I've got a cross-platform ViewController which exposes the same interface on both macOS and iOS, using a little bit of trickery:

#import <TargetConditionals.h>

#if TARGET_OS_OSX

#import <AppKit/AppKit.h>
typedef NSViewController NativeViewController;
typedef NSScrollView NativeScrollView;

#else

#import <UIKit/UIKit.h>
typedef UIViewController NativeViewController;
typedef UIScrollView NativeScrollView;

#endif

...

@interface FooViewController : NativeViewController

Now I'm wrapping this in UIViewControllerRepresentable and NSViewControllerRepresentable but I'm not sure if there's a way to achieve this without duplicating the code, one for macOS and one for iOS. Because the classes are so similar, the only code differences between the iOS and macOS versions are references to the types and function signatures. This kind of kills me :) Any ideas on how to improve this situation?