This code is a copy (roughly reconstructed from reformatted and reordered parts in an abandoned Eclipse project) of the code that was originally in http://jonasboner.com/2008/10/06/real-world-scala-dependency-injection-di.html#comment-7789930.

scala-di-from-comment-7789930.scala

trait BeeperDevice {
  def beep(beepSound: String)
}

trait DependsOnBeeper {
  protected val beeper: BeeperDevice
}


class Beeper extends BeeperDevice {
  def beep(beepSound: String) = println("beep: " + beepSound)
}

// =======================
// An aspect that can be mixed in with any TriggeredDevice. It also just
// extends the DependsOn interface instead of using self-typing.
trait BeepWhenTriggeredAspectDependencies extends DependsOnBeeper {
  trait BeepWhenTriggeredAspect extends TriggeredDevice {
    abstract override def trigger = {
      beeper.beep(name + " triggered")
      super.trigger
    }
  }
}

class EmergencyShutoffSwitch extends TriggeredDevice {
  protected val name = "emergency switch"
  def trigger = {
    println("EMERGENCY SHUTOFF")
  }
}

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

trait OnOffDevice {
  def on: Unit
  def off: Unit
}

trait DependsOnOnOffDevice {
  protected val onOff: OnOffDevice
}

class PotSensor extends SensorDevice {
  def isCoffeePresent = true
}

trait SensorDevice {
  def isCoffeePresent: Boolean
}

trait DependsOnSensorDevice {
  protected val sensor: SensorDevice
}

// =======================
// instantiate the services in a module
object ServiceRegistry extends WarmerDependencies with BeepWhenTriggeredAspectDependencies {

  // onOff and sensor are required by the compiler because ServiceRegistry
  // extends WarmerDependencies. They are injected into warmer. They are
  // protected, because, in this example, ServiceRegistry consumers don't
  // need to know about them
  protected val onOff = new Heater
  protected val sensor = new PotSensor

  // beeper is required by the compiler because ServiceRegistry mixes in
  // BeepWhenTriggeredAspectDependencies. It is injected into warmer
  // and emergencySwitch. It is public so ComponentyRegistry consumers can
  // us it
  val beeper = new Beeper

  // A warmer that beeps when it's triggered
  val warmer = new Warmer with BeepWhenTriggeredAspect

  // An emergency shutdown switch that beeps when it's triggered
  val emergencySwitch = new EmergencyShutoffSwitch with BeepWhenTriggeredAspect
}

// =======================
// instantiate the services in a module
object ServiceRegistrySimple extends WarmerDependencies {

  // onOff and sensor are required by the compiler because ServiceRegistry
  // extends WarmerDependencies. They are injected into warmer. They are
  // protected, because, in this example, ServiceRegistry consumers don't
  // need to know about them
  protected val onOff = new Heater
  protected val sensor = new PotSensor

  // A warmer that beeps when it's triggered
  val warmer = new Warmer
}

trait TriggeredDevice {
  protected val name: String
  def trigger
}

//TODO: add 'DependsOn...' for consistency

// =======================
// Service declaring two dependencies that it wants injected. It just extends
// and mixes in the DependsOn interfaces instead of using self-typing, so that
// users of WarmerDependencies aren't required to explicitly mix in the
// DependsOn interfaces
trait WarmerDependencies extends DependsOnOnOffDevice with DependsOnSensorDevice {
  class Warmer extends TriggeredDevice {
    protected val name = "warmer"
    def trigger = {
      if (sensor.isCoffeePresent) onOff.on
      else onOff.off
    }
  }
}