How to mock a Scala singleton object?

Question

I am trying to mock a Scala singleton object. In particular, I need to mock the object play.api.libs.ws.WS used inside a service component (class under test). U

Answer 1

Why you don't use the cake pattern?

It's a little verbose but it will solve your mocking problem.

trait GeolocationServiceComponent {
   val geolocationService:GeolocationService
   trait GeolocationService {      
      def wsClient
      def getPath(origin:Location, destination: Location): Future[Route]   
   }
}

trait GeolocationServiceComponentImpl {
   val geolocationService = new GeolocationService {
      override def wsClient = WS
   }
}


class DefaultGeolocationService extends GeolocationServiceComponent ...

//Defined into your test class
trait MockGeolocationServiceComponent {
       val geolocationService = Mock[GeolocationService]
//Here you define your mock logic       
}

You can also use monads to do this, but I have never implemented, it's described here: Scala dependency injection: alternatives to implicit parameters

Answer 2

I don't like to change the design to accommodate this kind of limitation. basically what I can get is all to change the design and then use the mock framework

  //since mocking frameworks on SCALA is not support mocking singleton
  //hacks are either required to change the design or ...
  //I'd rather putting the mocking logic here by myself
  var testUser:Option[User] = None;

  def getCurrentUser(request: Request[Object]) = {
    if( testUser != None ){
    testUser;
  }

hope it helps.

Answer 3

Mocking singleton objects using ScalaMock 3 is not possible, however Paul Butcher expects to reintroduce this feature in ScalaMock 4 (see http://paulbutcher.com/2014/04/15/scalamock-status-report/)

Answer 4

Don't mock the singleton. Instead of WS, make your service component depend on a thin facade hiding it:

trait GeolocationService {
  def ws: (String, Seq[String]) => Promise[Response] = { (url, params) =>
    wsClient.url(serviceProviderEndpoint.get).withQueryString(params: _*).get()
  }
  def getPath(origin: Location, destination: Location): Future[Route]
}

and in your test, just override ws method with a mock, which is now easy to create:

val mockedWs = mock[(String, Seq[String]) => Promise[Response]]
// TODO specify mock's behavior here
val service = new DefaultGeolocationService() {
  override def ws = mockedWs
}