NodeMCU lua function “conn”

Question

I have maybe a weird question but I need to know this. Im creating a lua script to connect esp8266 and my mqtt broker. The example script is very straight forward.

m = mqtt.Client("clientid", 120, "user", "password")

m:lwt("/lwt", "offline", 0, 0)

m:on("connect", function(con) print ("connected") end)
m:on("offline", function(con) print ("offline") end)

m:on("message", function(conn, topic, data) 
  print(topic .. ":" ) 
  if data ~= nil then
    print(data)
  end
end)

m:connect("192.168.11.118", 1880, 0, function(conn) 
    print("connected") 
end)

m:subscribe("/topic",0, function(conn) 
    print("subscribe success") 
end)

m:publish("/topic","hello",0,0, function(conn) 
    print("sent") 
end)

m:close();

But... there is one thing I can't get over. That is the "con" and "conn" parameter? It looks like an instance or something, but there is no such a thing defined. Can somebody explain this to me?

Answer 1

From the docs you can find that the client's on() method registers a callback where the first param is the client itself. For your convenience (and in case the link dies) I have copied the relevant information below:

mqtt.client:on()

Description

register callback function to event.

Syntax

mqtt:on(event, function(client, [topic], [message]))

Parameters

event
string, which can be: "connect", "message", "offline"

function(client, [topic], [message])
callback function. The first param is the client. If event is "message", the 2nd and 3rd param are received topic and message in string.

Returns

nil.