i have to use a rather terrible java api thats basically 1. set callback 2. call function and the ca...

# coroutines

[Nikky]

i have to use a rather terrible java api thats basically
1. set callback
2. call function
and the callback gives me the return value
how can i wrap such a structure in a suspend fun ?

[streetsofboston]

suspend fun fromSomeManagerCallback(): SomeResult = suspendCancellableCoroutine { cont ->
    someManager.setCallback { result ->
        cont.resume(result)
    }
}

[Nikky]

thanks

[streetsofboston]

Be sure that the callback, provided in the example to setCallback will be called no more than once.

[Nikky]

what should i put for onCancellation in the resule call ?

[Nikky]

seems like that is required.. should i just rethrow ?

[streetsofboston]

If the callback also can return a Throwable, you can do cont.resumeWithException(error)

[Nikky]

i mean there seems to be no resume call without providing onCancellation callback

[streetsofboston]

I don’t understand…. You mean an onCancellation callback to the someManager in my example-snippet?

[streetsofboston]

The call to cont.resume just needs one parameter; the result value (an instance of SomeResult in my example-snippet).

[streetsofboston]

I see… there are two overloaded versions of resume on a Continuation. Use the one that does not have the onCancellation parameter. It is the one that is an extension function on Continuation:

public inline fun <T> Continuation<T>.resume(value: T): Unit

[Nikky]

sometimes i really don't understand why some stuff is in extensions in the core library

[streetsofboston]

Yup….
This keeps happening, made worse by non-wildcard imports that most folks use.
If wildcard imports were used, the compiler would prioritize extension functions over class members (in this case, the auto-complete would have given you the extension function initially).
IIRC, this will change and improve a bit in a near future version of Kotlin, where if an overloaded extension function exists for a given receiver in addition to a class method, the extension function will be the first one being suggested.

[Nikky]

but the default IDE settings turn wirldcard imports into specific ones

[streetsofboston]

I know… frustrating.
Hopefully, regardless of the import settings of the IDE, that new version of the Kotlin plugin will makes things a bit better.
Same issue holds for the collect function on a `Flow`….