ScalaでDIというかServiceLocator的な名状しがたい何か

AppInjector.scala

trait ApiInjector {
  var twitter : TwitterApi = new TwitterApiImpl;
  //他にもいろいろなサービス
}

TwitterApi.scala

trait TwitterApi {
  def publicTimeLines : List[String]
  //他にも沢山の api
}

TwitterApiImpl.scala

class TwitterApiImpl extends TwitterApi {
  def publicTimeLines : List[String] = {
    //Twitter API つかってごにょごにょするはず
    List("本当は", "リアルに", "public", "timeline", "取得する")
  }
}

TwitterClient.scala

class TwitterClient extends ApiInjector {

  def indexedPublicTimeLine : List[String] =  {
    twitter.publicTimeLines.zipWithIndex
      .map{ case (s, i) => (i + 1) + " " + s}
  }
}

TwitterClientTest.scala

import org.specs2.mutable._

trait TestApiInjector extends ApiInjector {
  twitter = new TwitterApi {
    def publicTimeLines = {
      List ("dummy", "public", "timeline")
    }
  }
}

class TwitterClientTest extends Specification {

  "Twitter Api Client" should {
    val client = new TwitterClient with TestApiInjector
    val indexedPublicTimeLine = client.indexedPublicTimeLine
    "indexedPublicTimeLine return size " in {
      indexedPublicTimeLine.size must_== 3
    }

    "indexedPublicTimeLine return indexedLine" in {
      indexedPublicTimeLine(0) must_== "1 dummy"
    }

  }

}

override val とかできるので var じゃなくても差し替えられますよ。

trait ApiInjector {
  val twitter : TwitterApi = new TwitterApiImpl
}

trait TestApiInjector extends ApiInjector {
  override val twitter = new TwitterApi {
    def publicTimeLines = {
      List ("dummy", "public", "timeline")
    }
  }
}