I recently experienced a crash on iOS with the fol...

# serialization

[Justin]

I recently experienced a crash on iOS with the following message:

Instances of kotlin.Error, kotlin.RuntimeException and subclasses aren't propagated from Kotlin to Objective-C/Swift.

Other exceptions can be propagated as NSError if method has or inherits @Throws annotation.

Uncaught Kotlin exception: kotlinx.serialization.MissingFieldExcpetion: Field 'deviceId' is required, but it was missing

As the message suggests, the KMP library failed to deserialize to:

@Serializable data class Details(val id: String, val deviceId: String)

...because the JSON string didn't contain a deviceId field, hence the crash on the iOS app using the library.

I have already fixed this specific crash by modifying the model as follows:
@Serializable data class Details(val id: String, val deviceId: String? = null)

But my question is this:

Is there a way to have kotlinx.serialization propagate any/all exceptions it throws to the implementing platform (iOS/Android/Web/etc.) to decide how to handle it?

In this case, it would have been preferable to have caught this error in iOS, logged it using the iOS logging service, and not-crashed.

Any ideas would be much appreciated, thanks!

[Sam]

I'm not going to claim that this is the best idea but it has worked for me. I use a custom sealed`Result` class for kotlin code that can generate an exception. The exceptions are caught and then returned in a Result.Failure object. For my iOS code, I have some extension functions on Result.Failure that will convert the Exception to an NSError

The Result class is a stripped down version of the one found at https://github.com/michaelbull/kotlin-result.

The extension to make an NSError object isn't too complicated though you probably want to make other extensions for your own custom exceptions.

fun <V, E : Error> Result<V, E>.nserrorOrNull(): NSError? {
    val err = this.failureOrNull() ?: return null
    val localizedDescription = err.message
    val userInfo = mapOf<Any?, Any?>(NSLocalizedDescriptionKey to localizedDescription)
    return NSError("my app core", -20, userInfo)
}

[sandwwraith]

I've created an issue for that: https://github.com/Kotlin/kotlinx.serialization/issues/564 I'll see what can be done. Probably annotating all StringFormat members with @Throws would be enough to solve the problem