CredentialService.swift

import KeychainAccess
import shared

class CredentialService : PlatformCredentialService {

    private let keychain: Keychain

    public init(_ id: String) {
        keychain = Keychain(service: id)
    }

    public override func getString(key: String) -> String? {
        try? keychain.getString(key) ?? nil
    }

    public override func setString(key: String, value: String?) {
        if (value == nil) {
            try? keychain.remove(key)
        } else {
            try? keychain.set(value!, key: key)
        }
    }

    public override func clear() {
        try? keychain.removeAll()
    }
}

PlatformServices.kt

object PlatformSpecificServices() {
	
	lateinit var credentialService: PlatformCredentialService
	
	lateinit var locationService: PlatformLocationService
	
}

abstract class PlatformCredentialService {
	
	/**
	 * Platform required implementation for getting a credential value.
	 * @return The value or null if the credential does not exist.
	 */
	abstract fun getString(name: String): String?
	
	/**
	 * Platform required implementation for setting a credential value.
	 * A value of null clears the credential.
	 */
	abstract fun setString(name: String, value: String?)
	
	/**
	 * Platform required implementation for clearing all values.
	 */
	abstract fun clear()
	
	/*
	 * Methods implemented in Kotlin, that hide the complexity and reduce the need for other platfrom specific methods.
	 */
	
	fun containsKey(name: String): Bool = getCredential(name) != null
	
	fun remove(name: String) {
		setString(name, null)
	}
}

abstract class PlatformLocationService {
	
	/**
	 * Platform required implementation for updating the user's location.
	 */
	abstract fun updateLocation(latitude: Double, longitude: Double, time: Long, speed: Double, bearing: Double)
	
	/*
	 * Methods implemented in Kotlin, that hide the complexity and reduce the need for other platfrom specific methods.
	 */
	 
	 fun getHistory(since: Long): List<YourLocationObject> {
	 	// Your shared code here
	 }
	
}

YourApp.swift

import shared
import SwiftUI

@main
struct YourApp: Themeable, App {

	@ObservedViewModel private var model: AppViewModel = {
		KoinKt.doInitKoin()

		// Set the platform services to your platform specific implementations
		let platformServices: PlatformSpecificServices = koin.get()
		platformServices.credentialService = CredentialService("yourapp.keycainServiceId")
    	platformServices.locationService = LocationService()

		return koin.get()
	}()

    var body: some Scene {
        WindowGroup {			
            ZStack {
              // Your UI Here
            }
		}
    }
}

Note that Swift does not like Kotlin defaulted arguments and overrides on those so best to avoid those in these APIs. The CredentialService swift implementation is actually based on working code using the KeychainAccess swift package at https://github.com/kishikawakatsumi/KeychainAccess