Attempting to do dependency injection using the Cake Pattern – `Car` has an `Engine`. Choice of two different engines that's wired with a car under "Example usage" at the bottom.

CakePatternTest.scala

/*
  Dependency injection test using the Cake Pattern
  
  Notes:
  * `Component` wrapper traits
    * Allows multiple `Component`s to be composed together
  * Each of these classes must have their own `Component` trait:
      * Dependency interface
      * Dependency implementation
      * Dependent implementation (class using a dependency)
  * `Component` dependency instance
    * Each dependency interface component must declare an abstract instance of the dependency interface
  * A container singleton object wires up all the dependencies

*/

object CakePatternTest {
  /***** Dependency interface *****/
  trait EngineComponent {
    val engine: Engine
    trait Engine {
      def start: Unit
    }
  }
  
  /***** Dependency implementations *****/
  trait FourCylinderEngineComponent extends EngineComponent {
    class FourCylinderEngine extends Engine {
      def start: Unit = println("FourCylinderEngine.start: vroom")
    }
  }
  
  trait EightCylinderEngineComponent extends EngineComponent {
    class EightCylinderEngine extends Engine {
      def start: Unit = println("EightCylinderEngine.start: VROOM")
    }
  }
    
  
  /***** Dependent implementation *****/
  trait CarComponent {
    this: EngineComponent =>  // Makes CarComponent depend on an EngineComponent
    
    class Car {
      def turnKey: Unit = {
        println("Attempting to start the car...")
        engine.start
      }
    }
  }

  /***** Containers *****/
  object FourCylinderContainer extends CarComponent with FourCylinderEngineComponent {
    val engine = new FourCylinderEngine
    val car = new Car
  }
    
  object EightCylinderContainer extends EightCylinderEngineComponent with CarComponent {
    val engine = new EightCylinderEngine
    val car = new Car
  }
  
  
  /***** Example usage *****/
  val fourCylinderCar = FourCylinderContainer.car
  fourCylinderCar.turnKey

  val eightCylinderCar = EightCylinderContainer.car
  eightCylinderCar.turnKey
}