Creating a retrofit-like service based on annotations.

JsonRpc.kt

fun <T> createJsonRpcService(service: Class<T>, client: JsonRpcClient): T {

    val classLoader = service.classLoader
    val interfaces = arrayOf<Class<*>>(service)
    val invocationHandler = createInvocationHandler(service, client)

    @Suppress("UNCHECKED_CAST")
    return Proxy.newProxyInstance(classLoader, interfaces, invocationHandler) as T
}

private fun <T, B> createInvocationHandler(service: Class<T>, client: JsonRpcClient<B>): InvocationHandler {
    return object : InvocationHandler {

        val requestId = AtomicLong(0)

        override fun invoke(proxy: Any, method: Method, args: Array<Any?>?): Any {
            val methodAnnotation =
                    method.getAnnotation(JsonRpc::class.java) ?: return method.invoke(this, args)

            if (!method.returnsSingle) {
                error("Only io.reactivex.Single<T> is supported as return type")
            }

            val id = requestId.incrementAndGet()
            val methodName = methodAnnotation.value
            val parameters = method.jsonRpcParameters(args, service)

            val request = JsonRpcRequest(id, methodName, parameters)
            val returnType = method.resultGenericTypeArgument

            return client.call<Any>(request, returnType)
        }
    }
}

private fun Method.jsonRpcParameters(args: Array<Any?>?, service: Class<*>): Map<String, Any?> {
    return parameterAnnotations
            .map { it?.firstOrNull { JsonRpcParam::class.java.isInstance(it) } }
            .mapIndexed { i, a ->
                when (a) {
                    is JsonRpc -> a.value
                    else -> error("Argument #$i of ${service.name}#$name()" +
                            " must be annotated with @${JsonRpc::class.java.simpleName}")
                }
            }
            .mapIndexed { i, name -> name to args?.get(i) }
            .associate { it }
}

private val Method.returnsSingle: Boolean
    get() = returnType.canonicalName == Single::class.java.canonicalName

private val Method.resultGenericTypeArgument: Type
    @Suppress("CAST_NEVER_SUCCEEDS")
    get() = (this.genericReturnType as ParameterizedType).actualTypeArguments.first()

private class NullJsonRpcCallResultException : Exception()