How to call Kotlin suspending coroutine function from Java 7

Question

I'm trying to call Kotlin function from Java 7. I'm using coroutines and this called function is suspending, for example:

suspend fun suspendingFunction(): Boolean {
    return async { longRunningFunction() }.await()
}

suspend fun longRunningFunction() : Boolean {
    delay(400)
    return true
}

I was using coroutines in version 0.25.3 and I could emulate simple Java callback style by passing Continuation<U> instance as an argument to suspending function, e.g.

CoroutinesKt.suspendingFunction(new Continuation<Boolean>() {
    @Override
    public CoroutineContext getContext() {
        return EmptyCoroutineContext.INSTANCE;
    }

    @Override
    public void resume(Boolean value) {
        doSomethingWithResult(value);
    }

    @Override
    public void resumeWithException(@NotNull Throwable throwable) {
        handleException(throwable);
    }
});

However, after updating to fully stable 1.0.1 release, I think it's no longer possible. Let's say updated version of suspending function looks like that:

suspend fun suspendingFunction(): Boolean {
    return GlobalScope.async { longRunningFunction() }.await()
}

Continuation<U> now uses Result class, which seems to be unusable from Java (which makes sense as it is inline class). I was trying to use some subclass of Continuation from coroutines but they are all internal or private.

I know that usually it is advised to transform coroutine to CompletableFuture, but I'm on Android, which means Java 7 only. Simple Future on the other hand is too dumb as I don't want to check periodically if function is finished - I just want to be called when it is finished. And I would really like to avoid adding new libraries or many additional classes/methods.

Is there any simple way to call suspending function directly from Java 7?

As Kotlin tries to be very interoperable with Java I would imagine there would be some easy way to do that, but I'm yet to find it.

Answer 1

You have several options depending on your environment.

1. If you are using RxJava2 in the project, the module kotlinx-coroutines-rx2 has utility functions to convert back and forth between coroutines and Rx datatypes.

Example

suspend fun sayHello(): String {
    delay(1000)
    return "Hi there"
}

fun sayHelloSingle(): Single<String> = GlobalScope.rxSingle { sayHello() }

2. Otherwise, you could add a new Continuation class that matches the definition of the old one and is also useful in the Java side.

Example (Kotlin side)

abstract class Continuation<in T> : kotlin.coroutines.Continuation<T> {
    abstract fun resume(value: T)
    abstract fun resumeWithException(exception: Throwable)
    override fun resumeWith(result: Result<T>) = result.fold(::resume, ::resumeWithException)
}   

Example (Java side)

sayHello(new Continuation<String>() {
    @Override
    public CoroutineContext getContext() {
        return EmptyCoroutineContext.INSTANCE;
    }

    @Override
    public void resume(String value) {
        doSomethingWithResult(value);
    }

    @Override
    public void resumeWithException(@NotNull Throwable throwable) {
        doSomethingWithError(throwable);
    }
});