WebSocket

val client = OkHttpClient()

val request = Request.Builder()
    .url("wss://your.server.com/ws")
    .build()

val listener = object : WebSocketListener() {
    override fun onOpen(webSocket: WebSocket, response: Response) {
        // connected
        webSocket.send("hello")
    }

    override fun onMessage(webSocket: WebSocket, text: String) {
        // received text message
    }

    override fun onMessage(webSocket: WebSocket, bytes: ByteString) {
        // received binary message
    }

    override fun onClosing(webSocket: WebSocket, code: Int, reason: String) {
        webSocket.close(1000, null)
    }

    override fun onClosed(webSocket: WebSocket, code: Int, reason: String) {
        // fully closed
    }

    override fun onFailure(webSocket: WebSocket, t: Throwable, response: Response?) {
        // error — connection lost, server unreachable, etc.
    }
}

val webSocket = client.newWebSocket(request, listener)