How to resolve a list of futures in Scala

Question

I have a call that returns a Future. However, I need to make n calls so I will get back n futures. I am wondering how I would get the futures to all resolve before proceeding (without blocking the server)

For example,

while(counter < numCalls){     val future = call(counter)     future.map{  x =>         //do stuff     }     counter += 1  }  //Now I want to execute code here after ALL the futures are resolved without  //blocking the server 

Answer 1

You can use Future.sequence(futureList) to convert a List[Future[X]] to a Future[List[X]]. And since the latter is just a simple Future, you can wait for it to finish with the help of the Await.ready or similar helpers.

So you would have to keep a list of the futures you generate. Something like:

val futures = new ListBuffer[Future[X]] while(counter < numCalls) {     val future = call(counter)     futures += future     future.map { x =>         //do stuff     }     counter += 1  } val f = Future.sequence(futures.toList) Await.ready(f, Duration.Inf) 

which you could also write as:

val futures = (1 to numCalls).map(counter => {     f = call(counter)     f.map(x => ...)     f }) Await.ready(Future.sequence(futures), Duration.Inf)