async/await in Swift

All the features you need to know about

We’ll cover:

1. async functions
2. async get properties
3. async let
4. Parallel execution using withTaskGroup and withThrowingTaskGroup
5. Legacy APIs and withCheckedContinuation and withCheckedThrowingContinuation
6. Create arrays of events or AsyncSequence with AsyncStream
7. actor and nonisolated and why we need them

Grab the code here 👊

Foundations

Why?

Let’s start with why. Why the hell do we need it? Have a look:

Boy, this looks ugly. And it’s a typical async code we had in our apps, until now.

The first async function

Let’s make it pretty with async/await, and you’ll see why we need it.

Let’s keep this ugly monster here and create a new function.

First, we have to mark the function as async, it always goes before throws:

Many Apple APIs now have async alternatives for the functions we know.

Instead of passing the result through a callback, the function simply returns it and errors are thrown. That’s something we do in our function, too:

Now, when we call a function like that, we have to await it on the caller's side. It's done with await keyword which always goes after try. try is needed here to catch errors.

Here we see a further benefit — now we can use throw to return from a function with an error. You can’t do it inside the callback!

image.byPreparingThumbnail is just another async function provided for us by Apple. Nice!

Compare the 2 now. Which one is easier to read?

Let’s actually use the function now, let’s go to the ViewModel and replace the old onAppear():

Now the function has to be marked async, and we handle errors with a do/catch block.

Let’s go to the view now!

'async' call in a function that does not support concurrency screams at us!

How can we convert .onAppear {} to be async? - We can't! We use Task instead!

We dispatch a task (aka call an async function) this way from non-async context. It's a bridge between async and non-async worlds (and not only that).

Let’s run the app now to see it work!

Not only functions (async get)

Btw. functions are not the only things that can be marked async. You can have async get properties now too!

Let’s use it in fetchThumbnail:

Here’s our async/await series:

Part 1. (this one) async/await functions and async get

Part 2. async let and TaskGroup in Swift

Part 3. Convert legacy APIs to async/await

Part 4. How to use Async Sequence and AsyncStream in Swift

Part 5. Actors and Data Races in Swift