Does anyone know how to convert a `KotlinCValue&lt...

# kotlin-native

[Darron Schall]

Does anyone know how to convert a KotlinCValue<AnyObject> from a KMM library to the actual CValue type when using it in Swift?

(I'll turn this into a thread; the question setup / background is a little complicated.)

[Darron Schall]

Background:

Shared Library Setup

I have a KMM project with a shared library interacting with Android and iOS mapping frameworks. In the shared library, I have a simple Location Kotlin data class abstraction that looks like this:

// commonMain / Location.kt
data class Location(val longitude: Double, val latitude: Double)

I also have an iOS extension of Location to help interact with `platform.CoreLocation`:

// iosMain / LocationExtensions.kt
fun Location.asCoordinate(): CValue<CLLocationCoordinate2D> = CLLocationCoordinate2DMake(latitude, longitude)

This works pretty well in the shared library. Any time the platform CoreLocation or MapKit needs a CLLocationCoordinate2D, the CValue<CLLocationCoordinate2D> is the right type to use in Kotlin. I've paired this simple data class with a simple abstracton for map annotations:

// commonMain / MapMarker.kt
expect class MapMarker(location: Location)

The MapMarker is an expect / actual so that I can feed these obejcts directly to Apple's MapKit. The iOS actual for MapMarker looks somewhat like this (trimmed for brevity):

actual class MapMarker actual constructor(
    private val location: Location
) : NSObject(), MKAnnotationProtocol {
    override fun coordinate(): CValue<CLLocationCoordinate2D> {
        return location.asCoordinate()
    }
}

Usage in Swift

On the Swift side, I'm able to pair MapMarker directly and easily without issue with MKMapView, like this:

// Simple data setup for illustration purposes,
// this returns an array of `MapMarker` in the shared
// library that we're able to treat as an array of
// `[MKAnnotation]` from within the iosApp.
var annotations: [MKAnnotation] = MockData().mockAnnotations

// It's straightforward to display the annotations
let mapView = MKMapView()
let pointAnnotationViews = annotations.map {
	let pointAnnotation = MKPointAnnotation()
	// Note: MKAnnotation `coordinate` delegates to our
	// shared `Location` `asCoordinate` helper, which
	// returns a `CLLocationCoordinate2D`, as expected.
    pointAnnotation.coordinate = $0.coordinate
    return pointAnnotation
}
view.addAnnotations(pointAnnotationViews)

The problem I'm having is that the CValue<CLLocationCoordinate2D> Kotlin return type is treated as a CLLocationCoordinate2D in Swift from within the MKAnnotationProtocol context (a class extending NSObject). But, the same return type from a Kotlin class (not extending NSObject), doesn't return a CLLocationCoordinate2D. Instead, I (unexpectedly) get a `KotlinCValue<AnyObject>`:

// in Swift, this returns a KotlinCValue<AnyObject>
var value = MockData().defaultMapLocation.asCoordinate()

Again, that's not a CLLocationCoordinate2D, and it won't cast to one. I can't figure out how turn it into one, either; I'm new to Kotlin/Native cinterop and my understanding is still incomplete. Maybe I need to use Unmanaged like the comments from https://youtrack.jetbrains.com/issue/KT-39407/KotlinNative-Swift-interop-iOS-CGImage suggest... I tried a few different iterations of that approach, unsuccessfully.

Back to the original question

What's the best "right" way to get the CLLocationCoordinate2D out of the KotlinCValue<AnyObject>? Or, to ask it another way, how can I have Location.asCoordiante() return a CLLocationCoordinate2D that Swift can use?

I think this is related to improving Core Foundation support per https://youtrack.jetbrains.com/issue/KT-29256/Improve-support-for-Core-Foundation-types since Core Location is another C framework.

I think the "right" way would be to convert my Location to an expect / actual that extends NSObject in iosMain, and conforms to a protocol for "asCoordinate" that I need to make and declare in a nativeinterop def file (so that I can send the message to the NSObject to prevent "unrecognized selector sent to instance" errors, since NSObject subclasses appear to lose method definitions in the generated header file).

But, what I'd really like to have happen is have this "just work" as-is keeping Location as a Kotlin data class, maybe with improved compiler support. It's unexpected to me that returning CValue<CLLocationCoordinate2D> "works" sometimes, but not others, and the documentation and discoverability isn't good enough yet to sort through it.

For what it's worth, I've also found a different workaround doing a little more on the Kotlin side, but I don't think it's treating memory correctly (specifically, I'm not sure if the MemScope() below gives me an auto-released object that ARC captures on the Swift side of things to keep it around):

// In commonMain LocationExtensions.kt - Exchange the CValue for a CPointer.
fun Location.asCoordinatePointer(): CPointer<CLLocationCoordinate2D> = asCoordinate().getPointer(MemScope())

I can get the CLLocationCoordinate2D in Swift by calling this as location.asCoordinatePointer().load(as: CLLocationCoordinate2D.self).. but, again, this doesn't feel quite right to me.

Another workaround, which I don't like, is to re-declare the method in Swift:

// Location+Extensions.swift
extension Location {
    func asCoordinate() -> CLLocationCoordinate2D {
        return CLLocationCoordinate2D(latitude: latitude, longitude: longitude)
    }
}

This ends up making two asCoordiante() methods; the Kotlin one returning KotlinCValue<AnyObject> and the Swift one returning CLLocationCoordinate2D, and the compiler picks the right one based on the return type. This also duplicates the code itself (making the CLLocationCoordinate2D), and feels kind of gross.

I guess a final way of asking this is: What is special about the NSObject(), MKAnnotationProtocol setup that let's me treat returning CValue<CLLocationCoordinate2D> as a CLLocationCoordinate2D in Swift, when a Kotlin data class that returns a CValue<CLLocationCoordinate2D> returns it to Swift as KotlinCValue<AnyObject>?

[Jonas Hiltl]

Hi, I encountered the exact same issue. Did you manage to return a CValue<CLLocationCoordinate2D> from the asCoordinate function inside the compiled objective-c code? I also posted my specific question here stackoverflow, I have pretty much the same setup as you did.

[Darron Schall]

No, I never did figure this one out unfortunately. I'm still using the Swift extension workaround.

[Jonas Hiltl]

Ok, thanks for the quick reply. If I figure it out, I will let you know.