I'm working on a KMM library, using cinterop and h...

# multiplatform

[Ciaran Sloan]

I'm working on a KMM library, using cinterop and having some issues when I export my library to a framework 🧵

[Ciaran Sloan]

I cant seem to export a typealias to the headers of my framework. Heres an example to demonstrate:

Consider an expect interface is defined in `commonMain`:

// commonMain
expect interface MyCoolInterface

And the implementations look like this:

// androidMain
actual interface MyCoolInterface

// iosMain
actual typealias MyCoolInterface = MyCinteropProtocol

Neither MyCoolInterface nor MyCinteropProtocol is exported to the headers.

I've found this YouTrack issue which confirms that this is not yet supported. However, wondering if anyone has any suggestions for working around this?

[Ciaran Sloan]

What I've tried:
• Defining MyCoolInterface as an interface on iosMain (rather than a typealias) (fails with error Only classes are supported as subtypes of Objective-C types)
• Defining MyCoolInterface as a class in all modules. (fails with error Kotlin implementation of Objective-C protocol must have Objective-C superclass (e.g. NSObject)

[Landry Norris]

What is the end goal? Are there any common methods of the interface that you want to be able to call from common code? Are you trying to detect if an object implements this interface?

[Ciaran Sloan]

Yes I want to detect if an object implements this interface. My KMM library MyLibrary exposes the interface MyInterface which can be implemented in the common code of a KMM application MyApp. I'm also writing some swift utilities in a separate swift library SwiftUtils which is dependant on the MyLibrary framework, and uses MyInterface for example:

class MySwiftClass {
    func doWorkWith(_ object: MyInterface) {}
}

The issue I have is that the ObjC declaration of MyInterface is slightly different in MyApp (iOS) vs SwiftUtils (iOS), I suspect due to how the frameworks are generated. (they have different prefixes).

So I'm trying to generate the interface in ObjC, then using cinterop add it to both the library and the swift utils. Hopefully this makes sense!

[Ciaran Sloan]

I guess the other alternative would be if theres a way to specify what the ObjC name of a class/interface should be?

[Ciaran Sloan]

or if theres a way to prevent the baseName prefix being added to transitive dependencies?

[Rick Clephas]

Your cinterop approach sounds good. Why do you need the KMM library to expose the ObjC protocol?
Can't you add a dependency to the ObjC code from your Swift library?

[Rick Clephas]

FYI in Kotlin 1.8.0 you can control the exact names with the @ObjCName annotation.

[Rick Clephas]

Though I think in this case the cinterop approach would be better.

[Ciaran Sloan]

thanks @Rick Clephas! I need both the swift library and the app implementing the KMM library to have access to the same protocol. I did not know about @ObjName that might solve my problems 🤔

[Ciaran Sloan]

my issue however is when I implement the KMM library in a KMM app, the protocol gets named SharedModuleNameProtocolName (so long as i set transitiveExport = true) but in the swift library its getting named LibraryNameProtocolName . Hence I started looking into cinterop to get around this

[Rick Clephas]

In that case you could depend on an ObjC library with the protocol from KMM and Swift. I did something similar with KMM-ViewModel.

[Rick Clephas]

KMMViewModelCoreObjC declares a protocol (KMMVMViewModelScope) which is used in Kotlin via cinterop and in the Swift library KMMViewModelCore.

[Ciaran Sloan]

got it thanks for the reference. I think thats what I'm trying to do actually

[Ciaran Sloan]

I'll go back and take a look at where I might be going wrong 🙌

[Ciaran Sloan]

nice to know of the @ObjCName annotation coming up in 1.8 though!

[Rick Clephas]

Indeed, I think it can be quite useful (especially combined with the @HidesFromObjC and @ShouldRefineInSwift annotations)!

[Roberto Fernandez Montero]

I've been reading this @Ciaran Sloan, were u able to make this work? I'm a bit lost with ShouldRefineInSwift annotation..

[Ciaran Sloan]

hey Roberto 👋🏼 I was able to get c-interop working for me, but it unfortunately didn't suit my use case here for what I was trying to achieve. I didn't end up needing the annotations