Interface, which declare PUT, GET, POST etc methods, which Retrofit (Http network client) will use to create network requests

RetrofitService.kt

interface RetrofitService {
  companion object {
          fun create(): DevGetVoilaService {
              val clientBuilder = OkHttpClient.Builder().apply {
                  val loggingInterceptor = HttpLoggingInterceptor()
                  loggingInterceptor.level = HttpLoggingInterceptor.Level.BODY
                  addInterceptor(loggingInterceptor)
                  addInterceptor(AuthenticationInterceptor())
                  addInterceptor(AuditInterceptor())
              }

              val retrofit = Retrofit.Builder()
                      .addCallAdapterFactory(RxJava2CallAdapterFactory.create())
                      .client(clientBuilder.build())
                      .baseUrl(BuildConfig.API_BASE_URL)
                      .addConverterFactory(GsonConverterFactory.create())
                      .build()

              return retrofit.create(DevGetVoilaService::class.java)
          }

          private class AuthenticationInterceptor : Interceptor {
              override fun intercept(chain: Interceptor.Chain): Response {
                  val originalRequest = chain.request()

                  var response: Response = chain.proceed(originalRequest)
                  LocalDataSource.getInstance().getToken()
                          .subscribe(
                                  { token ->
                                      val newRequest = originalRequest.newBuilder()
                                              .header(Header.TOKEN.headerName, token.toNullable())
                                              .build()

                                      response = chain.proceed(newRequest)
                                  },
                                  { response = response } //if error, then there is no token, so use the original response
                          )
                  return response
              }
          }

          private class AuditInterceptor : Interceptor {
              override fun intercept(chain: Interceptor.Chain): Response {
                  val originalRequest = chain.request()

                  val newRequest = originalRequest.newBuilder()
                          .header(Header.APP_PLATFORM.headerName, "android")
                          .header(Header.APP_VERSION.headerName, BuildConfig.VERSION_CODE.toString())
                          .build()
                  return chain.proceed(newRequest)
              }
          }

      }

      //... some other methods
  
      @FormUrlEncoded
      @POST("authenticate/")
      fun authenticate(@Field("username") username: String,
                       @Field("password") password: String):
              Flowable<AuthenticateResult>

      @POST("sessions/conversation/v2/")
      fun getConversationId(): Flowable<ConversationId>

      @POST("layer/identity/")
      fun getIdentityToken(@Body identityTokenBody: IdentityTokenBody):
              Flowable<IdentityToken>

      @POST("flight/task/select/{flightId}")
      fun selectFlightWidget(@Path("flightId") flightId: Int): Completable

      @POST("hotel/task/select/{hotelId}")
      fun selectHotelWidget(@Path("hotelId") hotelId: Int): Completable

      @POST("car/task/select/{carId}")
      fun selectCarWidget(@Path("carId") carId: Int): Completable

      @POST("train/task/select/{trainId}")
      fun selectTrainWidget(@Path("trainId") trainId: Int): Completable

      @POST("flight/task/select")
      fun selectFlightWidgetWithCF(@Body selectCFBody: SelectCFBody): Completable

      @POST("hotel/task/select")
      fun selectHotelWidgetWithCF(@Body selectCFBody: SelectCFBody): Completable

      @POST("car/task/select")
      fun selectCarWidgetWithCF(@Body selectCFBody: SelectCFBody): Completable
  
      //... some other methods

 }