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Encoder ECC200 in Font ACTORS AND CONCURRENCY

CHAPTER 6 ACTORS AND CONCURRENCY
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Our Actor event handler receives the Changed message and invokes the private changed method.
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def act = loop { react { case Changed(f, cnt) => changed(f, cnt) } }
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The changed method sends an Access message to eventFrom if the count is less than 10:
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private def changed(eventFrom: AFoo, count: Int): Unit = if (count < 10) eventFrom ! Access }
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The listener also looks very similar. However, the behavior is different. The message handling is asynchronous, so we ll never run out of stack space, because the mutating happens when the Access message is processed and does not wait for the listener to process the Changed message. You ll also see that the synchronization is gone from the classes. We ve exchanged a little extra syntax and type-safety for a different and in many ways more powerful concurrency mechanism.7 Let s run the code:
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scala> val af = new AFoo
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af: AFoo = AFoo@1aca5e2
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scala> af.start
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res4: scala.actors.Actor = AFoo@1aca5e2
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scala> val afl = new AFooListener(af)
afl: AFooListener = AFooListener@12ee2a
7. Actors can receive anything as a message. They are like duck-typed objects in Ruby or Python. Thus, the compiler does not check to see whether the particular message that you are sending to an Actor is one that the Actor can or could process.
CHAPTER 6 ACTORS AND CONCURRENCY
scala> afl.start
res5: scala.actors.Actor = AFooListener@12ee2a
scala> af ! Access
No problem it works.
Actors: More Than Write-Only
So far, we ve seen Actors that are pretty much write-only. They know about their state, but unless you re a listener, you have no clue about their state. This is generally less than optimal. It s possible to send a message to an Actor and synchronously wait for a reply:
ChatServer2 ! GetMessages
This will send a GetMessages message to ChatServer2 and wait until ChatServer2 replies. We can pattern match the result:
val msgs: Option[List[String]] = ChatServer2 ! GetMessages match { case Messages(msg) => Some(msg) case _ => None }
We can also send the message and wait for a reply. If the reply comes within the timeout period, the method returns Some(response), otherwise the method returns None:
val msgs2: Option[List[String]] = ChatServer2 ! (500, GetMessages) match { case Some(Messages(msg)) => Some(msg) case Some(_) => None // got a response we didn't understand case None => None // timeout }
In the previous example, we send the GetMessages message to ChatServer2 and wait 500 milliseconds for a response. Let s look at the messages and the ChatServer2 implementation:
case object GetMessages case class Messages(msg: List[String]) object ChatServer2 extends Actor { private var chats: List[String] = Nil
CHAPTER 6 ACTORS AND CONCURRENCY
def act = loop { react { case s: String => chats = s :: chats case GetMessages => reply(Messages(chats)) } } this.start // make sure we start the chat server }
The GetMessages message is processed by reply(Messages(chats)). The paradigm is pretty simple. It s much like getters in standard OOP, except it is heavier-weight both in terms of syntax and execution speed. In general, if you keep most of the state in your Actor in immutable data structures and the query messages return all or substantially all of the state as a single request, then the overhead is incurred less frequently than repeatedly calling granular getters on an object:
for (i <- 0 until chats.count) yield chats.message(i)
Additionally, the difference in the calling syntax triggers something in my brain that says, This call may time out, so make sure you re testing the return value. This is an important value of the syntactic differences between object method invocation and Actor message sending. The difference says to the developer, Calculate the costs of invocation and probability of failure differently than for a normal method invocation. Additionally, during code reviews, it s much easier to see where Actors are being accessed.
Beyond the Basics
So far, we ve seen the basics of creating Actors, sending messages to Actors, and receiving replies from Actors. In this section, we re going to explore how Actors can change the messages they process depending on their state. This will lead us to a couple of ways to do transactions with Actors. Finally, we ll talk about how exception handling in Actors differs from normal exception processing.
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