I'm completely at a loss here as to how to use `Continuation` in my Java code when trying to run as...

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[luke]

I'm completely at a loss here as to how to use Continuation in my Java code when trying to run async coroutines. Anyone have any resources?

[diesieben07]

As far as I know you shouldn't. Use the higher-level "glue code" from the coroutines integration libraries:
https://github.com/Kotlin/kotlinx.coroutines/blob/master/integration/README.md

That allows you to convert from coroutines to e.g. CompletableFuture or ListenableFuture.

[luke]

Thank you.

[diesieben07]

(and vice versa allows you to await on a CompletableFuture, etc.)

[luke]

I'm needing to target API 19 in Android, so that's a no go for me.

[luke]

Thank you anyway.

[diesieben07]

Well, you can (in theory) use Continuation from Java by calling resumeWith on it, but that could be problematic because that requires an inline-class parameter. You'd have to write a helper method in Kotlin:
fun <T> resumeContinuation(continuation: Continuation<T>, value: T) = continuation.resume(value)

[diesieben07]

And then call that from Java.

[diesieben07]

And same for resumeWithException

[luke]

Thank you.

So this seems to be working OK, but does the Continuation instance have to be so verbose?

Object addresses = geo.reverseGeocodeAsync(location.getLatitude(), location.getLongitude(), 5).await(new Continuation<List<Address>>() {
            @NotNull
            @Override
            public CoroutineContext getContext() {
                return GlobalScope.INSTANCE.getCoroutineContext();
            }

            @Override
            public void resumeWith(@NotNull Object o) {

            }
        });

[luke]

And should anything be done in resumeWith?

[diesieben07]

honestly you should not be implementing continuation manually. Instead provide a java-adapted version from kotlin.

fun myFunctionAsync(): Deferred<Result> {

}

fun myFunctionForJava(callback: Consumer<Result>) {
    // adapt deferred to callback here
}

[diesieben07]

However this is not a trivial task, as you need to properly handle cancellation, exceptions, etc.
I highly urge you to check out https://github.com/Kotlin/kotlinx.coroutines/blob/master/integration/kotlinx-coroutines-guava/src/ListenableFuture.kt to see how not-easy this is.

[luke]

Thank you. What a nightmare. Been at it for hours.

[diesieben07]

If at all possible, consider using Guava and ListenableFuture. Or just write all your async code in Kotlin K

[luke]

Yeah, I tried that initially but when calling suspend functions from Java, the return type is Object

[diesieben07]

Yes, you cannot reasonably call suspend functions from Java. You need adapters (written in Kotlin) that translate to something Java can handle (like ListenableFuture)

[luke]

Cool, thank you.

[ghedeon]

you can always wrap suspended code in a kotlin class and pass the scope and a good ol' callback there