How do I get Elixir Websockex to subscribe to a websocket?

Question

I'm a newbie to Elixir/Erlang. By way of example, I'm trying to learn how to subscribe to a websocket feed.

I've copied the Websockex basic example:

defmodule WebSocketExample do
  use WebSockex

  def start_link(url, state) do
    WebSockex.start_link(url, __MODULE__, state)
  end

  def handle_frame({type, msg}, state) do
    IO.puts "Received Message - Type: #{inspect type} -- Message: #{inspect msg}"
    {:ok, state}
  end

  def handle_cast({:send, {type, msg} = frame}, state) do
    IO.puts "Sending #{type} frame with payload: #{msg}"
    {:reply, frame, state}
  end
end

and am trying to instantiate it in iex with:

WebSocketExample.start_link "wss://www.bitmex.com/realtime?subscribe=instrument,quote:XBTUSD", state

(CompileError) iex:3: undefined function state/0

however I get the error that state is undefined.

I'm not sure what state should be (surely that's empty and gets passed around a loop?). Any tips that help me understand would be much appreciated!

Dogbert Dogbert · Accepted Answer · 2018-06-14T09:37:39

state here can be any value. You can use it to store information about the socket instance which can later be accessed in the handle functions. For example, you can pass a user id to start_link and access that from handle_cast and handle_info. If you don't want to keep any state, just pass any value, e.g. :ok:

WebSocketExample.start_link "wss://...", :ok

How do I get Elixir Websockex to subscribe to a websocket?

1 Answers