How to convert an atypical completion handler to Swift Concurrency

Question

I have in my code some asynchronous method written with a completion handler, and I want to convert it to the new Swift Concurrency syntax.

Usually this is simply a matter of wrapping the call in withCheckedThrowingContinuation and calling either resume(throwing:) or resume(returning:) instead of, respectively completion(nil, error) or completion(value, nil) .

However I have in my code a method that may call the completion handler with a value AND an error (the use case is: outdated cache, but use the value if.network is unreachable), aka completion(value, CacheError.outdatedCache) .

What approach may I use to convert this to a throwing async method?

Answer 1

There are many alternatives. Here are a few:

1. You could have a non-throwing rendition that returns a Response :

    enum Response<Value, Failure: Error> { case success(Value) case failure(Failure) case failureWithCachedValue(Failure, Value) }

2. You could have non-throwing rendition that returns a tuple of (Foo?, Error?) .