How do I wait for a Scala future's onSuccess callback to complete?

Question

In Scala, I can use Await to wait for a future to complete. However, if I have registered a callback to run upon completion of that future, how can I wait not

Answer 1

Don't use an onSuccess callback, but instead do the side effect in a Future.map call. That way, you have a Future[Unit] to use Await on.

import scala.concurrent.ExecutionContext.Implicits.global
import scala.concurrent.duration.Duration
import scala.concurrent.{ Await, Future }

object Main {
  def main(args: Array[String]): Unit = {
    val f: Future[Int] = Future(0)
    val f2: Future[Unit] = f.map { x =>
      Thread.sleep(10000)
      println("The program waited patiently for this callback to finish.")
    }

    Await.ready(f2, Duration.Inf)
  }
}

Note that if you want to execute a side effect only in case of success (like in your example), map is appropriate. If you want to execute a side effect also in case of failure, andThen is the right method to use. See this post from Roland Kuhn on scala-user.

Also, please don't use Thread.sleep anywhere near production code.