在Akka中保留类型参数 [英] Preserving type arguments in Akka receive

问题描述

Roland Kuhn在此帖子中已经回答了该问题。但是，尽管有几条评论要求提供详细信息，他还是不愿意分享完整的答案。

This question has kind of been answered by Roland Kuhn in this post, however, despite several comments asking for detail, he didn't bother to share the complete answer.

这就是我想要做的：我有一个包装类 case类Event [T]（t：T）我将实例发送给Akka演员。然后，在该参与者的 receive 方法中，我想区分 Event [Int] 和 Event [String] ，由于类型擦除，这显然不是那么简单。

Here's what I want to do: I have a wrapper class case class Event[T](t: T) of which I send instances to an Akka actor. In the receive method of that actor, I then want to distinguish between Event[Int] and Event[String], which obviously isn't so simple due to type erasure.

Roland Kuhn在提到的帖子中分享的是确实有一种方法可以做到，即在消息中体现类型信息。所以我这样做了：

What Roland Kuhn shares in the mentioned post is that "there is exactly one way to do it", that is, embodying the type information within the message. So I did this:

case class Event[T](t: T)(implicit val ct: ClassTag[T])

尽管有人提出要求，但Roland Kuhn并没有说在然后接收方法。这是我尝试的方法。

Even though asked by different people to provide it, Roland Kuhn does not say what to actually do within the receive method then. Here's what I tried.

def receive = {
  case e: Event =>
    if (e.ct.runtimeClass == classOf[Int])
      println("Got an Event[Int]!")
    else if (e.ct.runtimeClass == classOf[String])
      println("Got an Event[String]!")
    else
      println("Got some other Event!")
  case _ =>
    println("Got no Event at all!")
}

此这是我能想到的最好的选择，因为很难将自己的头包裹在Scala的反射丛林中。

This is the best I could come up with as it's hard to wrap one's head around Scala's reflection jungle. It's not compiling, though:

value ct is not a member of Any
else if (e.ct.runtimeClass == classOf[String])
           ^

因此，我要特别询问接收方法应如下所示。

Thus, I am asking specifically about what the receive method should look like.

推荐答案

修复错误<$之后c $ c>事件采用类型参数：

def receive = {
  case e: Event[_] =>
    if (e.ct.runtimeClass == classOf[Int])
      println("Got an Event[Int]!")
    else if (e.ct.runtimeClass == classOf[String])
      println("Got an Event[String]!")
    else
      println("Got some other Event!")
  case _ =>
    println("Got no Event at all!")
}

代码编译。通过不查看 ClassTag 的内部，可以稍微简化一下（当然， ClassTag＃equals 的实现正在比较类）：

the code compiles. It can be slightly simplified by not looking inside the ClassTags (of course, the implementation of ClassTag#equals is going to compare the classes):

import scala.reflect.{ClassTag, classTag}

def receive = {
  case e: Event[_] =>
    if (e.ct == ClassTag.Int) // or classTag[Int]
      println("Got an Event[Int]!")
    else if (e.ct == classTag[String])
      println("Got an Event[String]!")
    else
      println("Got some other Event!")
  case _ =>
    println("Got no Event at all!")
}

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