Akka

private[scheduler] class DAGSchedulerEventProcessActor(dagScheduler: DAGScheduler)
  extends Actor with Logging {

  def receive = {
    case completion @ CompletionEvent(task, reason, _, _, taskInfo, taskMetrics) =>
      dagScheduler.handleTaskCompletion(completion)

意思是把CompletionEvent赋值给completion,这是scala定义的@操作符