Concurrency, Combine, and Swift 5.5

I started a post that brings together all the moving parts that have been discussed in the various concurrency proposals that are going into Swift 5.5. They’re all accessible through GitHub, and the discussions in the public forums. The combined view of all the moving parts is complex. I was aiming to post something this weekend, but in the end I trashed the draft after I read Paul Hudson’s What’s New in Swift 5.5. He has outdone himself with a beautiful job of describing, and pulling together, all the moving parts, and I think there’s little at this stage that I could add — anything I wrote would be a poor shadow, repeating the work he put into that overview. If you haven’t yet read it, do so. It’s a stellar overview and detailed description (with examples!) of a lot of the moving parts that we’ll see this year. And I’m sure it sets the foundation for quite a bit more to come!

I started closely following the updates to the Swift language, partially because there’s an obvious overlap with what has been proposed and how Combine operates. Some of what/how Combine does it’s processing is getting getting pushed down into the language, which is frankly amazing. I don’t know how Apple’s Combine framework will evolve, but I fully expect it to embrace the async/await foundation, as well as the iteration, sequence, and stream-like structures that are in flight. There’s a massive collection of concurrency proposals and work, and if that alone was what Apple announced and shared at WWDC, it would be huge. I’m guessing that it’s only the foundation for quite a lot of other elements, as yet to be seen. So yeah – I’m one of the people thinking “This year’s gunna be a big one!” when it comes to WWDC announcements.

For me, the portion of the concurrency proposals that’s been most intriguing, exciting, and perhaps a little scary, is the addition of Actors and Global Actors. Building on top of the concurrency features, they provide some amazing coordination and memory-safety constructs that make it far easier to write safe code for an increasingly multi-core system. And I suspect that Actors aren’t going to be limited to a single device, but that it’ll be extended to communicating and coordinating across devices. There’s nothing yet in the public proposals, but I think there’s still quite a bit more coming.

I’m looking forward to these hitting the streets, and (hopefully) API use of Global Actors to help provide compiler-enforced expectations around callbacks and functions that need to interact on the main thread. Yeah – mostly that’s UI updates. The existing class of bugs related to UI updates on macOS and iOS have been helped by warnings from the amazing Clang Thread Sanitizer, but now it can be enforced – in Swift at least – within the compiler. I also expect a surge of complexity due to the annotations and new keywords for concurrency. I suspect it may be overwhelming for a lot of developers, especially new developers getting involved in the platform. While you probably don’t need it to learn swift, I’m guessing that it’ll be pretty front and center to a lot of core app development. And I wouldn’t be surprised to see a lot of discussion, and confusion, around the techniques of linking the earlier Cocoa delegate call-back style into async methods and functions. I think it may be challenging – at least at the start.

Like any new tool, I predict that Actors will get over-used in the near future, maybe for quite a while, before collapsing back to a more sane level of usage in constructing software. In any case, I think it’ll change a lot of conversations about how current software components work together. I hope that the concurrency elements come with equally excellent testing infrastructure and patterns. I’m still bummed that Combine-Schedulers wasn’t something that was included with the Combine framework directly, or at least with XCTest, while at the same time being immensely grateful to the Point•free crew for making that library available.

I’m not going to make any other WWDC predictions, and I’m clearing my mental decks for the conference updates and details to come. It’s getting about time for “sponge learning” mode. I’m looking forward to seeing what so many worked on, and tremendously excited about the potential this year’s Swift language updates are setting up.

Published by heckj

Developer, author, and life-long student. Writes online at

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