為什么Scala的Future.onComplete需要一個ExecutionContext

Question

Promise.complete不能只運行Future.onComplete的回調，而不能一直通過ExecutionContext並且據我所知，不能安排Future.onComplete的回調用於以后並可能在另一個線程中運行嗎？

Answer 1

您可以向onComplete提供自己的ExecutionContext ，它將在同一線程上運行代碼：

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

您甚至可以將其設為implicit ，並且對於希望在另一個線程中執行的情況，可以提供scala.concurrent.ExecutionContext.global或其他上下文。

這是一個測試，它如何工作：

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")
}

運行testContext()將打印

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

Answer 2

這是實現scala Futures的設計決策，所有操作（地圖，flatMap等）都需要隱式的ExecutionContext。

如果您想擁有更好的線程可重用性，並且線程之間的上下文切換更少，我建議您看一下scalaz Task，它對於延遲計算的抽象略有不同： http ://timperrett.com/2014/07/20/ scalaz-task-missing文檔/