如何等待 Scala 未来的 onSuccess 回调完成？

Question

在 Scala 中，我可以使用 Await 等待未来完成。但是，如果我注册了一个回调以在该未来完成时运行，我如何不仅等待未来完成，还等待该回调完成？

这里有一个最小但完整的程序来说明这个问题：

import scala.concurrent.ExecutionContext.Implicits.global
import scala.concurrent.duration.Duration
import scala.concurrent.{ Await, Future }

object Main {
  def main(args: Array[String]): Unit = {
    val f: Future[Int] = Future(0)
    f.onSuccess { case _ =>
      Thread.sleep(10000)
      println("The program waited patiently for this callback to finish.")
    }

    // This waits for `f` to complete but doesn't wait for the callback
    // to finish running.
    Await.ready(f, Duration.Inf)
  }
}

我希望输出是：

The program waited patiently for this callback to finish.

相反，没有输出；程序在回调结束前退出。

请注意，这与等待未来完成不是同一个问题，之前已在 this question 回答过。

Answer 1

不要使用 onSuccess 回调，而是在 Future.map 调用中产生副作用。这样，你就有了一个 Future[Unit] 来使用 Await。

import scala.concurrent.ExecutionContext.Implicits.global
import scala.concurrent.duration.Duration
import scala.concurrent.{ Await, Future }

object Main {
  def main(args: Array[String]): Unit = {
    val f: Future[Int] = Future(0)
    val f2: Future[Unit] = f.map { x =>
      Thread.sleep(10000)
      println("The program waited patiently for this callback to finish.")
    }

    Await.ready(f2, Duration.Inf)
  }
}

请注意，如果您只想在成功的情况下执行副作用（例如在您的示例中），则 map 是合适的。如果您想在失败的情况下也执行副作用，那么使用正确的方法。请参阅 Roland Kuhn 在 scala-user 上的 post。

另外，请不要在生产代码附近的任何地方使用 Thread.sleep。

Answer 2

import scala.concurrent.ExecutionContext.Implicits.global
import scala.concurrent.duration.Duration
import scala.concurrent.{ Await, Future }
import scala.util._

object Main {
  def main(args: Array[String]): Unit = {
    val f1: Future[Int] = Future(0)
    val f2 = f1 andThen {
      case Success(v) =>
        Thread.sleep(10000)
        println("The program waited patiently for this callback to finish.")
      case Failure(e) =>
        println(e)
    }

    Await.ready(f1, Duration.Inf)
    println("F1 is COMPLETED")
    Await.ready(f2, Duration.Inf)
    println("F2 is COMPLETED")
  }
}

打印：

F1 is COMPLETED
The program waited patiently for this callback to finish.
F2 is COMPLETED

使用 Promise 更加清晰：

import scala.concurrent.ExecutionContext.Implicits.global
import scala.concurrent.duration.Duration
import scala.concurrent._
import scala.util._

object Main {
  def main(args: Array[String]): Unit = {
    val f: Future[Int] = Future(0)
    val p = Promise[Unit]()
    p.future.onSuccess { case _ =>
      println("The program waited patiently for this callback to finish.")
    }
    f.onSuccess { case _ =>
      Thread.sleep(10000)
      p.success(())
    }

    Await.ready(f, Duration.Inf)
    println("F is COMPLETED")
    Await.ready(p.future, Duration.Inf)
    println("P is COMPLETED")
  }
}

打印：

F is COMPLETED
P is COMPLETED
The program waited patiently for this callback to finish.