我如何等待Scala future的onSuccess回调完成？ [英] How do I wait for a Scala future's onSuccess callback to complete?

问题描述

在Scala中，我可以使用 Await 等待将来的完成。但是，如果我已经注册了在将来完成时运行的回调，那么我不仅要等待将来完成，而且还要等待回调完成吗？

In Scala, I can use Await to wait for a future to complete. However, if I have registered a callback to run upon completion of that future, how can I wait not only for the future to complete but also for that callback to finish?

以下是一个最小但完整的程序来说明问题：

Here is a minimal but complete program to illustrate the problem:

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)
  }
}

我希望输出为：

The program waited patiently for this callback to finish.

相反，没有输出；

请注意，这与等待将来完成不一样，之前已经在这个问题。

Please note that this is not the same problem as waiting for a future to complete, which has been answered previously at this question.

推荐答案

不要使用onSuccess回调，而是在Future.map调用中产生副作用。这样，您就可以使用Future [Unit]。

Don't use an onSuccess callback, but instead do the side effect in a Future.map call. That way, you have a Future[Unit] to use Await on.

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，如果要执行边仅在成功的情况下才有效（例如您的示例），请使用map。如果您还想在发生故障的情况下执行副作用，则使用正确的方法。在scala-user上查看Roland Kuhn的帖子。

Note that if you want to execute a side effect only in case of success (like in your example), map is appropriate. If you want to execute a side effect also in case of failure, andThen is the right method to use. See this post from Roland Kuhn on scala-user.

另外，请 不要 在生产代码附近的任何地方使用Thread.sleep。

Also, please don't use Thread.sleep anywhere near production code.

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