WWDC Notes: Meet AsyncSequence

Published by donnywals on

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.

Note: AsyncStream does not appear to be present in Beta 1

Stay up to date with my weekly newsletter

Marketing permission: I give my consent to donnywals.com to be in touch with me via email using the information I have provided in this form for the purpose of receiving a weekly newsletter.

Practical Combine

Learn everything you need to know about Combine and how you can use it in your projects with my new book Practical Combine. You'll get thirteen chapters, a Playground and a handful of sample projects to help you get up and running with Combine as soon as possible.

The book is available as a digital download for just $34.99!

Get Practical Combine
Categories: WWDC21 Notes