Hi, I have a question about Kotlin Int to Swift an...

# touchlab-tools

[François]

Hi, I have a question about Kotlin Int to Swift and SKIE

//  On swift side is Int32 (OK)
    public var test: Int = 0

    // On swift side is SkieSwiftOptionalStateFlow<KotlinInt> (WHY?)
    private val _intNullValue = MutableStateFlow<Int?>(null)
    public val intNullValue: StateFlow<Int?> = _intNullValue

    //  On swift side is SkieSwiftStateFlow<KotlinInt> (WHY?)
    private val _intNotNullValue = MutableStateFlow(0)
    public val intNotNullValue: StateFlow<Int> = _intNotNullValue

So why the stateflow is not SkieSwiftStateFlow<Int32>? same thing for other integer types.

[Tadeas Kriz]

SKIE is currently not doing changes to how generic types are represented. When you have StateFlow<Int> in Kotlin, SKIE makes it available using SkieKotlinStateFlow<Int> which produces SkieKotlinStateFlow<KotlinInt> for Objective-C/Swift. Then we bridge that to SkieSwiftStateFlow, but KotlinInt has no direct bridging. So SKIE would have to manually map the type in runtime which has added complexity. This is on our list of possible features, but that list is long, so much to do 😄

[François]

I see, currently it’s kind of downgrade, as by default the Kotlin int is replaced by ObjC Int32 automatically, but when I’m using the StateFlow is not.
So, I need to do the job manually, I don’t want to know I’m woking with KotlinType.
Let’s wait and see 😄

[Tadeas Kriz]

Kotlin's Int is replaced by Int32 only when it's not in places where NSObject type is required (generics, some lambdas etc)

[François]

Yes, I understand that case. I’m using it inside ViewModel, I don’t have this issue. I just want to avoid using KotlinType.

[Tadeas Kriz]

I might be misunderstanding. Is there a different solution that allows you to use StateFlow with Int32 from Swift? We could take inspiration from it

[François]

Oh, my bad. Inside a Shared Kotlin ViewModel
The Kotlin StateFlowInt is converted to a SkieSwiftStateFlowKotlinInt.
If I’m using a directly a Kotlin Int, it’s converted to a ObjC int32_t, the compiler use the expected type.

[Tadeas Kriz]

I see, but then you can't observe its changes, so it's either Int32 but no observation, or KotlinInt but observing, correct?

[François]

yes, a classical ViewModel

[François]

I don’t have this issue with String or Data Class, I didn’t try every type. but Int is a basic one.

[Tadeas Kriz]

All types that are not NSObject will behave this way. For example even Kotlin's Unit is translated to Swift's Void, but when it's used in generics it'll be KotlinUnit because Objective-C's void is a primitive, same as int32, int64, float etc. It's a limitation of Objective-C generics and pointers. For example the same problem is when you make that Kotlin Int in your VM into Int?. You'll now see KotlinInt? in Swift.

[François]

Oh, I see.
I wrap an Int and an Int? inside a data class, I got the following : Int32 and KotlinInt?

[Tadeas Kriz]

Yeah, basically there's no way to represent int32 that'd be nullable in Objective-C without making it an object (NSObject). In real Objective-C, we used to just put NSNumber where we needed nullable int32, but Kotlin's doing better by introducing KotlinInt so it's at least visible that it's supposed to be an Int and not any number. Although it's still backed by NSObject (and it's a descendant) so you can call .floatValue on KotlinInt instance and I'll convert it to float, which is weird but again, Objective-C legacy.

[François]

I hope the SwiftExport will make the things easier.

[Tadeas Kriz]

Some, yeah

[Tadeas Kriz]

But things like generics will be very different and possibly more difficult to use

[François]

Ok, then the main issue is about the Generic.
Currently SwiftExport use Int32 and Int32? , so no use of KotlinInt, which is great

[Tadeas Kriz]

Yeah, but even Int32 is not great for Swift, where you'd want to use just regular Int. But that's a different discussion

[François]

Well, that’s fine, Good to know some limitation about the iOS dev experiences,

[François]

The main reason of the first question is, I’m working on a macro for wraping a Kotlin ViewModel into a SwiftUI Viewmodel
I got something like this and the integer type was kind of problematic, but now I have a better understanding, I have fixed my issue 😄

@sharedViewModel(ofType: MainScreenViewModel.self,
                 publishing:
                    (\.mainScreenUIState, MainScreenUIState.self),
                 (\.userId, String?.self),
                 (\.intNotNullValue, Int.self),
                 (\.intNullValue, Int?.self)
)
class MyMainScreenViewModel: ObservableObject {}