How to properly cancel Swift async/await function

Question

I have watched Explore structured concurrency in Swift video and other relevant videos / articles / books I was able to find (swift by Sundell, hacking with swift, Ray Renderlich), but all examples there are very trivial - async functions usually only have 1 async call in them. How should this work in real life code?

For example:

...
        task = Task {
            var longRunningWorker: LongRunningWorker? = nil

            do {
                var fileURL = state.fileURL
                if state.needsCompression {
                    longRunningWorker = LongRunningWorker(inputURL: fileURL)
                    fileURL = try await longRunningWorker!.doAsyncWork()
                }

                let urls = try await ApiService.i.fetchUploadUrls()

                if let image = state.image, let imageData = image.jpegData(compressionQuality: 0.8) {
                    guard let imageUrl = urls.signedImageUrl else {
                        fatalError("Cover art supplied but art upload URL is nil")
                    }

                    try await ApiService.i.uploadData(url: imageUrl, data: imageData)
                }

                let fileData = try Data(contentsOf: state.fileUrl)
                try await ApiService.i.uploadData(url: urls.signedFileUrl, data: fileData)

                try await ApiService.i.doAnotherAsyncNetworkCall()
            } catch {
                longRunningWorker?.deleteFilesIfNecessary()
                throw error
            }


        }
...

Then at some point I will call task.cancel() .

Whose responsible for cancelling what? Examples I've seen so far would use try Task.checkCancellation() , but for this code that line should appear every few lines - is that how it should be done?

If API service uses URLSession the calls will be cancelled on iOS 15, but we don't use async variant of URLSession code so we have to cancel the calls manually. Also this applies to all the long running worker code.

I am also thinking that I could add this check within each of async functions, but then basically all async functions would have the same boilerplate code which again seems wrong and I haven't seen that done in any of the videos.

EDIT: I have removed callback calls as those are irrelevant to the question.

Answer 1

There are two basic patterns for the implementation of our own cancelation logic:

1. Use withTaskCancellationHandler(operation:onCancel:) to wrap your cancelable asynchronous process.

   This is useful when calling a cancelable legacy API and wrapping it in a Task . This way, canceling a task can proactively stop the asynchronous process in your legacy API, rather than waiting until you reach a manual isCancelled or checkCancellation call. This pattern works well with iOS 13/14 URLSession API, or any asynchronous API that offers a cancelation method.

2. Periodically check isCancelled or try checkCancellation .

   This is useful in scenarios where you are performing some manual, computationally intensive process with a loop.

   Many discussions about handling cooperative cancelation tend to dwell on these methods, but when dealing with legacy cancelable API, the aforementioned withTaskCancellationHandler is generally the better solution.

So, I would personally focus on implementing cooperative cancelation in your methods that wrap some legacy asynchronous process. And generally the cancelation logic will percolate up, frequently not requiring additional checking further up in the call chain, often handled by whatever error handling logic you might already have.

Answer 2

Examples I've seen so far would use try Task.checkCancellation() , but for this code that line should appear every few lines - is that how it should be done?

Basically yes. Cancellation is a totally voluntary venture. The runtime doesn't know what cancellation means for your particular task, so it just leaves it up to you. You look at Task.isCancelled , or, if your intention is to throw just in case the task is cancelled, you can call Task.checkCancellation .

Note that if, within your task, you are calling (with try ) any async material that throws when cancelled, you do not need to any cancellation work with regard to that material, because when it throws due to cancellation, you will throw due to cancellation automatically.

---

Having said all that, I have to add, as a footnote, that your code is extremely strange. Callbacks and async/await are opposites ; the idea that you would do a do/catch and call a callback within a Task is extremely weird and I would advise against it. You are basically negating all the advantages of a Task by doing that, as well as making untrue the thing I just said about the throw trickling up and out of your task.