How to use MQTT and Json on ESP8266 or ESP32

The main program remains essentially identical to the previous example. The only difference is that we have added the dependency on the ArduinoJson library.

#include <WiFi.h>
#include <AsyncMqttClient.h>
#include <ArduinoJson.h>

#include "config.h"  // Replace with your network data
#include "MQTT.hpp"
#include "ESP32_Utils.hpp"
#include "ESP32_Utils_MQTT_Async.hpp"

void setup(void)
{
  Serial.begin(115200);
  delay(500);

  WiFi.onEvent(WiFiEvent);
  InitMqtt();

  ConnectWiFi_STA();
}

void loop()
{
  PublishMqtt();

  delay(1000);
}

What has changed, logically, is the MQTT.hpp file. Remember that in this file we have grouped the specific functionality of our project in terms of MQTT.

As we can see, instead of sending a simple piece of data as in the previous example, where we sent the value of millis(), this time we are sending a text string encoded as Json.

In this Json, we are encoding a ‘data’ field with the value of millis(). Obviously, the Json could be anything else. But, to show the use and test the communication, it is sufficient.

On the other hand, in the OnMqttReceived callback function, we are receiving the MQTT message, decoding the Json, and displaying the ‘data’ field, which contains the value of the sent millis(), by serial port.

#pragma once

const IPAddress MQTT_HOST(192, 168, 1, 150);
const int MQTT_PORT = 1883;

AsyncMqttClient mqttClient;

String GetPayloadContent(char* data, size_t len)
{
  String content = "";
  for(size_t i = 0; i < len; i++)
  {
    content.concat(data[i]);
  }
  return content;
}

void SuscribeMqtt()
{
  uint16_t packetIdSub = mqttClient.subscribe("hello/world", 0);
  Serial.print("Subscribing at QoS 2, packetId: ");
  Serial.println(packetIdSub);
}

void PublishMqtt()
{
  String payload = "";

  StaticJsonDocument<300> jsonDoc;
  jsonDoc["data"] = millis();
  serializeJson(jsonDoc, payload);

  mqttClient.publish("hello/world", 0, true, (char*)payload.c_str());
}

void OnMqttReceived(char* topic, char* payload, AsyncMqttClientMessageProperties properties, size_t len, size_t index, size_t total)
{
  Serial.print("Received on ");
  Serial.print(topic);
  Serial.print(": ");

  String content = GetPayloadContent(payload, len);

  StaticJsonDocument<200> doc;
  DeserializationError error = deserializeJson(doc, content);
  if(error) return;

  unsigned long data = doc["data"];
  Serial.print("Millis:");
  Serial.println(data);
}