一篇写的比较好的关于scala中trait特性的文章

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我的新博客地址：http://cuipengfei.me/blog/2013/10/13/scala-trait/

我在Coursera上跟了一门叫做Functional Programming Principles in Scala的课程，是由Scala的作者Martin Odersky讲授的。其中第三周的作业中使用到了Scala的trait这个语言特性。

我以前熟知的语言都没有类似的特性（Ruby的mixin和Scala的trait很像，但是Ruby我不熟），所以这周的博客就分析一下这个语言特性是如何实现的。

trait

在讲trait的实现机制之前，先看一个使用trait的例子。假设我们有以下几个类：

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abstract class Plant {  def photosynthesis = println("Oh, the sunlight!")}class Rose extends Plant {  def smell = println("Good!")  def makePeopleHappy = println("People like me")}class Ruderal extends Plant {  def grow = println("I take up all the space!")}abstract class Animal {  def move = println("I can move!")}class Dog extends Animal {  def bark = println("Woof!")  def makePeopleHappy = println("People like me")}class Snake extends Animal {  def bite = println("I am poisonous!")}

植物家族有玫瑰和杂草。

动物家族有狗和毒蛇。

仔细观察可以发现，玫瑰和狗有一个共同的行为，它们都可以取悦人类，这个行为是用完全一样的代码实现的。

如何把Rose和Dog中的重复代码消除掉呢？有一种潜在的解决方案：把makePeopleHappy提取到一个类中去，让植物和动物都继承自它。

这么做虽然消除了重复代码但有两个明显的缺点：

植物和动物继承自同一个类，不太合理
杂草和毒蛇也具有了取悦于人的能力，也不太合理

这时我们就可以使用trait，它没有上面提到的两个缺点。

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trait PeoplePleaser {  def makePeopleHappy = println("People like me")}class Rose extends Plant with PeoplePleaser {  def smell = println("Good!")}class Dog extends Animal with PeoplePleaser {  def bark = println("Woof!")}

我们定义一个trait，把makePeopleHappy置于其中，让Rose和Dog都with这个trait。然后就可以写这样的代码来调用它们了：

  new Rose().makePeopleHappy  new Dog().makePeopleHappy

这样我们就解决了重复代码的问题，而且没有触及已存在的继承关系。

现在看看trait的实现机制吧，我们开始反编译！

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public abstract interface PeoplePleaser{  public abstract void makePeopleHappy();}public abstract class PeoplePleaser$class{  public static void makePeopleHappy(PeoplePleaser $this)  {    Predef..MODULE$.println("People like me");  }  public static void $init$(PeoplePleaser $this)  {  }}public class Rose extends Plant  implements PeoplePleaser{  public void makePeopleHappy()  {    PeoplePleaser$class.makePeopleHappy(this);  }  public void smell() { Predef..MODULE$.println("Good!"); }  public Rose()  {    PeoplePleaser.class.$init$(this);  }}public class Dog extends Animal  implements PeoplePleaser{  public void makePeopleHappy()  {    PeoplePleaser$class.makePeopleHappy(this);  }  public void bark() { Predef..MODULE$.println("Woof!"); }  public Dog()  {    PeoplePleaser.class.$init$(this);  }}

真相大白了，PeoplePleaser被编译成了一个接口加一个抽象类。Rose和Dog实现这个接口，并通过调用抽象类中的静态方法来实现了makePeopleHappy。

很有趣的一点是Rose和Dog在调用静态方法时都把this传了进去，为什么呢？我们把原来的代码改成这样来看：

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trait PeoplePleaser {  val moreMessage = ""  def makePeopleHappy = println("People like me. " + moreMessage)}class Rose extends Plant with PeoplePleaser {  override val moreMessage = "Because I smell nice."  def smell = println("Good!")}class Dog extends Animal with PeoplePleaser {  override val moreMessage = "Because I fetch balls."  def bark = println("Woof!")}

我们给makePeopleHappy加上一段额外的信息。现在再次反编译。

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public abstract interface PeoplePleaser{  public abstract void objsets$PeoplePleaser$_setter_$moreMessage_$eq(String paramString);  public abstract String moreMessage();  public abstract void makePeopleHappy();}public abstract class PeoplePleaser$class{  public static void makePeopleHappy(PeoplePleaser $this)  {    Predef..MODULE$.println(new StringBuilder()    .append("People like me. ")    .append($this.moreMessage()).toString());  }  public static void $init$(PeoplePleaser $this)  {    $this.objsets$PeoplePleaser$_setter_$moreMessage_$eq("");  }}public class Rose extends Plant  implements PeoplePleaser{  private final String moreMessage;  public void objsets$PeoplePleaser$_setter_$moreMessage_$eq(String x$1)  {  }  public void makePeopleHappy()  {    PeoplePleaser$class.makePeopleHappy(this);  }  public String moreMessage() { return this.moreMessage; }  public void smell() {    Predef..MODULE$.println("Good!");  }  public Rose()  {    PeoplePleaser.class.$init$(this);    this.moreMessage = "Because I smell nice.";  }}public class Dog extends Animal  implements PeoplePleaser{  private final String moreMessage;  public void objsets$PeoplePleaser$_setter_$moreMessage_$eq(String x$1)  {  }  public void makePeopleHappy()  {    PeoplePleaser$class.makePeopleHappy(this);  }  public String moreMessage() { return this.moreMessage; }  public void bark() {    Predef..MODULE$.println("Woof!");  }  public Dog()  {    PeoplePleaser.class.$init$(this);    this.moreMessage = "Because I fetch balls.";  }}

现在就清楚了，抽象类中的静态方法可能会依赖于各个实例不同的状态，所以需要把this传递进去。这样我们才能够给makePeopleHappy加上一段额外的信息。

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