Dependency injection in Scala with Play 2: it’s free

1_Secured.scala

object MyApp extends Controller {
  // Home page
  val index = Action {
    Ok(views.html.index())
  }

  // Login action: here I just bind a (non empty) username from the query and put it in the session
  val login = Action { implicit request =>
    Form(mapping("username" -> nonEmptyText)(identity)(Some(_))).bindFromRequest.fold(
      noUser => redirectToIndex,
      username => redirectToIndex.withSession("username" -> username)
    )
  }
  
  // Logout action: clear the session and redirect to the home page
  val logout = Action {
    redirectToIndex.withNewSession
  }

  // Convenient shortcut
  val redirectToIndex = Redirect(routes.MyApp.index)
}

object Secured {
  // Authentication check: executes the wrapped action only if a username is found in the session
  def Authenticated[A](action: Action[A]): Action[A] = Action(action.parser) { request =>
    request.session.get("username") match {
      case Some(user) => action(request)
      case None => Unauthorized(views.html.unauthorized())
    }
  }
}

2_Authenticated.scala

val index = Secured.Authenticated {
  Action {
    Ok(views.html.index())
  }
}

3_MockSecured.scala

object MockSecured {
  def Authenticated[A](action: Action[A]) = action
}

4_Security.scala

trait Security {
  def Authenticated[A](action: Action[A]): Action[A]
}

trait Secured extends Security {
  override def Authenticated[A](action: Action[A]) = Action(action.parser) { request =>
    // …
  }
}

trait MockedSecured extends Security {
  override def Authenticated[A](action: Action[A]) = action
}

5_Dependency.scala

trait MyApp extends Controller {
  this: Security =>

  val index = Authenticated {
    Action {
      Ok(views.html.index())
    }
  }

  // … login and logout actions are not changed
}

6_bootstrap.scala

package object myApp {
  
  import play.api.Play._
  
  val MyApp = if (isDev) {
    new controllers.MyApp with controllers.MockSecured
  } else {
    new controllers.MyApp with controllers.Secured
  }
}