concurrently, such as fetching data, processing files, or querying databases. By using async let, you can write clean, readable, and efficient concurrent code without needing to manage the complexities of threads or GCD.

Next time you’re dealing with asynchronous code, think about whether you can leverage async let to improve your app’s performance and responsiveness.

Try it out in Swift Playgrounds

Dive deeper into Swift Concurrency: Read ‘Understanding Swift’s Async/Await’.