从类[A]中获取TypeTag [A]

Question

我有createOld方法,我需要覆盖,我无法改变它.我想使用TypeTag模式匹配提供的类型createNew.我们的目标是找出如何调用createNew的createOld.我目前的理解是编译器没有足够的关于Ain createOld方法的类型信息,如果它还没有TypeTag[A].

object TypeTagFromClass {
  class C1
  class C2

  // How to get TypeTag[A] needed by createNew?
  def createOld[A](c: Class[A]): A = createNew ???

  def createNew[A : TypeTag]: A = {
    val result = typeOf[A] match {
      case a if a =:= typeOf[C1] => new C1()
      case a if a =:= typeOf[C2] => new C2()
    }
    result.asInstanceOf[A]
  }
}

Answer 1

它可以创建一个TypeTag从Class使用Scala的反射,虽然我不知道这是否执行TypeCreator是绝对正确的:

import scala.reflect.runtime.universe._

def createOld[A](c: Class[A]): A = createNew {
  val mirror = runtimeMirror(c.getClassLoader)  // obtain runtime mirror
  val sym = mirror.staticClass(c.getName)  // obtain class symbol for `c`
  val tpe = sym.selfType  // obtain type object for `c`
  // create a type tag which contains above type object
  TypeTag(mirror, new TypeCreator {
    def apply[U <: Universe with Singleton](m: api.Mirror[U]) =
      if (m eq mirror) tpe.asInstanceOf[U # Type]
      else throw new IllegalArgumentException(s"Type tag defined in $mirror cannot be migrated to other mirrors.")
  })    
}

但是,TypeTag如果您不需要检查通用参数和完整的Scala类型信息,则实际上并不需要完整.您可以使用ClassTags:

def createNew[A: ClassTag]: A = {
  val result = classTag[A].runtimeClass match {
    case a if a.isAssignableFrom(classOf[C1]) => new C1()
    case a if a.isAssignableFrom(classOf[C2]) => new C2()
  }
  result.asInstanceOf[A]
}

或者用一些隐含的糖:

implicit class ClassTagOps[T](val classTag: ClassTag[T]) extends AnyVal {
  def <<:(other: ClassTag[_]) = classTag.runtimeClass.isAssignableFrom(other.runtimeClass)
}

def createNew[A: ClassTag]: A = {
  val result = classTag[A] match {
    case a if a <<: classTag[C1] => new C1()
    case a if a <<: classTag[C2] => new C2()
  }
  result.asInstanceOf[A]
}

您可以使用普通的旧Java newInstance()方法进一步简化:

def createNew[A: ClassTag]: A = classTag[A].runtimeClass.newInstance().asInstanceOf[A]

当然,这只有在您不需要不同类的不同构造函数参数时才有效.

调用这个createNew从createOld比用一个简单得多TypeTagS:

def createOld[A](c: Class[A]): A = createNew(ClassTag[A](c))