Scala Dependency Injection: an improved cake pattern

cake_pattern_di.scala

//harry huang [[email protected]]
//
//After reading the original post [http://jboner.github.com/2008/10/06/real-world-scala-dependency-injection-di.html, ]
//the original  cake pattern seems quite verbose for me, and it is quite invasive, so I spent a bit time
//and come up with an improved version of cake pattern, which I call it "Auto Cake DI". It is working 
//well with any POST(plain old scala trait)/POSO(plain old scala object) which means that anything can be
//injected without modification or introducing new traits.  

/*---------inject trait---------*/
trait Inject[+T] { def inject: T }

object Cake {
  // =======================
  def main(s: Array[String]) {
    val warmer = ComponentRegistry.warmer
    warmer.trigger
  }
}

// =======================
// service interfaces
trait OnOffDevice {
  def on: Unit
  def off: Unit
}

trait SensorDevice {
  def isCoffeePresent: Boolean
}

class Heater extends OnOffDevice {
  def on = println("heater.on")
  def off = println("heater.off")
}
class PotSensor extends SensorDevice {
  def isCoffeePresent = true
}

// =======================
// service declaring two dependencies that it wants injected
trait WarmerComponentImpl {
  this: Inject[(OnOffDevice, SensorDevice)] =>
  class Warmer {
    def trigger = {
      val (onOff, sensor) = inject

      if (sensor.isCoffeePresent) onOff.on
      else onOff.off
    }
  }
}

// =======================
// instantiate the services in a module
object ComponentRegistry extends WarmerComponentImpl with Inject[(OnOffDevice, SensorDevice)] {
  val inject = (new Heater, new PotSensor)
  val warmer = new Warmer
}