Koru

Automatically generates wrappers for suspend functions and Flow for easy access from Swift code in Kotlin Multiplatform projects.

Inspired by https://touchlab.co/kotlin-coroutines-rxswift/ by Russell Wolf.

Note: this library is in alpha state - the API may change.

Getting started

To get started, consult the Basic example below, read introductory article or check out the example repo.

Basic example

Let's say you have a class in the shared module, that looks like this:

@ToNativeClass(name = "LoadUserUseCaseIos")
class LoadUserUseCase(private val service: Service) {

    suspend fun loadUser(username: String) : User? = service.loadUser(username)
    
}

Such use case can be easily consumed from Android code, but in Kotlin Native (e.g. iOS) suspend functions generate a completion handler which is a bit of a PITA to work with.

When you add @ToNativeClass annotation to the class, a wrapper is generated:

public class LoadUserUseCaseIos(private val wrapped: LoadUserUseCase) {

  public fun loadUser(username: String): SuspendWrapper<User?> = 
      SuspendWrapper(null) { wrapped.loadUser(username) }
  
}

Notice that in place of suspend function, we get a function exposing SuspendWrapper. When you expose LoadUserUseCaseIos to your Swift code, it can be consumed like this:

loadUserUseCaseIos.loadUser(username: "foo").subscribe(
            scope: coroutineScope, //this can be provided automatically, more on that below
            onSuccess: { user in print(user?.description() ?? "none") },
            onThrow: { error in print(error.description())}
        )

From here it can be easily wrapped into RxSwift Single<User?> or Combine AnyPublisher<User?, Error>.

Generated functions - Suspend, Flow and regular

The wrappers generate different return types based on the original function signature

Original	Wrapper
`suspend` fun returning `T`	fun returning `SuspendWrapper<T>`
fun returning `Flow<T>`	fun returning `FlowWrapper<T>`
fun returning `T`	fun returning `T`

So, for example, this class:

@ToNativeClass(name = LoadUserUseCaseIos)
class LoadUserUseCase(private val service: Service) {

    suspend fun loadUser(username: String) : User? = service.loadUser(username)
    
    fun observeUser(username: String) : Flow<User?> = service.observeUser(username)
    
    fun getUser(username: String) : User? = service.getUser(username)
}

becomes:

public class LoadUserUseCaseIos(private val wrapped: LoadUserUseCase) {

    public fun loadUser(username: String): SuspendWrapper<User?> =
        SuspendWrapper(null) { wrapped.loadUser(username) }

    public fun observeUser(username: String): FlowWrapper<User?> =
        FlowWrapper(null, wrapped.observeUser(username))
        
    public fun getUser(username: String): User? = wrapped.getUser(username)
    
}

More options

Customizing generated names

You can control the name of the generated class or interface:

@ToNativeClass(name = "MyFancyIosClass")
@ToNativeInterface(name = "MyFancyIosProtocol")

You can also omit the name parameter and use the defaults:

@ToNativeClass Foo becomes FooNative
@ToNativeInterface Foo becomes FooNativeProtocol

Provide the scope automatically

One of the caveats of accessing suspend functions / Flows from Swift code is that you still have to provide CoroutineScope from the Swift code. This might upset your iOS team ;). In the spirit of keeping the shared code API as business-focused as possible, we can utilize @ExportScopeProvider to handle scopes automagically.

First you need to show the suspend wrappers where to look for the scope, like this:

@ExportedScopeProvider
class MainScopeProvider : ScopeProvider {

    override val scope = MainScope()
    
}

And then you provide the scope like this

@ToNativeClass(launchOnScope = MainScopeProvider::class)

Thanks to this, your Swift code can be simplified to just the callbacks, scope that launches coroutines is handled implicitly.

loadUserUseCaseIos.loadUser(username: "some username").subscribe(
            onSuccess: { user in print(user?.description() ?? "none") },
            onThrow: { error in print(error.description())}
        )

What happens under the hood?

Under the hood, a top level property val exportedScopeProvider_mainScopeProvider = MainScopeProvider() is created. Then, it is injected into the SuspendWrappers and FlowWrappers as the default scope that launches the coroutines. Remember, that you can always provide custom scope if you need to.

fun flow(foo: String) = FlowWrapper(exportedScopeProvider_mainScopeProvider, wrapped.flow(foo))
fun suspending(foo: String) = SuspendWrapper(exportedScopeProvider_mainScopeProvider) { wrapped.suspending(foo) }

Generate interfaces from classes and classes from interfaces

Usually you will just need to use @ToNativeClass on your business logic class like in the basic example. However, you can get more fancy, if you want.

Generate interface from class

Say, you want to expose to Swift code both the class and an interface (which translates to protocol in Swift), so that you can use the protocol to create a fake impl for unit tests.

@ToNativeClass(name = "FooIos")
@ToNativeInterface(name = "FooIosProtocol")
class Foo

This code will create an interface and a class extending it.

interface FooIosProtocol

class FooIos(private val wrapped: Foo) : FooIosProtocol

Generate interface from interface

If you already have an interface, you can reuse it just as easily:

@ToNativeInterface(name = "FooIosProtocol")
interface IFoo

@ToNativeClass(name = "FooIos")
class Foo : IFoo

This will also create an interface and a class and automatically match them:

interface FooIosProtocol

class FooIos(private val wrapped: Foo) : FooIosProtocol

Generate class from interface

*Not sure what the use case might be, nevertheless, it's also possible"

@ToNativeClass(name = "FooIos")
interface Foo

Will generate:

class FooIos(private val wrapped: Foo)

Handling in Swift code

You can consume the coroutine wrappers directly as callbacks. But if you are working with Swift Combine, you can wrap those callbacks using simple global functions (extension functions are not supported for Kotlin Native generic types at this time).

Then, you can call them like this:

createPublisher(wrapper: loadUserUseCase.loadUser(username: "noone special"))
    .sink(
        receiveCompletion: { completion in print("Completion: \(completion)") },
        receiveValue: { user in print("Hello from the Kotlin side \(user?.name)") }
    )
    .store(in: &cancellables)

Similar helper functions can be easily created for RxSwift.

Download

The artifacts are available on Maven Central.

To use the library in a KMM project, use this config in the build.gradle.kts:

plugins {
    kotlin("multiplatform")
    kotlin("kapt")
    ...
}

kotlin {

  ...
  
  sourceSets {
        
        ...
  
        val commonMain by getting {
            dependencies {
                ...
                implementation("com.futuremind:koru:0.3.4")
                configurations.get("kapt").dependencies.add(
                    org.gradle.api.internal.artifacts.dependencies.DefaultExternalModuleDependency(
                        "com.futuremind", "koru-processor", "0.3.4"
                    )
                )

            }
        }
        
        val iosMain by getting {
            ...
            kotlin.srcDir("${buildDir.absolutePath}/generated/source/kaptKotlin/")
        }
        
    }
    
}

klamborowski/koru