伴随对象的继承答案

【问题标题】：Inheritance with companion Objects伴随对象的继承
【发布时间】：2016-08-09 15:06:04
【问题描述】：

我正在尝试重构一些我目前看起来像这样的模型：

case class Person(name: String, age: Int)

object Person {
    implicit val reads: Reads[Person] = (
        (JsPath \ "name").read[String] and
        (JsPath \ "age").read[Int] 
    )(Person.apply _)
}

到看起来像这样的东西：

abstract class BaseModel {
    val pk: String  // Some stuff here that will be common
}

object BaseModel {
    implicit val reads: Reads[BaseModel] // Not sure what to do here
}

所以我可以这样做：

trait MyTrait[Model <: BaseModel] {

    // More code here
    val body: JsObject = ...
    val parsed = body.validate[Model]  // Error: There is no implicit value defined for Model

}


case class NewPerson extends BaseModel {...}
object NewPerson {...} // Maybe need to extend something here


class MyController extends MyTrait[NewPerson]

我希望每个模型都定义一个隐式读取值，但我不确定如何在抽象类的伴随对象中指出这一点。

【问题讨论】：

标签： scala generics inheritance companion-object

【解决方案1】：

没有一种语言功能会强制将抽象类/伴侣对一起扩展。通过使抽象类的“伴侣”成为特征，我克服了这个缺失的环节。像这样的：

abstract class BaseModel {
     val pk: String
}

trait ModelCompanion[A <: BaseModel] { self: Singleton =>
    implicit val reads: Reads[A]
}

case class Person(name: String, age: Int) extends BaseModel

object Person extends BaseModel[Person] {
    ...
}

不幸的是，这仍然没有告诉MyTrait（在 OP 中定义）Model <: BaseModel 有一个可以找到隐式Reads[Model] 的同伴。同样，需要手动创建链接，要求 MyTrait 持有对模型伴随对象的引用。

trait MyTrait[Model <: BaseModel] {

    def companion: ModelCompanion[Model]

    implicit def reads: Reads[Model] = companion.reads

    // More code here
    val body: JsObject = ...
    val parsed = body.validate[Model]  // Now this would work

}

object MyTraitImpl extends MyTrait[Person] {
    def companion = Person
}

【讨论】：