[RESOLVED :white_check_mark:] Hi ! I am writing a shared interface with a suspend function. In iOS i...

# multiplatform

[Nicolas Verinaud]

[RESOLVED ✅] Hi ! I am writing a shared interface with a suspend function. In iOS it generates a protocol with a method with a completionHandler which is great. Problem is how can I handle cancellation ?

My shared interface :

interface CanRetrieveAuthorizationToken {
    suspend fun getToken(): String
}

The corresponding protocol :

public protocol CanRetrieveAuthorizationToken {
   
    /**
     @note This method converts instances of CancellationException to errors.
     Other uncaught Kotlin exceptions are fatal.
    */
    func getToken(completionHandler: @escaping (String?, Error?) -> Void)
}

[mbonnin]

You can look at https://github.com/rickclephas/KMP-NativeCoroutines

[Nicolas Verinaud]

Interesting, thanks @mbonnin. Problem is, my interface is not used by native code (implemented by though), so the native code does not need the cancellation mechanism, it assumes the implementation can manage the cancellation correctly.

[Nicolas Verinaud]

Maybe I should pass the cancellation mechanism to the getToken method ? Or add a cancelGetToken method to the interface ?

[mbonnin]

Not an expert by any means but the above library integrates with RxSwift and/or Combine so I guess it has a way to handle cancellation from them and if you don't want to use these frameworks, it should be doable too. Maybe not easy though

[Nicolas Verinaud]

No because the suspend function is not used by swift but implemented by swift. The swift code needs a way to know when to cancel its job.

[mbonnin]

Ah got it!

[mbonnin]

Maybe expose something that uses a CancellableContinuation instead?

[Nicolas Verinaud]

Great, I check this 😉

[Nicolas Verinaud]

Okay, I ended up with the following solution : I did transform the getToken method like this :

fun getToken(onSuccess: (String) -> Unit, onError: (Throwable) -> Unit): Disposable

This allows swift code to be cancellable by returning an instance of Disposable which is a class I created inspired by C# (wrapping a method executed when the disposable is disposed aka cancelled).

I then wrapped the call to the new getToken method in a suspend function using suspendCancellableCoroutine to make it usable alongside other suspending function (in my case http calls using ktor). More info here : https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/suspend-cancellable-coroutine.html.

Thanks @mbonnin for the hint about CancellableContinuation 🙌

[Nicolas Verinaud]

Merci cher confrère 😉