Building an AsyncSequence with AsyncStream.makeStream

In this video you'll learn how to use Swift 5.9's AsyncStream.makeStream method to build custom async sequences. You'll learn about its usage as well as its caveats.
This video is a companion for this blog post: www.donnywals....
Other blog posts worth reading:
Understanding Swift Concurrency's AsyncStream: www.donnywals....
Understanding Swift Concurrency's AsyncSequence: www.donnywals....
Iterating over web socket messages with async / await in Swift: www.donnywals....
Comparing lifecycle management for async sequences and publishers: www.donnywals....

[@priceringo1756]

Thank for such great, simple but not trivial, examples!

[@indiekiduk]

I think makeStream is an anti-pattern because I ran into issues when using like cancellation not working. Once I redesigned my async code and made AsyncStream own the object that yields values all my problems were solved!

[@DonnyWalsdev]

Any chance you have a gist that would demonstrate the issues you've had? The tests I ran involved moving from the older AsyncStream init to makeStream and I didn't see these issues.

[@indiekiduk]

@@DonnyWalsdev ok I'm trying to work on that. I noticed you had onTermination = { [weak self] that must have been to work around the retain cycle issue.

[@indiekiduk]

I've been investigating this all day and I think makeStream is fine. By using a weak var in the parent object and using onTerminate to capture a strong reference to the stream's object I can make cancellation deinit the stream's object properly. My test harness has a task group with 2 tasks that call the same stream property on its containing object (to make sure the first stream is cancelled). I can only have one stream working at a time despite the object's stream property being a computed var that returns a new stream, so I wanted it to also cancel a previous stream.