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)