[Edit December 18th: Swift Playground version - Should work on iPad with the latest Swift Playgrounds app: UKCovidData.swiftpm.zip

Note that because of the lag in local level figures by age which the app focuses on it isn't especially useful for tracking the current situation right now because the Omicron growth is so fast that week old data may be four doublings behind - maybe a factor of 16 off. It is still potentially useful for looking at the historic data and will catch up with the picture as Omicron levels off - as it must because the whole population will have had it pretty soon if current growth rates continue.]

[Edit November 10th: App link updated to fix a crash. Source code not currently updated because I can't currently participate in open source. macOS 12 is no longer in beta so it should be available to most people that way.]

A small app providing some data about case rates at the most local level available in England for particular age groups.

This app didn’t really started as planned for release (and it ended up that I couldn’t release on iOS) at least. The intention was partly to play with Swift’s structured concurrency in a real context and partly to get the UK cases data into a form where I could handle it and update it more easily than I was managing with a spreadsheet.

If you want to try the app you either need to have a Mac with macOS Monterey (12.0) which is currently only in Beta release (download app here) or to have Xcode to build and install and install it from source code on your iPhone or iPad (Github project).

Using the App

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One of the interesting things from WWDC this year in not just that Swift is adopting AsyncSequence but that Apple is fully behind it rather than further expanding support for Combine. This raises some interesting questions for the future of Combine, whether it is just destined to be a niche feature just supporting SwiftUI in particular ways, gets completely deprecated in future. This series (possibly just pair) of blog posts is going to compare the two by means of implementing mappers in both ways between them.

[Update: Marin Todorov has a nice post about the reverse if you want to get ahead on that.] 

if you want to have

While I haven't had the opportunity to use it in a real project yet I'm really liking the look of AsyncSequence and the whole of the structured concurrency approach.

These mappers will be implemented as raw conformances because the point is for me to learn and understand more. There may be better, more efficient or simpler approaches but I want to look into the details of how they work. I'll follow up if I find significantly better ways to do it.

Resuming continuations from actors doesn't work (Beta1-2)

One thing that has delayed this post is having hit some non-deterministic bugs I lost confident in my understanding of the Structured Concurrency and didn't want to share widely until I had established that the issues I was seeing were related to Swift/MacOS bugs rather a deadlock I was introducing by doing something substantially wrong.

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[Update: Compiler but in Xcode 13 beta 1 confirmed. See below update]

[Update2: New post up with some rough and ready benchmarking of AsyncSequence file reading using URLSession]

So what I wanted to know was how to efficiently and asynchronously read a file into a Data object using the new structured concurrency approach in Swift. The short version is that I didn't manage to get the async approaches to file reading working, not sure yet if I was doing it wrong or there are currently problems with the frameworks (I have filed a bug report with Apple - FB9177012). Either way the async approaches were crashing for me. And I do welcome responses letting me know what I'm doing wrong.

I couldn't see any exact APIs that looked right for asynchronously reading a file. The Meet AsyncSequence talk from WWDC gave one possibility although it didn't look very efficient of handling the URL as an AsyncSequence of bytes but I thought I would do some investigation to check. I later as you can see at the end of this post realised that URLSession provides the method I need for a one shot request (that also wasn't working). I still would like to test how the performance of all these approaches varies.

Update

Confirmation that it is a beta 1 from someone who worked on some of the AsyncSequence stuff at Apple.

Yeah there’s a compiler bug in b1, you can use the URLSession version for now. Sorry about that

— David Smith (@Catfish_Man) June 16, 2021

(Specifically, public actors don’t get along well with library evolution mode)

— David Smith (@Catfish_Man) June 16, 2021

End update

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