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.
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Answer
You have several options depending on your environment.
- If you are using
RxJava2
in the project, the modulekotlinx-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() }
- 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); } });