Meet AsyncSequence

Written by Donny Wals

Description: Iterating over a sequence of values over time is now as easy as writing a “for” loop. Find out how the new AsyncSequence protocol enables a natural, simple syntax for iterating over anything from notifications to bytes being streamed from a server. We'll also show you how to adapt existing code to provide asynchronous sequences of your own. To get the most out of this session, we recommend first watching “Meet async/await in Swift.”

Map, filter, reduce, dropFirst all work in async sequences:

for try await someThing in async.dropFirst() {

For example.

AsyncSequence suspends on each element and receives values asynchronously from the iterator. AsyncSequences either complete with success or stop when an error is thrown.

Implementing an AsyncSequence follows all the rules that a normal sequence follows. Its next() returns nil when it’s completed for example.

An async iterator also consumes its underlying collection.

Things like break and continue work in async sequences too.

You can cancel an iteration by holding on to its Task.Handle when you wrap it in async:

let handle = async {
  for await thing in list {
    // ...


Reading files from a URL is commonly done async. You can use URL.lines for this. It works for network and local resources.

URLSession has a bytes(_:) function to enable roughly the same but with more control.

NotificationCenter notifications can be awaited too. You can even await one notification:

// listens for 1 notification only
let center = NotificationCenter.default
let notification = await center.notifications(named: .SomeNotification).first { notification in 

Callbacks that are called multiple times, and some delegates are good candidates for async sequences.

Start / stop / handle pattern is a good candidate. Sounds similar to location managers.

let stream = AsyncStream(Output.self) { continuation in 
  let object = SomeObject()
  object.handler = { element in 

  object.onTermination = { _ in 
  // starts producing values

AsyncStream is the easiest way to create your own asynchronous sequences. They also handle buffering (not sure what that means in this case).

There’s also a throwing version: AsyncThrowingStream.

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Missing anything? Corrections? Contributions are welcome 😃


Written by

Donny Wals

Donny Wals

Passionate iOS developer at Disney Streaming Services. Loves learning and sharing knowledge. ❤️ Dorien -- #swift #ios #swiftlang #iosdev -- opinions are my own