KMP-NativeCoroutines

A library to use Kotlin Coroutines from Swift code in KMP apps.

Why this library?

Both KMP and Kotlin Coroutines are amazing, but together they have some limitations.

The most important limitation is cancellation support.
Kotlin suspend functions are exposed to Swift as functions with a completion handler.
This allows you to easily use them from your Swift code, but it doesn't support cancellation.

[!NOTE] While Swift 5.5 brings async functions to Swift, it doesn't solve this issue.
For interoperability with ObjC all functions with a completion handler can be called like an async function.
This means starting with Swift 5.5 your Kotlin suspend functions will look like Swift async functions.
But that's just syntactic sugar, so there's still no cancellation support.

Besides cancellation support, ObjC doesn't support generics on protocols.
So all the Flow interfaces lose their generic value type which make them hard to use.

This library solves both of these limitations 😄.

Compatibility

The latest version of the library uses Kotlin version 2.0.21.
Compatibility versions for older and/or preview Kotlin versions are also available:

You can choose from a couple of Swift implementations.
Depending on the implementation you can support as low as iOS 9, macOS 10.9, tvOS 9 and watchOS 3:

Installation

The library consists of a Kotlin and Swift part which you'll need to add to your project.
The Kotlin part is available on Maven Central and the Swift part can be installed via CocoaPods or the Swift Package Manager.

Make sure to always use the same versions for all the libraries!

Kotlin

For Kotlin just add the plugin to your build.gradle.kts:

plugins {
    id("com.google.devtools.ksp") version "2.0.21-1.0.25"
    id("com.rickclephas.kmp.nativecoroutines") version "1.0.0-ALPHA-37"
}

and make sure to opt in to the experimental @ObjCName annotation:

kotlin.sourceSets.all {
    languageSettings.optIn("kotlin.experimental.ExperimentalObjCName")
}

Swift (Swift Package Manager)

The Swift implementations are available via the Swift Package Manager.
Just add it to your Package.swift file:

dependencies: [
    .package(url: "https://github.com/rickclephas/KMP-NativeCoroutines.git", exact: "1.0.0-ALPHA-37")
],
targets: [
    .target(
        name: "MyTargetName",
        dependencies: [
            // Swift Concurrency implementation
            .product(name: "KMPNativeCoroutinesAsync", package: "KMP-NativeCoroutines"),
            // Combine implementation
            .product(name: "KMPNativeCoroutinesCombine", package: "KMP-NativeCoroutines"),
            // RxSwift implementation
            .product(name: "KMPNativeCoroutinesRxSwift", package: "KMP-NativeCoroutines")
        ]
    )
]

Or add it in Xcode by going to File > Add Packages... and providing the URL: https://github.com/rickclephas/KMP-NativeCoroutines.git.

[!NOTE] The version for the Swift package should not contain the Kotlin version suffix (e.g. -new-mm or -kotlin-1.6.0).

[!NOTE] If you only need a single implementation you can also use the SPM specific versions with suffixes -spm-async, -spm-combine and -spm-rxswift.

Swift (CocoaPods)

If you use CocoaPods add one or more of the following libraries to your Podfile:

pod 'KMPNativeCoroutinesAsync', '1.0.0-ALPHA-37'    # Swift Concurrency implementation
pod 'KMPNativeCoroutinesCombine', '1.0.0-ALPHA-37'  # Combine implementation
pod 'KMPNativeCoroutinesRxSwift', '1.0.0-ALPHA-37'  # RxSwift implementation

[!NOTE] The version for CocoaPods should not contain the Kotlin version suffix (e.g. -new-mm or -kotlin-1.6.0).

IntelliJ / Android Studio

Install the IDE plugin from the JetBrains Marketplace to get:

• Annotation usage validation
• Exposed coroutines warnings
• Quick fixes to add annotations

Usage

Using your Kotlin Coroutines code from Swift is almost as easy as calling the Kotlin code.
Just use the wrapper functions in Swift to get async functions, AsyncStreams, Publishers or Observables.

Kotlin

The plugin will automagically generate the necessary code for you! 🔮
Just annotate your coroutines declarations with @NativeCoroutines (or @NativeCoroutinesState).

Flows

Your Flow properties/functions get a native version:

import com.rickclephas.kmp.nativecoroutines.NativeCoroutines

class Clock {
    // Somewhere in your Kotlin code you define a Flow property
    // and annotate it with @NativeCoroutines
    @NativeCoroutines
    val time: StateFlow<Long> // This can be any kind of Flow
}

StateFlows

Using StateFlow properties to track state (like in a view model)?
Use the @NativeCoroutinesState annotation instead:

import com.rickclephas.kmp.nativecoroutines.NativeCoroutinesState

class Clock {
    // Somewhere in your Kotlin code you define a StateFlow property
    // and annotate it with @NativeCoroutinesState
    @NativeCoroutinesState
    val time: StateFlow<Long> // This must be a StateFlow
}

Suspend functions

The plugin also generates native versions for your annotated suspend functions:

import com.rickclephas.kmp.nativecoroutines.NativeCoroutines

class RandomLettersGenerator {
    // Somewhere in your Kotlin code you define a suspend function
    // and annotate it with @NativeCoroutines
    @NativeCoroutines
    suspend fun getRandomLetters(): String { 
        // Code to generate some random letters
    }
}

Interface limitations

Unfortunately extension functions/properties aren't supported on Objective-C protocols.

However this limitation can be "overcome" with some Swift magic.
Assuming RandomLettersGenerator is an interface instead of a class we can do the following:

import KMPNativeCoroutinesCore

extension RandomLettersGenerator {
    func getRandomLetters() -> NativeSuspend<String, Error, KotlinUnit> {
        RandomLettersGeneratorNativeKt.getRandomLetters(self)
    }
}

Exposed coroutines checks

When suspend functions and/or Flow declarations are exposed to ObjC/Swift, the compiler and IDE plugin will produce a warning, reminding you to add one of the KMP-NativeCoroutines annotations.

You can customise the severity of these checks in your build.gradle.kts file:

nativeCoroutines {
    exposedSeverity = ExposedSeverity.ERROR
}

Or, if you are not interested in these checks, disable them:

nativeCoroutines {
    exposedSeverity = ExposedSeverity.NONE
}

Swift Concurrency

The Async implementation provides some functions to get async Swift functions and AsyncSequences.

Async functions

Use the asyncFunction(for:) function to get an async function that can be awaited:

import KMPNativeCoroutinesAsync

let handle = Task {
    do {
        let letters = try await asyncFunction(for: randomLettersGenerator.getRandomLetters())
        print("Got random letters: \(letters)")
    } catch {
        print("Failed with error: \(error)")
    }
}

// To cancel the suspend function just cancel the async task
handle.cancel()

or if you don't like these do-catches you can use the asyncResult(for:) function:

import KMPNativeCoroutinesAsync

let result = await asyncResult(for: randomLettersGenerator.getRandomLetters())
if case let .success(letters) = result {
    print("Got random letters: \(letters)")
}

for Unit returning functions there is also the asyncError(for:) function:

import KMPNativeCoroutinesAsync

if let error = await asyncError(for: integrationTests.returnUnit()) {
    print("Failed with error: \(error)")
}

AsyncSequence

For Flows there is the asyncSequence(for:) function to get an AsyncSequence:

import KMPNativeCoroutinesAsync

let handle = Task {
    do {
        let sequence = asyncSequence(for: randomLettersGenerator.getRandomLettersFlow())
        for try await letters in sequence {
            print("Got random letters: \(letters)")
        }
    } catch {
        print("Failed with error: \(error)")
    }
}

// To cancel the flow (collection) just cancel the async task
handle.cancel()

Combine

The Combine implementation provides a couple functions to get an AnyPublisher for your Coroutines code.

[!NOTE] These functions create deferred AnyPublishers.
Meaning every subscription will trigger the collection of the Flow or execution of the suspend function.

[!NOTE] You must keep a reference to the returned Cancellables otherwise the collection will be cancelled immediately.

Publisher

For your Flows use the createPublisher(for:) function:

import KMPNativeCoroutinesCombine

// Create an AnyPublisher for your flow
let publisher = createPublisher(for: clock.time)

// Now use this publisher as you would any other
let cancellable = publisher.sink { completion in
    print("Received completion: \(completion)")
} receiveValue: { value in
    print("Received value: \(value)")
}

// To cancel the flow (collection) just cancel the publisher
cancellable.cancel()

You can also use the createPublisher(for:) function for suspend functions that return a Flow:

let publisher = createPublisher(for: randomLettersGenerator.getRandomLettersFlow())

Future

For the suspend functions you should use the createFuture(for:) function:

import KMPNativeCoroutinesCombine

// Create a Future/AnyPublisher for the suspend function
let future = createFuture(for: randomLettersGenerator.getRandomLetters())

// Now use this future as you would any other
let cancellable = future.sink { completion in
    print("Received completion: \(completion)")
} receiveValue: { value in
    print("Received value: \(value)")
}

// To cancel the suspend function just cancel the future
cancellable.cancel()

RxSwift

The RxSwift implementation provides a couple functions to get an Observable or Single for your Coroutines code.

[!NOTE] These functions create deferred Observables and Singles.
Meaning every subscription will trigger the collection of the Flow or execution of the suspend function.

Observable

For your Flows use the createObservable(for:) function:

import KMPNativeCoroutinesRxSwift

// Create an observable for your flow
let observable = createObservable(for: clock.time)

// Now use this observable as you would any other
let disposable = observable.subscribe(onNext: { value in
    print("Received value: \(value)")
}, onError: { error in
    print("Received error: \(error)")
}, onCompleted: {
    print("Observable completed")
}, onDisposed: {
    print("Observable disposed")
})

// To cancel the flow (collection) just dispose the subscription
disposable.dispose()

You can also use the createObservable(for:) function for suspend functions that return a Flow:

let observable = createObservable(for: randomLettersGenerator.getRandomLettersFlow())

Single

For the suspend functions you should use the createSingle(for:) function:

import KMPNativeCoroutinesRxSwift

// Create a single for the suspend function
let single = createSingle(for: randomLettersGenerator.getRandomLetters())

// Now use this single as you would any other
let disposable = single.subscribe(onSuccess: { value in
    print("Received value: \(value)")
}, onFailure: { error in
    print("Received error: \(error)")
}, onDisposed: {
    print("Single disposed")
})

// To cancel the suspend function just dispose the subscription
disposable.dispose()

Customize

There are a number of ways you can customize the generated Kotlin code.

Name suffix

Don't like the naming of the generated properties/functions?
Specify your own custom suffixes in your build.gradle.kts file:

nativeCoroutines {
    // The suffix used to generate the native coroutine function and property names.
    suffix = "Native"
    // The suffix used to generate the native coroutine file names.
    // Note: defaults to the suffix value when `null`.
    fileSuffix = null
    // The suffix used to generate the StateFlow value property names,
    // or `null` to remove the value properties.
    flowValueSuffix = "Value"
    // The suffix used to generate the SharedFlow replayCache property names,
    // or `null` to remove the replayCache properties.
    flowReplayCacheSuffix = "ReplayCache"
    // The suffix used to generate the native state property names.
    stateSuffix = "Value"
    // The suffix used to generate the `StateFlow` flow property names,
    // or `null` to remove the flow properties.
    stateFlowSuffix = "Flow"
}

CoroutineScope

For more control you can provide a custom CoroutineScope with the NativeCoroutineScope annotation:

import com.rickclephas.kmp.nativecoroutines.NativeCoroutineScope

class Clock {
    @NativeCoroutineScope
    internal val coroutineScope = CoroutineScope(job + Dispatchers.Default)
}

[!NOTE] Your custom coroutine scope must be either internal or public.

If you don't provide a CoroutineScope the default scope will be used which is defined as:

internal val defaultCoroutineScope = CoroutineScope(SupervisorJob() + Dispatchers.Default)

[!NOTE] KMP-NativeCoroutines has built-in support for KMP-ObservableViewModel.
Coroutines inside your ViewModel will (by default) use the CoroutineScope from the ViewModelScope.

Ignoring declarations

Use the NativeCoroutinesIgnore annotation to tell the plugin to ignore a property or function:

import com.rickclephas.kmp.nativecoroutines.NativeCoroutinesIgnore

@NativeCoroutinesIgnore
val ignoredFlowProperty: Flow<Int>

@NativeCoroutinesIgnore
suspend fun ignoredSuspendFunction() { }

Refining declarations in Swift

If for some reason you would like to further refine your Kotlin declarations in Swift, you can use the NativeCoroutinesRefined and NativeCoroutinesRefinedState annotations.
These will tell the plugin to add the ShouldRefineInSwift annotation to the generated properties/function.

[!NOTE] This currently requires a module-wide opt-in to kotlin.experimental.ExperimentalObjCRefinement.

You could for example refine your Flow property to an AnyPublisher property:

import com.rickclephas.kmp.nativecoroutines.NativeCoroutinesRefined

class Clock {
    @NativeCoroutinesRefined
    val time: StateFlow<Long>
}

import KMPNativeCoroutinesCombine

extension Clock {
    var time: AnyPublisher<KotlinLong, Error> {
        createPublisher(for: __time)
    }
}