Scala 运行时反射获取特定类型的所有成员，即使对于内部类也是如此

Question

使用 scala 2.12.10

假设我想在运行时隐式转换一个案例类，在本例中是 Special 到一个案例类 SpecialString。隐式转换由特征 External 提供。 SpecialString 的名称应该是 Special 类的声明名称。

import scala.reflect.runtime.universe.{runtimeMirror, typeOf}
import scala.reflect.runtime.universe


case class Special(i: Int)
case class SpecialString(s: String)

trait External {
  val rm = runtimeMirror(getClass.getClassLoader)
  val im = rm.reflect(this)
  val members = im.symbol.typeSignature.members
  def specials: Iterable[universe.Symbol] = members.filter(_.typeSignature <:< typeOf[Special] )
  implicit def SpecialIntToString(s: Special): SpecialString = {
    val name = im.reflectField(specials.filter(x => im.reflectField(x.asTerm).get.asInstanceOf[Special] == s).head.asTerm).symbol.toString.replace("value ", "")
    SpecialString(s"name = $name")
  }
}

目前，我能够隐式转换在扩展 External 特征的类中声明的转换成员。

class MyClass extends External {
  val firstSpecial = Special(1)
  val two = 2
  val specialS: SpecialString = firstSpecial
}

class MySecondClass extends MyClass {
  val specialS2: SpecialString = firstSpecial
}
val myClass = new MyClass
print(myClass.specialS) // SpecialString(name = firstSpecial)

但是我无法转换在父类(super class)中声明的成员

class MyClass {
  val firstSpecial = Special(1)
  val two = 2
  val specialS: SpecialString = firstSpecial
}

class MySecondClass extends MyClass with External {
  val specialS2: SpecialString = firstSpecial
}
val myClass = new MyClass
print(myClass.specialS)
val mySecondClass = new MySecondClass
print(mySecondClass.specialS2) // java.util.NoSuchElementException: next on empty iterator

有什么帮助吗？

Answer 1

如果您通过名称而不是 typeSignature 找到必要的成员(然后它实际上找到了)并打印 specials.head.typeSignature 和 typeOf[Special] 你会明白为什么一个不是另一个的子类型

trait External {
  ...
  def specials: Iterable[universe.Symbol] =
    members.filter(_.name == universe.TermName("firstSpecial") )
    //members.filter(_.typeSignature.resultType <:< typeOf[Special] )
  println(s"specials.head.typeSignature=${specials.head.typeSignature}=${universe.showRaw(specials.head.typeSignature)}")
  println(s"typeOf[Special]=${typeOf[Special]}=${universe.showRaw(typeOf[Special])}")
  println(s"specials.head.typeSignature <:< typeOf[Special]=${specials.head.typeSignature <:< typeOf[Special]}")
  ...
}

//specials.head.typeSignature=pckg.App.Special=NullaryMethodType(TypeRef(ThisType(pckg.App), pckg.App.Special, List()))
//typeOf[Special]            =pckg.App.Special=TypeRef(ThisType(pckg.App), pckg.App.Special, List())
//specials.head.typeSignature <:< typeOf[Special]=false

返回 Special 的 nullary 方法的类型不是 Special 的子类型。

您应该添加resultType。替换

trait External {
  ...
  def specials: Iterable[universe.Symbol] =
    members.filter(_.typeSignature <:< typeOf[Special] )

与

trait External {
  ...
  def specials: Iterable[universe.Symbol] =
    members.filter(_.typeSignature.resultType <:< typeOf[Special])

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