onComplete method in Scala's Fututre monod - scala

Question

I am learning Future in Scala and have the following code snippet. I am generating random marks based on the first letter in the name.
For the following scenario, I expect a list to be printed by the onComplete method. But it does not print anything.
def randomMark(name:String) = name(0) match {
case 'R'|'A'|'J'|'S' => Thread.sleep(500); 99
case _ => Thread.sleep(500); 80
}
import scala.concurrent._
import concurrent.ExecutionContext.Implicits.global
val returns = Future sequence List( Future(randomMark("Rob")), Future(randomMark("Andy")), Future(randomMark("George")) )
Thread.sleep(550)
returns onComplete { e => { val y1 = e.getOrElse("Error"); println(y1) } }
//This println statement does not execute. I expect a list List(99,99,80) to be printed
Can someone please help me to understand why the functional literal I supply for onComplete method does not execute?
Thanks!

Answer 1

Most likely because you need to wait for the result because onComplete is an async operation.
import scala.concurrent.duration._
import scala.concurrent._
def randomMark(name:String) = name(0) match {
case 'R'|'A'|'J'|'S' => Thread.sleep(500); 99
case _ => Thread.sleep(500); 80
}
import scala.concurrent._
import concurrent.ExecutionContext.Implicits.global
val returns = Future sequence List( Future(randomMark("Rob")), Future(randomMark("Andy")), Future(randomMark("George")) )
Thread.sleep(550)
returns onComplete { e => { val y1 = e.getOrElse("Error"); println(y1) } }
println("Waiting futures to be completed")
Await.ready(returns, 5.seconds)
println("Futures to be completed")
Print's out:
Waiting futures to be completed
List(99, 99, 80)
Futures to be completed
Scatie: https://scastie.scala-lang.org/SWv18p8RTtuo7nMNHNHMoQ

Answer 2

Since callbacks are executed asynchronously, you'll need to wait for your callback to complete before the program exits. In this case, if you want to wait for an onComplete callback, you need to signal its completion somehow. In the following example I'm using a Promise:
def randomMark(name:String) = name(0) match {
case 'R'|'A'|'J'|'S' => Thread.sleep(500); 99
case _ => Thread.sleep(500); 80
}
import scala.concurrent._
import scala.concurrent.duration._
import ExecutionContext.Implicits.global
val returns = Future sequence List( Future(randomMark("Rob")), Future(randomMark("Andy")), Future(randomMark("George")) )
val returns: scala.concurrent.Future[List[Int]] = Future(<not completed>)
val p = Promise[Unit]()
returns onComplete { e => { val y1 = e.getOrElse("Error"); println(y1); p.success(()) } }
Await.ready(p.future, 5.seconds)
But, you could, instead use andThen as follows:
def randomMark(name:String) = name(0) match {
case 'R'|'A'|'J'|'S' => Thread.sleep(500); 99
case _ => Thread.sleep(500); 80
}
import scala.concurrent._
import scala.concurrent.duration._
import ExecutionContext.Implicits.global
val returns =
Future sequence List( Future(randomMark("Rob")), Future(randomMark("Andy")), Future(randomMark("George")) ) andThen
{ case e => { val y1 = e.getOrElse("Error"); println(y1) } }
Await.ready(returns, 5.seconds)