为什么 Scala 的 Future.onComplete 需要一个 ExecutionContext

Question

不能 Promise.complete 只是运行 Future.onComplete 的回调，而不是一直通过 ExecutionContext 并且，据我所知，安排 Future.onComplete 稍后的回调，可能运行它在另一个线程中？

Answer 1

这是实现 scala Futures 的设计决策，所有操作（map、flatMap 等...）都需要隐式 ExecutionContext。

如果您想要更好的线程可重用性和线程之间更少的上下文切换，我建议您看一下 scalaz Task，延迟计算的抽象略有不同：http://timperrett.com/2014/07/20/scalaz-task-the-missing-documentation/

Answer 2

您可以将自己的 ExecutionContext 提供给 onComplete，这将运行同一线程上的代码：

val immediateContext: ExecutionContext = new ExecutionContext {
  def execute(runnable: Runnable) {
    runnable.run()
  }
  def reportFailure(cause: Throwable) {}
}

您甚至可以将其设置为 implicit，并且对于您希望在另一个线程中执行的情况，您可以提供 scala.concurrent.ExecutionContext.global 或其他一些上下文。

这是一个测试，它是如何工作的：

val immediateContext: ExecutionContext = new ExecutionContext {
  override def reportFailure(cause: Throwable): Unit = {}
  override def execute(runnable: Runnable): Unit = {
    println("Executing")
    runnable.run()
    println("Executed")
  }
}

def testContext(): Unit = {
  println("Scheduling on an uncompleted future")
  val p = Promise[Int]()
  println("Scheduling")
  p.future.onComplete { _ => println("Completed") }(immediateContext)
  println("Scheduled")
  p.complete(Success(5))

  println()

  println("Scheduling on an already completed future")
  val p2 = Promise[Int]().complete(Success(5))
  println("Scheduling")
  p2.future.map { n =>
    println("Mapping")
    n * 2
  }(immediateContext).onComplete{
    case Success(n) => println(s"Completed: $n") 
    case _ =>
  }(immediateContext)
  println("Scheduled")

  println()

  println("Using scala.concurrent.ExecutionContext.global for comparison")
  val p3 = Promise[Int]().complete(Success(5))
  println("Scheduling")
  p3.future.onComplete {
    _ => println("Completed")
  }(concurrent.ExecutionContext.global)
  println("Scheduled")
}

运行 testContext() 将打印

Scheduling on an uncompleted future
Scheduling
Scheduled
Executing
Completed
Executed

Scheduling on an already completed future
Scheduling
Executing
Mapping
Executed
Executing
Completed: 10
Executed
Scheduled

Using scala.concurrent.ExecutionContext.global for comparison
Scheduling
Scheduled
Completed

为什么 Scala 的 Future.onComplete 需要一个 ExecutionContext

Why does Scala's Future.onComplete needs an ExecutionContext

scala

concurrency