Pattern for responding with async result - scala

Question

I am trying to get the following example working:
def asyncTest = Action {
val willBeInt = Future {
Thread.sleep(5000)
100
}
willBeInt.onComplete({
case Success(value) => Ok(s"Value = $value")
case Failure(e) => Failure(e)
})
}
But I am getting an error about overloading a method:
Overloaded method value [apply] cannot be applied to (Unit)
I'm coming from a background in NodeJS and am struggling to figure out how these callbacks are supposed to work while simultaneously returning a result to appease the method signature.

Answer 1

Think of Action as a function that returns a promise, rather than as a function that accepts a callback. In scala terminology, you'll be returning a Future. Play's internals will be calling onComplete (or something similar) on their own (analagous to a javascript Promise's then function).
Specifically, your compilation error is due to the fact that onComplete returns Unit, when the Action block is expecting you to return a Future. You can use map to transform your willBeInt future into what Play is looking for:
def asynTest = Action.async {
val willBeInt = Future {
Thread.sleep(5000)
100
}
// note you will probably need to
// import scala.concurrent.ExecutionContext.Implicits.global
// to be able to call `map` here
willBeInt map { value =>
Ok(s"Value = $value")
} recover {
case e: Throwable => InternalServerError(e.toString)
}
}
For some extra reading, check out the docs for Future, and the docs for Action