KMM iOS crashes because `Flow` does not correct throw, Exception doesn't match @Throws-specified class

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I have the following method returning Flow

@Throws(Throwable::class) fun getPokemon(): Flow<PokemonList> {
        try {
            return flow {
                throw Exception("Hello")
                emit(api.getFirstPagePokedex())
            }
        } catch (e: Throwable) {
            throw e
        }
    }

where api.getFirstPagePokedex is a suspend fun with signature suspend fun getFirstPagePokedex(): PokemonList.

When I call getPokemon() in Swift, I'm faced with this error

Exception doesn't match @Throws-specified class list and thus isn't propagated from Kotlin to Objective-C/Swift as NSError.
It is considered unexpected and unhandled instead. Program will be terminated.
Uncaught Kotlin exception: kotlin.Exception: Hello

If I move throw Exception("Hello") outside flow block then the iOS version works fine. But it would not catch the error from api.getFirstPagePokedex() as it should.


@Throws(Throwable::class) fun getPokemon(): Flow<PokemonList> {
        try {
            throw Exception("Hello")
            return flow {
                emit(api.getFirstPagePokedex())
            }
        } catch (e: Throwable) {
            throw e
        }
    }

How can we catch exception from flow block correctly for Swift?

Please note that all cases works as expected on Android with no crash.

kotlin-multiplatform kotlin swift
1 Answers

When you call getPokemon(), all you're doing is creating the flow. The code inside the flow doesn't get executed until later, when you collect the flow.

You can actually remove the @Throws annotation from getPokemon(), as that method itself won't throw the exception. You should instead add the annotation to any method where you collect items from the flow.

user_46183310
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