获取有关Scala中案例类字段的完整信息

Question

请考虑以下类和方法:

case class User(id: Long, name: String) {
  private var foo = "Foo" // shouldn't be printed
  val bar = "bar" // also shouldn't be printed
}
case class Message(id: Long, userId: Long, text: String)

def printInfo[E](o: E)(implicit tt: TypeTag[E]) = {

}

我想让这个方法打印任何案例类的每个字段的名称,类型和值,即

printInfo(User(1, "usr1")) // prints something like "(id, Long, 1), (name, String)"
printInfo(Message(1, 1, "Hello World")) // prints "(id, Long, 1), (userId, Long, 1), (text, String, "Hello World")"

为字段添加一些自定义注释也是相当可观的.

Answer 1

您可以通过检查类型标记列出的成员并使用其镜像进行反映来完成此操作:

import scala.reflect.ClassTag
import scala.reflect.runtime.universe.TypeTag

def printInfo[A](a: A)(implicit tt: TypeTag[A], ct: ClassTag[A]): String = {
  val members = tt.tpe.members.collect {
    case m if m.isMethod && m.asMethod.isCaseAccessor => m.asMethod
  }

  members.map { member =>
    val memberValue = tt.mirror.reflect(a).reflectMethod(member)()
    s"(${ member.name }, ${ member.returnType }, $memberValue)"
  }.mkString(", ")
}

哪个会像这样工作:

scala> case class User(id: Long, name: String) {
     |   private var foo = "Foo" // shouldn't be printed
     |   val bar = "bar" // also shouldn't be printed
     | }
defined class User

scala> case class Message(id: Long, userId: Long, text: String)
defined class Message

scala> printInfo(User(1, "usr1"))
res0: String = (name, String, usr1), (id, scala.Long, 1)

scala> printInfo(Message(1, 1, "Hello World"))
res1: String = (text, String, Hello World), (userId, scala.Long, 1), (id, scala.Long, 1)

(如果你想要Long而不是scala.Long它,那么从你得到的类型中删除前缀并不会太难member.returnType,但我会将其作为练习留给读者.)

如果没有使用Shapeless进行任何运行时反射,这也不难做到这一点:

import shapeless.{ ::, HList, HNil, LabelledGeneric, Typeable, Witness }
import shapeless.labelled.FieldType

trait PrettyPrintable[A] {
  def apply(a: A): List[(String, String, String)]
}

object PrettyPrintable {
  implicit val hnilPrettyPrintable: PrettyPrintable[HNil] =
    new PrettyPrintable[HNil] {
      def apply(a: HNil): List[(String, String, String)] = Nil
    }

  implicit def hconsPrettyPrintable[K <: Symbol, H, T <: HList](implicit
    kw: Witness.Aux[K],
    ht: Typeable[H],
    tp: PrettyPrintable[T]
  ): PrettyPrintable[FieldType[K, H] :: T] =
    new PrettyPrintable[FieldType[K, H] :: T] {
      def apply(a: FieldType[K, H] :: T): List[(String, String, String)] =
        (kw.value.name, ht.describe, a.head.toString) :: tp(a.tail)
    }

  implicit def genPrettyPrintable[A, R <: HList](implicit
    ag: LabelledGeneric.Aux[A, R],
    rp: PrettyPrintable[R]
  ): PrettyPrintable[A] = new PrettyPrintable[A] {
    def apply(a: A): List[(String, String, String)] = rp(ag.to(a))
  }

  def printInfo[A](a: A)(implicit pp: PrettyPrintable[A]) = pp(a).map {
    case (memberName, memberType, memberValue) =>
      s"($memberName, $memberType, $memberValue)"
  }.mkString(", ")
}

然后:

scala> PrettyPrintable.printInfo(User(1, "usr1"))
res2: String = (id, Long, 1), (name, String, usr1)

scala> PrettyPrintable.printInfo(Message(1, 1, "Hello World"))
res3: String = (id, Long, 1), (userId, Long, 1), (text, String, Hello World)

除了其他的东西,这给你的声明顺序的字段,我认为应该可以使用类型标记方法,但我尽可能多地避免使用该API,所以我不确定.