具有 stashing 和 unstashing 的 Akka FSM Actor

Question

我想使用 FSM Akka Acctor 进行存储/取消存储。我不知道把stash() 和unstashAll() 放在哪里。

我有一个简化的例子如下：

import akka.actor.{ActorSystem, FSM, Props, Stash}

trait TestState
case object StateA extends TestState
case object StateB extends TestState

case class TestData()

case class MessageA(msg: String)
case class MessageB(msg: String)
case object ChangeState

class TestFSM extends FSM[TestState, TestData] with Stash {

  startWith(StateA, TestData())

  when(StateA) {
    case Event(MessageA(msgA), _) =>
      println(s"In StateA: $msgA")
      stay()
    case Event(ChangeState, _) =>
      println("Changing state from A to B")
      goto(StateB)
  }

  when(StateB) {
    case Event(MessageB(msgB), _) =>
      println(s"In StateB: $msgB")
      stay()
  }

  whenUnhandled {
    case Event(e, _) =>
      println(s"Unhandled event: $e")
      stay()
  }
}

object TestFSM extends App {
  val system = ActorSystem("test-system")
  val actor = system.actorOf(Props[TestFSM])

  actor ! MessageA("Apple 1")
  actor ! MessageB("Banana 1")
  actor ! MessageA("Apple 2")

  actor ! ChangeState

  actor ! MessageB("Banana 2")
}

初始状态为StateA。 当在StateA 中时，actor 应该只处理MessageA 类型的消息。如果它收到任何其他类型的消息（ChangeState 除外），它应该将其隐藏。收到消息ChangeState 后，actor 应更改为StateB。 从StateA 更改为StateB 后，它应该取消隐藏所有消息。 在 StateB 中时，actor 应该只处理 MessageB 类型的消息。

我不确定在哪里使用stash() 和unstashAll() 来实现这一点。

我运行的输出是：

In StateA: Apple 1
Unhandled event: MessageB(Banana 1)
In StateA: Apple 2
Changing state from A to B
In StateB: Banana 2

我想看到的输出是：

In StateA: Apple 1
In StateA: Apple 2
Changing state from A to B
In StateB: Banana 1
In StateB: Banana 2

非常感谢。

Answer 1

您可以通过在FSM 上使用onTransition 方法来实现此目的。这会在状态发生变化时执行，我们可以利用那一刻来取消存储所有消息。至于存储，您需要在whenUnhandled 方法中进行。我还做了一个小更新，以便您可以在状态之间循环：

class TestFSM extends FSM[TestState, TestData] with Stash {
  startWith(StateA, TestData())

  when(StateA) {
    case Event(MessageA(msgA), _) =>
      println(s"In StateA: $msgA")
      stay()
    case Event(ChangeState, _) =>
      println("Changing state from A to B")
      goto(StateB)
  }

  when(StateB) {
    case Event(MessageB(msgB), _) =>
      println(s"In StateB: $msgB")
      stay()
    case Event(ChangeState, _) =>
      println("Changing state from B to A")
      goto(StateA)
  }

  /**
    * Here we can stash all messages. For example when we're in state A,
    * we handle both `MessageA` and `ChangeState` messages, but we don't
    * handle `MessageB` instances which will end up here. The opposite
    * happens when we're in state B.
    */
  whenUnhandled {
    case _: Event =>
      stash()
      stay()
  }

  // When transitioning into another state, unstash all messages.
  onTransition {
    case StateA -> StateB => unstashAll()
    case StateB -> StateA => unstashAll()
  }
}

如果您还有其他问题，请告诉我:)