Optionally generate Client Stubs about akka-grpc

[johanandren]

Throwing sounds like the path to go to me. I can't think of any benefits from returning an empty default response message in a test scenario?

[alexklibisz]

I opened the original Lightbend support ticket requesting this. Thanks for opening here.

I think just throwing is fine. I'm trying to think why we used the Empty pattern, and can't remember a compelling reason. I just tried replacing Empty with Unimplemented in a couple places and it works fine.

One more consideration: in some cases we've also found it useful to have an object and/or method as follows:

object SomeApiClientStub {

  trait Unimplemented extends SomeApiClient {
    def someRpc(in: SomeRpcRequest): Future[SomeRpcResponse] = ???
  }

  object Unimplemented extends Unimplemented

  def unimplemented: SomeApiClient = new Unimplemented
}

This will be extremely helpful for reducing boilerplate in test suites. Thanks :)

[alexklibisz]

It might also be useful to throw a custom exception, roughly, final class AkkaGrpcStubNotImplementedException extends NotImplementedError, to make it more obvious that a stubbable, unimplemented method was called.