无法理解lagom中的执行流程

Question

我正在学习本教程 - https://www.lagomframework.com/documentation/1.3.x/scala/ServiceImplementation.html

我创建了一个记录服务

//logged takes a ServerServiceCall as argument (serviceCall) and returns a ServerServiceCall.
  //ServerServiceCall's compose method creates (composes) another ServerServiceCall
  //we are returing the same ServerServiceCall that we received but are logging it using println
  def logged[Request,Response](serviceCall:ServerServiceCall[Request,Response]) = {
    println("inside logged");

    //return new ServerServiceCall after logging request method and uri
    ServerServiceCall.compose({
    requestHeader=>println(s"Received ${requestHeader.method} ${requestHeader.uri}")
    serviceCall
  }
  )}

我使用记录如下：

  override def hello3 = logged(ServerServiceCall
  { (requestHeader,request) =>
    println("inside ssc")
    val incoming:Option[String] = requestHeader.getHeader("user");

    val responseHeader = ResponseHeader.Ok.withHeader("status","authenticated")
    incoming match {
      case Some(s) => Future.successful((responseHeader,("hello3 found "+s)))
      case None => Future.successful((responseHeader,"hello3 didn't find user"))
    }
  })

我预计会先打印inside ssc，然后在logged中打印，结果却相反。传递给函数的参数不应该首先被评估吗？

我明白了。为什么？

inside logged Received POST /hello3 inside ssc

Answer 1

logged 是您编写的一个函数，它接受一个 ServiceCall 并用它自己的 ServiceCall 修饰它。稍后，Lagom 使用请求 header 调用服务调用。您在服务调用被修饰时记录 inside logged，在它返回到 Lagom 之前，因此在它被调用之前，这就是它首先被调用的原因。这或许可以解释：

def logged[Request, Response](serviceCall:ServerServiceCall[Request, Response]) = {
  println("3. inside logged method, composing the service call")
  val composed = ServerServiceCall.compose { requestHeader=>
    println("6. now Lagom has invoked the logged service call, returning the actual service call that is wrapped")
    serviceCall
  }

  println("4. Returning the composed service call")
  composed
}

override def hello3 = {
  println("1. create the service call")
  val actualServiceCall: ServerServiceCall[Request, Response] = ServerServiceCall { (requestHeader, request) =>
    println("7. and now the actual service call has been invoked")
    val incoming:Option[String] = requestHeader.getHeader("user");

    val responseHeader = ResponseHeader.Ok.withHeader("status","authenticated")
    incoming match {
      case Some(s) => Future.successful((responseHeader,("hello3 found "+s)))
      case None => Future.successful((responseHeader,"hello3 didn't find user"))
    }
  }

  println("2. wrap it in the logged service call")
  val loggedServiceCall = logged(actualServiceCall)

  println("5. return the composed service call to Lagom")
  loggedServiceCall
}

需要记住的重要一点是hello3方法的调用不是服务调用的调用，hello3仅仅是returns Lagom 将用来调用它的 ServiceCall。

无法理解lagom中的执行流程

unable to understand execution flow in lagom

lagom