Scala：密封的特征对象在 case 语句中不起作用：Akka 演员答案

【问题标题】：Scala: sealed trait objects are not working inside case statement: Akka actorsScala：密封的特征对象在 case 语句中不起作用：Akka 演员
【发布时间】：2018-03-04 06:02:55
【问题描述】：

我一直在努力处理 case 语句中与密封特征对象使用相关的以下错误。这是我与 akka 演员相关的代码。我正在使用伴随对象并定义密封特征，如下面的 scala 文件所示。

ActorCreation.scala

import akka.actor.Actor
import akka.actor.ActorSystem
import akka.actor.Props

object MusicController{
  sealed trait ControllerMsg
  case object Play extends ControllerMsg
  case object Stop extends ControllerMsg

  }

class MusicController extends Actor{
  def receive ={
   case Play =>
     println("Music Started.....")
   case Stop =>
     println("Music Stopped.....")
 }
}

object MusicPlayer{
  sealed trait PlayMsg
  case object StopMusic extends PlayMsg
  case object StartMusic extends PlayMsg
}

class MusicPlayer extends Actor {
  def receive ={
   case StopMusic =>
     println("I don't Stop Music now.....")
   case StartMusic =>
     val controller = context.actorOf(Props[MusicController],"controller")
     controller ! Play
   case _ =>
     println("Unknown Message")
 }
}

object Creation extends App{

 val system = ActorSystem("creation")

 val player = system.actorOf(Props[MusicPlayer],"player")

 player ! StartMusic
}

这是我在为各个类的伴随对象中定义它们之后与密封特征对象相关的错误堆栈。

[info] Updating {file:/C:/Akka%20Actors/chapter02/}chapter02...
[info] Resolving jline#jline;2.12.1 ...
[info] Done updating.
[info] Compiling 1 Scala source to C:\Akka Actors\chapter02\target\scala-2.11\classes...
[error] C:\Akka Actors\chapter02\src\main\scala\ActorCreation.scala:14: not found: value Play
[error]    case Play =>
[error]         ^
[error] C:\Akka Actors\chapter02\src\main\scala\ActorCreation.scala:16: not found: value Stop
[error]    case Stop =>
[error]         ^
[error] C:\Akka Actors\chapter02\src\main\scala\ActorCreation.scala:30: not found: value StopMusic
[error]    case StopMusic =>
[error]         ^
[error] C:\Akka Actors\chapter02\src\main\scala\ActorCreation.scala:32: not found: value StartMusic
[error]    case StartMusic =>
[error]         ^
[error] C:\Akka Actors\chapter02\src\main\scala\ActorCreation.scala:34: not found: value Play
[error]      controller ! Play
[error]                   ^
[error] C:\Akka Actors\chapter02\src\main\scala\ActorCreation.scala:46: not found: value StartMusic
[error]  player ! StartMusic
[error]           ^
[error] 6 errors found
[error] (compile:compileIncremental) Compilation failed
[error] Total time: 10 s, completed Sep 22, 2017 11:12:37 AM
>

供您参考，这里是 build.sbt

name := "akka-actors"

version := "1.0"

scalaVersion := "2.11.8"

libraryDependencies +=
    "com.typesafe.akka" % "akka-actor_2.11" % "2.4.0"

【问题讨论】：

标签： scala traits companion-object akka-actor

【解决方案1】：

您只需要将这些对象从伴生对象导入到演员中，这里是一个使用 MusicController 的示例：

object MusicController {
  sealed trait ControllerMsg
  case object Play extends ControllerMsg
  case object Stop extends ControllerMsg

}

class MusicController extends Actor{
  import MusicController._  // <--- Add this

  def receive = {
   case Play =>
     println("Music Started.....")
   case Stop =>
     println("Music Stopped.....")
  }
}

或者，您可以将导入语句添加到文件顶部，以便所有内容都可以访问它们：

import akka.actor.Actor
import akka.actor.ActorSystem
import akka.actor.Props
import example.MusicPlayer._       // new
import example.MusicController._   // new

【讨论】：