Hi everyone! I have a question about interop of ge...

# multiplatform

[novikau]

Hi everyone! I have a question about interop of generics in ObjC/Swift.

If I have a class with a generic in kotlin, let's say GenericBox<T> :

class GenericBox<T : Any>(val value: T)

And I want to use List<String> as a generic type of this `GenericBox`:

fun getListOfStringsBox(): GenericBox<List<String>> {
    return GenericBox(listOf("List", "of", "strings"))
}

I noticed that in Objective C the interface is generated correctly and all the generics are there: GenericBox has T and the signature of getListOfStringsBox() has SharedGenericBox<NSArray<NSString *> *> * as a return type

__attribute__((objc_subclassing_restricted))
__attribute__((swift_name("GenericBox")))
@interface SharedGenericBox<T> : SharedBase
- (instancetype)initWithValue:(T)value __attribute__((swift_name("init(value:)"))) __attribute__((objc_designated_initializer));
@property (readonly) T value __attribute__((swift_name("value")));
@end

__attribute__((swift_name("Demo")))
@interface SharedDemo : SharedBase
+ (instancetype)alloc __attribute__((unavailable));
+ (instancetype)allocWithZone:(struct _NSZone *)zone __attribute__((unavailable));
+ (instancetype)demo __attribute__((swift_name("init()")));
@property (class, readonly, getter=shared) SharedDemo *shared __attribute__((swift_name("shared")));
- (SharedGenericBox<NSArray<NSString *> *> *)getListOfStringsBox __attribute__((swift_name("getListOfStringsBox()")));
@end

But when I try to access the value from GenericBox, the type is lost and it returns NSArray<Any> instead of NSArray<NSString>. This happens only to List -> NSArray interop and doesn't happen if there are two kotlin types with generics nested one to another, eg. GenericBox<ChildGenericBox<String>> (in this case both types will be preserved).

Is it a limitation of Objective C NSArray type (or lightweight generics supported there) or it's a KMP compiler bug? I couldn't find an issue reported to JB issue tracker (or maybe I searched it badly).

[novikau]

I prepared a simple demo project to show this issue in case somebody wants to play around: https://github.com/anthony-novikov/objc-list-kmm-binding-demo

[Jeff Lockhart]

See the docs:

Generics can only be defined on classes, not on interfaces (protocols in Objective-C and Swift) or functions.

Because List is an interface, the generic type is lost in the Objective-C interop.

[novikau]

I don't think this is entirely the reason. I would have understood this if kotlin List was exported to ObjC as @protocol KotlinList as it happens to Iterator<T> for example. KMP exports List<T> to Objective C as NSArray<T> and even in my sample app this construction will keep the generic type of a list item

class GenericListBox<T : Any>(val value: List<T>)

So I believe there is something else that doesn't allow to use List as a generic type.

[Jeff Lockhart]

Nested generics are also not supported. The generic type of a generic type will be lost.

[novikau]

no it won't.

GenericBox<ChildGenericBox<String>>

works as well