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library(mqtt)

mqtt_broker("bbf928f8-ef46-4614-893f-d36c43f7b8ce", "test.mosquitto.org", 1883L) %>%
  mqtt_silence(c("error", "log", "publish")) %>% 
  mqtt_subscribe("cvmartin", ~{
    if (topic == "cvmartin") {
      payload <- readBin(payload, "character")
      cat(crayon::cyan(topic), crayon::white(payload), "\n", sep=" ")
      # Here is where the magig happens. Modify the response!
      resp <- 8 * (as.numeric(payload))
      resp <- as.character(resp)
      con$publish_chr(0, "cvmartintest", resp, 0, FALSE)
    }
  }) %>% 
  mqtt_run() -> res
  
  # Then, in a Pi (or wherever terminal)
  # mosquitto_pub -h test.mosquitto.org -t cvmartin -m "50"
  # In another terminal, receive the message back: 
  # mosquitto_sub -d -h test.mosquitto.org -t cvmartintest