Comment by cube2222

Found an interesting article about the internals of async specifically[0], also the docs[1].

TLDR: Seems like it uses a global thread pool, where each async method call yields to the runtime. Additionally, you get actors which are special classes with serialized method calls, and everybody other than itself has to call those methods as async. Additionally, you can have custom executors, who, if I understand correctly, still only pass tasks to the global thread pool, but can influence the scheduling order. Finally, you can force functions/actors to run on the main thread (outside of the thread-pool) as iOS requires the UI to run on the main thread.

Interestingly (from an API evolution perspective), async protocol functions can be implemented in classes by synchronous functions, as async in swift means "this could be async". Which plays well with the fact that async functions can call sync ones, but not vice-versa.

Additionally, it has cooperative cancellation (you can basically check if you've been cancelled and react accordingly). Concurrency is by default structured, and cancelling a parent task will cancel its children.

[0]: https://swiftrocks.com/how-async-await-works-internally-in-s...

[1]: https://docs.swift.org/swift-book/documentation/the-swift-pr...