Hi there, my team has been working on a multiplatf...

# multiplatform

[Deniz Tumer]

Hi there, my team has been working on a multiplatform app for around a year now. We've run into a bit of a limitation that we can't seem to overcome. Was wondering if someone might be able to point us in the right direction.

We're using Firebase for our authentication layer (using the respective cocoapod and java packages for the iOS and JVM targets) and have an expect/actual in front of the SDK to provide a common interface between the platforms. Inside of the Firebase library for iOS there's a few places where an NSException can be raised and it doesn't seem like there's a way to "catch" that exception either in the iosMain code or propagate the error out to the Swift side of our codebase outside of our shared Kotlin framework.

Are we missing something simple here or is there a gap in Kotlin Native where NSException can't be implicitly wrapped into a Kotlin exception type that can be caught and re-thrown as needed?

[russhwolf]

I'm not sure why you wouldn't be able to catch from the Kotlin side, but you can add a @Throws annotation to the relevant methods, and they'll be exported as throws so you can try/catch from Swift.

[Deniz Tumer]

Yeah I tried that with @Throws(Throwable::class) but our app is crashing inside of the try/catch block. Seems like the NSException isn't being captured as an exception in Kotlin?

[russhwolf]

yeah, sounds like something isn't wired up right but I couldn't guess at what the issue is without being in the codebase

[Deniz Tumer]

I'm currently trying to opt into foreign exceptions to see if that could solve this? Seems like there's a beta api that needs to be opted into for wrapping NSExceptions in a ForeignException object?

extraOpts = listOf("-Xforeign-exception-mode", "objc-wrap")

[Jeff Lockhart]

NSException isn't possible to catch in Kotlin. It's also not possible in Swift. NSError can be propagated though. NSError is more commonly used for non-fatal errors in Objective-C, which can be handled in Kotlin.

[Deniz Tumer]

Right I think the issue here though is that we're using an external SDK so I don't necessarily have control over whether the NSException is thrown.

From the issue you posted it seems like these should still be propagated if marked with @Throws?

[Deniz Tumer]

I can confirm that the following opt in works

extraOpts = listOf("-Xforeign-exception-mode", "objc-wrap")

This allows me to do the following in kotlin iosMain module:

try {
  // some code that throws NSException
} catch (e: ForeignException) {
  // Catch!
}

[Deniz Tumer]

Seems like this is an api that's in Beta though. Not sure if it's just newer or if there's some known issues with it. Seems like it does exactly what I'm looking for

[Jeff Lockhart]

Annotating a Kotlin function with @Throws will create an Objective-C header with an NSError parameter. But you're working in the opposite direction, needing to handle an Objective-C error in Kotlin. The typical pattern for this is for the Objective-C code to similarly allow for passing an NSError reference to propagate any error with.

[Deniz Tumer]

Yeah I can't change the objective-c code though as it's a library owned by Google: https://github.com/firebase/firebase-ios-sdk

Ideally yeah that makes sense that NSError would be easier to work with, just in this circumstance I don't have that flexibility

[Jeff Lockhart]

That's great if the objc-wrap opt-in works around this for your use case here. It'd be good to communicate this on YouTrack so JetBrains is aware of your experience.

[Jeff Lockhart]

Maybe Google wraps the Objective-C NSException throwing code to handle as NSError in Swift somehow. Otherwise this would similarly be an issue using the APIs from Swift.

[Jeff Lockhart]

Note that the opposite reversed translation is not implemented yet: Swift/Objective-C error-throwing methods aren't imported to Kotlin as exception-throwing.

To adapt Objective-C NSError producing code, for APIs that handle errors this way, I created this useful wrapper that converts NSError results into a Kotlin Exception and throws. Example usage.

[Jiri Bruchanov]

hey guys, where do I put the extraOpts = listOf("-Xforeign-exception-mode", "objc-wrap") ?
if it's crashing from ios code directly, not from cocoapod 3rd party lib

[Deniz Tumer]

@Jiri Bruchanov I put this into the build gradle for the shared module inside the pod definition. Here's an example from our codebase:

pod("FirebaseAuth") {
    version = libs.versions.cocoapods.firebase.get()
    extraOpts = listOf("-Xforeign-exception-mode", "objc-wrap")
}

[Jiri Bruchanov]

@Deniz Tumer
yeah that would work for your case...
but I don't have any 3rd party cocoapod lib

[Deniz Tumer]

Oh apologies yeah that is if the code breaks in external lib. If it's in code that you own I would probably use @Jeff Lockhart's suggestion above and replace usages of NSException with NSError

[Deniz Tumer]

^ @Jiri Bruchanov

[Jiri Bruchanov]

@Deniz Tumer
Thanks, but I dont control the code. Its just regular ios api, not 3rd party lib.

[Deniz Tumer]

Oh whoa yeah that's tough maybe there's a way to add something into the iosMain module to capture this? I'm not experienced enough with this yet to know where it goes. Are there compiler options we can use when setting up a kotlin source set?

[Jiri Bruchanov]

I was wondering, if objc_begin_catch and objc_end_catch functions could be somehow useful,
but I'm terrible with the cinterop, so not sure what, how to create argument for the begin_catch