执行顺序返回未来的功能序列

Question

I have a sequence of functions that return a future. I want to execute them sequentially ie after the first function future is complete, execute the next function and so on. Is there a way to do it?

ops: Seq[() => Future[Unit]]

Answer 1

You can combine all the futures into a single one with a foldLeft and flatMap :

def executeSequentially(ops: Seq[() => Future[Unit]])(
  implicit exec: ExecutionContext
): Future[Unit] =
  ops.foldLeft(Future.successful(()))((cur, next) => cur.flatMap(_ => next()))

foldLeft ensures the order from left to right and flatMap gives sequential execution. Functions are executed with the ExecutionContext , so calling executeSequentially is not blocking. And you can add callbacks or await on the resulting Future when/if you need it.

If you are using Twitter Future s, then I guess you won't need to pass ExecutionContext , but the general idea with foldLeft and flatMap should still work.

Answer 2

If given a Seq[Future[T]] you can convert it to a Future[Seq[T]] like so:

Val a: Seq[Future[T]] = ???

val resut: Future[Seq[T]] = Future.sequence(a)

a little less boilerplate than the above :)

Answer 3

I believe this should do it:

import scala.concurrent.{Await, Future}
import scala.concurrent.duration.Duration

def runSequentially(ops: Seq[() => Future[Unit]]): Unit = {
  ops.foreach(f => Await.result(f(), Duration.Inf))
}

If you want to wait less then Duration.Inf , or stop at failure - should be easy to do.