How to use ktor ContentNegotiation for json deserialising for websocket client

Question

I am investigating ktor client in my current android project for consuming a websocket stream.

my ktor client is configured as follows:-

val client = HttpClient(OkHttp) {
    install(Logging) {
        logger = Logger.DEFAULT
        level = LogLevel.ALL
    }

    install(ContentNegotiation) {
        json(Json {
            ignoreUnknownKeys = false
            isLenient = true
            encodeDefaults = true
            coerceInputValues = true
            explicitNulls = false
        })
    }

    install(WebSockets) {
        pingInterval = 20_000

    }
}

with my websocket session configured as shown below:-

    client.webSocket(urlString = "wss://stream.org") {
        incoming.consumeEach { frame ->
            when (frame) {
                is Frame.Binary -> println("Frame.Binary = $frame")
                is Frame.Text -> {
                    println("Frame.Text = $frame")
                    val incoming = frame.readText()
                    println("\tincoming $incoming")

                    val eventResponse = json.decodeFromString<WebSocketResponse>(incoming)

                    when (eventResponse.event) {
                        Event.CONNECTED  -> println("Event.SUBSCRIPTION_CREATED = $incoming")
                        Event.SUBSCRIPTION_CREATED -> println("Event.SUBSCRIPTION_CREATED = $incoming")
                        Event.SUBSCRIPTION_DELETED -> println("Event.SUBSCRIPTION_DELETED = $incoming")
                        Event.TOPIC_ADDED -> println("Event.TOPIC_ADDED = $incoming")
                        Event.TOPIC_REMOVED -> println("Event.TOPIC_REMOVED = $incoming")
                        Event.TOPIC_UPDATED -> println("Event.TOPIC_UPDATED = $incoming")
                        else -> TODO("Unexpected event $eventResponse")
                    }
                }

                is Frame.Close -> println("Frame.Close = $frame")
                is Frame.Ping -> println("Frame.Ping = $frame")
                is Frame.Pong -> println("Frame.Pong = $frame")
                else -> TODO("Unexpected frame = $frame")
            }
        }
    }

the json in val eventResponse = json.decodeFromString<WebSocketResponse>(incoming) is a different json instance than the one configured in the ktor client, as i have it already declared for processing restful api calls via retrofit

its declared as follows:-

@OptIn(ExperimentalSerializationApi::class)
val json = Json {
    ignoreUnknownKeys = true
    isLenient = true
    encodeDefaults = true
    coerceInputValues = false
    explicitNulls = false
}

how do i use the json instance i configured for content negotiation?

Answer 1

You cannot access the Json instance from the ContentNegotiation plugin, but you can configure the plugin with the predefined Json object and use it for the WebSockets packet deserialization too.

@OptIn(ExperimentalSerializationApi::class)
val jsonInstance = Json {
    ignoreUnknownKeys = true
    isLenient = true
    encodeDefaults = true
    coerceInputValues = false
    explicitNulls = false
}

val client = HttpClient {
    install(ContentNegotiation) {
        json(jsonInstance)
    }
}

// ... 
// Usage of jsonInstance for WebSockets packet deserialization