Dependency Injection by Cake Pattern http://eed3si9n.com/ja/real-world-scala-dependency-injection-di

CakePattern.scala

case class User(username: String, password: String)

trait UserRepositoryComponent {
  val userRepository: UserRepository
  // ordinal class
  class UserRepository {
    def authenticate(user: User): User = {
      println("authenticating user: " + user)
      user
    }
    def create(user: User) = println("creating user: " + user)
    def delete(user: User) = println("deleting user: " + user)
  }
}

trait UserServiceComponent {
  // specify dependencies by self-type annotation
  self: UserRepositoryComponent =>
  val userService: UserService
  class UserService {
    def authenticate(username: String, password: String): User =
      userRepository.authenticate(new User(username, password))
    def create(username: String, password: String) =
      userRepository.create(new User(username, password))
    def delete(user: User) =
      userRepository.delete(user)
  }
}

// for Production
trait RealEnvironment
  extends UserServiceComponent
  with UserRepositoryComponent
{
  val userRepository = new UserRepository
  val userService = new UserService
}

object Main extends RealEnvironment {
  def main(args: Array[String]) {
    val user = userService.authenticate("foo", "bar")
    println("authenticated user is " + user)
  }
}

// for Test
import org.mockito._
import org.specs2.mutable._

trait TestEnvironment
  extends UserServiceComponent
  with UserRepositoryComponent
{
  // all dependant objects are mocked
  val userRepository = Mockito.mock(classOf[UserRepository])
  val userService = Mockito.mock(classOf[UserService])
}

class UserServiceSpec extends Specification with TestEnvironment {
  // create a non-Mock object that you want to test
  override val userService = new UserService
  "authenticate" should {
    "return user" in {
      val user = User("test", "test")
      // note that denpendant object `userRepository` is mocked
      Mockito.when(userRepository.authenticate(user)).thenReturn(user)
      userService.authenticate("test", "test") must_== user
    }
  }
}

$ scalac -cp mockito-all-1.9.5.jar:specs2_2.10-2.3.7.jar:scalaz-concurrent_2.10-7.0.4.jar:scalaz-core_2.10-7.0.0.jar CakePattern.scala
$ scala Main
authenticating user: User(foo,bar)
authenticated user is User(foo,bar)