Spark event dispatcher.

central event dispatcher

// version spark 1.0.0
// file org.apache.spark.scheduler.LiveListenerBus
  
  private val listenerThread = new Thread("SparkListenerBus") {
    setDaemon(true)
    override def run(): Unit = Utils.logUncaughtExceptions {
      while (true) {
        eventLock.acquire()
        // Atomically remove and process this event
        LiveListenerBus.this.synchronized {
          val event = eventQueue.poll
          if (event == SparkListenerShutdown) {
            // Get out of the while loop and shutdown the daemon thread
            return
          }
          Option(event).foreach(postToAll)
        }
      }
    }
  }