Demonstrates

OAuthApi.kt

val authApi: OAuthApi = Retrofit.newBuilder()
            .baseUrl(COINBASE_URL)
            .addConverterFactory(MoshiConverterFactory.create(moshi))
            .addCallAdapterFactory(RxJava2CallAdapterFactory.createWithScheduler(Schedulers.io()))
            .build()
            .create()

interface OAuthApi {

    /**
     * Calls onError() in the event that a network error 
     * (request didn't make it to the server or response didn't make
     * it back to the client) occurs. 
     */
    @POST("token")
    fun authToken(@Body body: TokenRequestBody): Single<Response<AccessToken>>
}