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Scalaの新しいリフレクションAPIを使用して、クラスのコンパニオンオブジェクトへの参照を取得することは可能ですか？私はこれらの線に沿って何かを考えています：

trait Base {
  def companion: MetaBase = someReflectionMagic(this).asInstanceOf[MetaBase]
}

trait MetaBase {
  // stuff
}

// ---

class Foo extends Base

object Foo extends MetaBase

assert(new Foo.companion == Foo)
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3 に答える 3

37

デイブ！新しい反射に興味を持っていただきありがとうございます。アーリー アダプターは、リフレクションとマクロの開発プロセスを大幅に推進してきました。私は、素晴らしいコミュニティの一員であることを非常に嬉しく思います。

質問に答える前に、免責事項から始めたいと思います。2.10.0-M4 では、Scala リフレクション API の基礎が築かれました。まだ報道されていないので、ドキュメントは非常に少なく、API は便利なものでいっぱいではありません。動作しますが、テストとフィードバックが必要です。確かに、リリース前の API をいじるのは面倒ですが、いつでもお手伝いします。

これまでのところ、将来リフレクション SIP になるもののドラフトがあります: https://docs.google.com/document/d/1Z1VhhNPplbUpaZPIYdc0_EUv5RiGQ2X4oqp0i-vz1qw/edit#heading=h.pqwdkl1226tc . すぐに読むか、最初に以下の私の回答にざっと目を通してください。

trait Base {
  def companion: MetaBase = {
    // runtime reflection is typically done
    // by importing things from scala.reflect.runtime package
    import scala.reflect.runtime._

    // the new Scala reflection API is mirror based
    // mirrors constitute a hierarchy of objects
    // that closely follows the hierarchy of the things they reflect
    // for example, for a class you'll have a ClassMirror
    // for a method you'll have a MethodMirror and so on
    // why go the extra mile?
    // because this provides more flexibility than traditional approaches
    // you can read more about mirror-based designs here:
    // https://dl.dropbox.com/u/10497693/Library/Computer%20Science/Metaprogramming/Reflection/mirrors.pdf
    // https://dl.dropbox.com/u/10497693/Library/Computer%20Science/Metaprogramming/Reflection/reflecting-scala.pdf

    // bottom line is that to do anything you will need a mirror
    // for example, in your case, you need a ClassMirror

    // remember I said that mirrors provide more flexibility?
    // for one, this means that mirror-based reflection facilities
    // might have multiple implementations
    // in a paper linked above, Gilad Bracha muses over a runtime
    // that loads things remotely over the network
    // in our case we might have different mirrors for JVM and CLR
    // well, anyways

    // the canonical (and the only one now) implementation of the mirror API
    // is Java-based reflection that uses out of the box classloaders
    // here's its root: https://github.com/scalamacros/kepler/blob/9f71e9f114c10b52350c6c4ec757159f06e55daa/src/reflect/scala/reflect/api/Mirrors.scala#L178
    // yeah, right, I've just linked a source file from trunk
    // we'll have Scaladocs for that soon, but for now take a look
    // this file is interfaces-only and is heavy on comments

    // to start with Java-based reflection implementation you need a classloader
    // let's grab one and instantiate the root mirror
    // btw, the same effect could be achieved by writing
    // `scala.reflect.runtime.currentMirror`
    val rootMirror = universe.runtimeMirror(getClass.getClassLoader)

    // now when we've finally entered the reflective world
    // we can get the stuff done
    // first we obtain a ClassSymbol that corresponds to the current instance
    // (ClassSymbols are to Scala the same as Classes are to Java)
    var classSymbol = rootMirror.classSymbol(getClass)

    // having a Scala reflection entity
    // we can obtain its reflection using the rootMirror
    val classMirror = rootMirror.reflectClass(classSymbol)

    // now we just traverse the conceptual hierarchy of mirrors
    // that closely follows the hierarchy of Scala reflection concepts
    // for example, a ClassMirror has a companion ModuleMirror and vice versa
    val moduleMirror = classMirror.companion.get

    // finally, we've arrived at our destination
    moduleMirror.instance.asInstanceOf[MetaBase]
  }
}

trait MetaBase {
  // stuff
}

// ---

class Foo extends Base

object Foo extends MetaBase

object Test extends App {
  assert(new Foo().companion == Foo)
}

アップデート。Daniel Sobral による優れた投稿 ( http://dcsobral.blogspot.ch/2012/07/json-serialization-with-reflection-in.html ) もご覧ください。

于 2012-06-14T10:45:36.770 に答える