Scala By Example: 拍卖会

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Scala的最大亮点就是Erlang风格的actor处理机制。但是第二个示例就涉及到Actor令我始料未及。不过这本书在Scala官网上是属于“进阶示例”，所以出手有些快也在所难免。（如果看不懂这个例子，最好先读铅笔书《Scala程序设计》（我假设你懂Java，否则你就直接选Erlang了，不是吗。书非常薄，如果有闭包之类的经验的话，三天就能看个差不多）。

   1: #!/bin/env scala   2: !#   3:     4: import scala.actors._   5: import Actor._   6: import java.util.Date   7:     8: // for each base class, there are a number of 'case classed' which define the   9: // format of marticular messages in the class  10: abstract class AuctionMessage  11: case class Offer(bid: Int, client: Actor) extends AuctionMessage  12: case class Inquire(client: Actor) extends AuctionMessage  13:    14: abstract class AuctionReply  15: case class Status(asked: Int, expire: Date) extends AuctionReply  16: case object BestOffer extends AuctionReply  17: case class BeatenOffer(maxBid: Int) extends AuctionReply  18: case class AuctionConcluded(seller: Actor, client: Actor) extends AuctionReply  19: case object AuctionFailed extends AuctionReply  20: case object AuctionOver extends AuctionReply  21:    22: class Auction(seller: Actor, minBid: Int, closing: Date) extends Actor {  23:     val timeToShutdown = 3600  24:     val bidIncrement = 10  25:    26:     println("Auction Started!")  27:     def act() {  28:         var maxBid = minBid - bidIncrement  29:         var maxBidder: Actor = null  30:         var running = true  31:         while(running) {  32:             println("Auction is still on")  33:             // the 'receiveWithin' method of class Actor takes as parameters a  34:             // time span given in milliseconds and a function that process  35:             // messages in the mailbox.  36:             val timeout = closing.getTime() - new Date().getTime()  37:             println("To be timeout in " + (timeout / 1000) + "s")  38:             receiveWithin(timeout) {  39:                 case Offer(bid, client) =>  40:                     if(bid >= maxBid + bidIncrement) {  41:                         if(maxBid >= minBid) maxBidder ! BeatenOffer(bid)  42:                         maxBid = bid; maxBidder = client; client ! BestOffer  43:                     } else {  44:                         client ! BeatenOffer(maxBid)  45:                     }  46:                 case Inquire(client) => client ! BeatenOffer(maxBid)  47:                 // if no messages are received in the meantime, this pattern is  48:                 // triggered after the time span which is passed as argument to  49:                 // the enclosing receiveWithin method.  50:                 case TIMEOUT =>  51:                     if(maxBid >= minBid) {  52:                         val reply = AuctionConcluded(seller, maxBidder)  53:                         maxBidder ! reply  54:                         seller ! reply  55:                     } else {  56:                         seller ! AuctionFailed  57:                     }  58:                     receiveWithin(timeToShutdown) {  59:                         case Offer(_, client) => client ! AuctionOver  60:                         case TIMEOUT => running = false  61:                     }  62:             }  63:         }  64:     }  65: }  66:    67: val seller = actor {  68:     println("I'm the seller")  69:     var running = true  70:     while(running) {  71:         println("Seller is still waiting...")  72:         receiveWithin(5000) {  73:             case AuctionConcluded(_, maxBidder) =>  74:                 println("Auction concluded! The max bidder is" + maxBidder)  75:                 running = false  76:             case AuctionFailed =>  77:                 println("F**k, it failed again!")  78:                 running = false  79:             case _ => println("Nothing happened")  80:         }  81:     }  82: }  83:    84: val date = new Date(new Date().getTime() + 20000)  85: val auction = new Auction(seller, 100, date)  86:    87: val bidder1 = actor {  88:   auction ! Inquire(self) // notice: auction is not yet started  89:   receiveWithin(1000) { case msg => println(msg) }  90: }  91:    92: auction.start()  93:    94: val bidder2 = actor {  95:   auction ! Offer(120, self)  96:   receiveWithin(1000) { case msg => println(msg) }  97: }  98:    99: val bidder3 = actor { 100:   auction ! Offer(150, self) 101:   receiveWithin(1000) { case msg => println(msg) } 102: } 103:   104: val bidder4 = actor { 105:   auction ! Offer(120, self) 106:   receiveWithin(1000) { case msg => println(msg) } 107: } 108:   109: receiveWithin(30000) { case _ => } // little trick:)

注意，在程序的一开始，我们看到两个什么也不做的抽象类和一堆派生的case类/对象，它们就是在Actor之间传递的消息。当然，也可以不定义这些类和对象，case同样可以接受元组形式的参数。注意，使用Scala的时候，你可以彻底忘掉Singleton（也许有人还在争论Singleton要不要做双重加锁呢），那几个用object声明的对象就是Scala中的单例对象。（有人曾经评价ruby这样的魔幻语言让许多经典模式都瞬时凌乱，我认为Groovy和Scala都是如此）

第二个重点则是act()的方法。尤其重要的是receiveWithin和TIMEOUT的配合使用。想想看，用JAVA来写的话你需要写多少行？

原书的代码只包含了类的定义，为了运行这些代码，我加了N多行。大家可以运行这个程序看看输出是什么样的顺序。体会一下Scala消息传递的“风格”。

最后需要指出的是最后一行代码。逻辑上这行代码什么都不做，但是由于所有Actor（包括auction, seller 和一群bidder）都在自己的进程里运行，所以当bidder4被创立之后，主进程就结束了（而此时拍卖会并没有结束）。因此，加上一行等待专用码还是有必要的。

PS. 虽然我以说一口华丽的中文为骄傲，但是当写代码的时候，输入中文永远令我纠结。今天我下定决心，以后在IDE里也好、VIM里也好，都用鸟语书写了。（嗯，顺便把练字放到我的日程里，想当年我的书法还是很漂亮的）

PS II. 虽然把Scala的语法大致过了遍，真正写程序的时候还是不熟练，所以我做了个练习项目，用主要Scala来写，不会的地方用Groovy和Java搭一把。结果Scala和Java相互调用没有问题，Groovy和Java相互调用也没有问题，放到一起就杯具了。Groovy可以调用Scala，但是混合编译的时候要先编译Groovy后编译Scala，否则就报错，至于反过来，完全没法混合编译（IDEA和gradle都一样），看来只有把不同的代码分别编译后才能相互调用了。

PS III. 一想到以后不用再输入synchronized就兴奋。

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