Scala Nested Futures

Question

I have a couple of futures. campaignFuture returns the [BigInt] list, and I want to be able to call a second future Future profile for each of the values ​​in the list returned from the first. The second future can be called only when the first is completed. How to achieve this in Scala?

campaignFuture(1923).flatMap?? (May be?) def campaignFuture(advertiserId: Int): Future[List[BigInt]] = Future { val campaignHttpResponse = getCampaigns(advertiserId.intValue()) parseProfileIds(campaignHttpResponse.entity.asString) } def profileFuture(profileId: Int): Future[List[String]] = Future { val profileHttpResponse = getProfiles(profileId.intValue()) parseSegmentNames(profileHttpResponse.entity.asString) } 

Answer 1

A for understanding is not applicable here, because we have a combination of List and Future's. So your friends are map and flatMap:

Future Responses

  import scala.concurrent.{Future, Promise, Await} import scala.concurrent.duration.Duration import scala.concurrent.ExecutionContext.Implicits.global def campaignFuture(advertiserId: Int): Future[List[BigInt]] = Future { List(1, 2, 3) } def profileFuture(profileId: Int): Future[List[String]] = { // delayed Future val p = Promise[List[String]] Future { val delay: Int = (math.random * 5).toInt Thread.sleep(delay * 1000) p.success(List(s"profile-for:$profileId", s"delayed:$delay sec")) } p.future } // Future[List[Future[List[String]]] val listOfProfileFuturesFuture = campaignFuture(1).map { campaign => campaign.map(id => profileFuture(id.toInt)) } // React on Futures which are done listOfProfileFuturesFuture foreach { campaignFutureRes => campaignFutureRes.foreach { profileFutureRes => profileFutureRes.foreach(profileListEntry => println(s"${new Date} done: $profileListEntry")) } } // !!ONLY FOR TESTING PURPOSE - THIS CODE BLOCKS AND EXITS THE VM WHEN THE FUTURES ARE DONE!! println(s"${new Date} waiting for futures") listOfProfileFuturesFuture.foreach{listOfFut => Await.ready(Future.sequence(listOfFut), Duration.Inf) println(s"${new Date} all futures done") System.exit(0) } scala.io.StdIn.readLine() 

To get the result of all futures at once

  import scala.concurrent.{Future, Await} import scala.concurrent.duration.Duration import scala.concurrent.ExecutionContext.Implicits.global def campaignFuture(advertiserId: Int): Future[List[BigInt]] = Future { List(1, 2, 3) } def profileFuture(profileId: Int): Future[List[String]] = Future { List(s"profile-for:$profileId") } // type: Future[List[Future[List[String]]]] val listOfProfileFutures = campaignFuture(1).map { campaign => campaign.map(id => profileFuture(id.toInt)) } // type: Future[List[List[String]]] val listOfProfileFuture = listOfProfileFutures.flatMap(s => Future.sequence(s)) // print the result //listOfProfileFuture.foreach(println) //scala.io.StdIn.readLine() // wait for the result (THIS BLOCKS INFINITY!) Await.result(listOfProfileFuture, Duration.Inf) 

---

• we use Future.sequence to convert the list [Future [T]] to Future [List [T]].
• flatMap to get the future [T] from the future [Future [T]]
• if you need to wait for the result (BLOCKING!), use Await to wait for the result