posts/converting-closure-based-code-into-async-await-in-swift/

[utterances-bot]

Converting closure-based code into async/await in Swift • Andy Ibanez

https://www.andyibanez.com/posts/converting-closure-based-code-into-async-await-in-swift/

[kimfucious]

This is a GREAT article. Thanks for the write up.

I am baffled sometimes when implementing this on occasion. Sometimes if just works, and other times I get an error.

Here's an example of when it doesn't work:

func getAppleMusicUserToken() async throws -> String {
    return try await withCheckedThrowingContinuation ({ (continuation: CheckedContinuation<String, Error>) in
        do {
            let tokenProvider = DefaultMusicTokenProvider()
            let options = MusicTokenRequestOptions()
            let devToken = try await tokenProvider.developerToken(options: options)
            let userToken = try await tokenProvider.userToken(for: devToken, options: options)
            continuation.resume(returning: userToken)
        } catch {
            print(error)
            continuation.resume(throwing: error)
        }
    })
}

The error is:

Cannot pass function of type '(CheckedContinuation<String, Error>) async -> Void' to parameter expecting synchronous function type

I know I'm doing something, dumb, but I just don't know what it is!

Any tips?

[kimfucious]

Dumb follow up: Why not just do this, or am I missing the point?

func getAppleMusicUserToken() async throws -> String {
        do {
            let tokenProvider = DefaultMusicTokenProvider()
            let options = MusicTokenRequestOptions()
            let devToken = try await tokenProvider.developerToken(options: options)
            print("devToken \(devToken)")
            let userToken = try await tokenProvider.userToken(for: devToken, options: options)
            print("userToken \(userToken)")
            return userToken
        } catch {
            print(error)
            throw error
        }
}

[AndyIbanez]

@kimfucious The example with the new Apple Music framework is not a good example because it is already async/await. Checked continuations help you migrate non async await code into async await code. You won't find many of these within Apple's SDKs, but very likely in third party libraries.

[kimfucious]

I knew I was being dumb, thanks!

[zhouhao27]

Is it possible to use async/await in iOS version earlier than iOS 15?

[alexryakhin]

Hey, I can't find any examples how to transform functions that have @espaping closure with empty parentheses. These functions usually do something and then call completion. If I make it async, it would return Void. And another problem is I tried to use continuations, like Apple gave an example in AsyncSequence WWDC video. But I use optional handlers as properties, and in my VC I create a class that contains those properties lazily and initialize handlers inside this block. Like this:

class Manager {
   var handler: (() -> Void)?

   func doSomething() {
      doSomethingElse()
      handler?()
   }

   func doSomethingElse() {
      ...
   }
}

class ViewController: UIVIewController {

   lazy var manager = {
      let manager = Manager()
      manager.handler = {
         doSomerhing()
      }
      return manager
   }()
}

[AndyIbanez]

@alexryakhin if you are talking about callbacks that are called after something is done executing, wouldn't you just put the code underneath the async call in the async/await version?

class Manager {
   var handler: (() -> Void)? // you would get rid of this

   func doSomething() async {
      doSomethingElse()
      //handler?()
   }

   func doSomethingElse() {
      ...
   }
}

let myManager = Manager()
await myManager.doSomething()
// Write the code you'd put inside the handler here...

Unless of course I'm misunderstanding something in your question.

[shurale85]

in chooseContactTouchUpInside you did just async block: async {...} I haven't seen such construction in previous articles. Can u pls give some explanation of it?

[AndyIbanez]

in chooseContactTouchUpInside you did just async block: async {...} I haven't seen such construction in previous articles. Can u pls give some explanation of it?

Thanks for pointing that out. It should be Task { ... } instead of async {...}. Earlier betas used the async {} construct instead of Task.

[jhokit]

Great article Andy, saved me a lot of time. 👏🏻