斯卡拉：與工廠

Question

繼承比方說，我有兩個特點，其中之一是一個工廠的另一個：

trait BaseT { 
    val name: String 
    def introduceYourself() = println("Hi, I am " +name) 
    // some other members ... 
} 
trait BaseTBuilder { 
    def build: BaseT 
} 

現在，我想延長巴塞特：

trait ExtendedT extends BaseT { 
    val someNewCoolField: Int 
    override def introduceYourself() = { 
    super.introduceYourself() 
    println(someNewCoolField) 
    } 
    // some other extra fields 

比方說，我知道如何初始化新字段，但我想用BaseTBuilder來初始化超類成員。是否有可能創建一個能夠以某種方式實例化ExtendedT的特徵？這種做法顯然失敗：

trait ExtendedTBuilder { self: TBuilder => 
    def build: ExtendedT = { 
    val base = self.build() 
    val extended = base.asInstanceOf[ExtendedT] // this cannot work 
    extended.someNewCoolField = 4 // this cannot work either, assignment to val 
    extended 
    } 
    def buildDifferently: ExtendedT = { 
    new ExtendedT(4) // this fails, we don't know anything about constructors of ExtendedT 
    } 
    def build3: ExtendedT = { 
    self.build() with {someNewCoolField=5} //that would be cool, but it cannot work either 
    } 
} 

我想有這樣一組特性（或對象），當有人提供具體落實BaseT和BaseTBuilder我可以寫instantiantiate ExtendedT的：

val extendedBuilder = new ConcreteBaseTBuilder with ExtendedTBuilder 
val e: ExtendedT = extendedBuilder.build 

ExtendedT可能包含BaseT類型的字段，但這時需要手動代理所有必要的方法和字段，這在我看來是違反DRY原則的。如何解決這個問題？

Answer 1

如何在你的ExtendBaseTBuilder

trait ExtendBaseTBuilder { self : BaseTBuilder => 
    def build: ExtendBaseT = { 
    new ExtendBaseT { 
     val someNewCoolField: Int = 3 
    } 
    } 

}