NodeMCU Callback-Subscribe
In our previous examples, we published data from nodemcu to our software.
Now we will explain how to use the data published from our software in nodemcu.
MQTT and wifi connection codes are the same as in our previous examples.
The message format published from our software to nodemcu is also the same.
Device name / data name / data value
Nodemcu must be subscribed to receive messages.
In order to receive messages published under his own name, we will subscribe to incoming messages under his own name.We should use subscribe operations within mqtt connection and reconnection blocks.
To subscribe we will use the following line of code for each data.
client.subscribe("devicename/topic");We will use the callback function to use the data when the subscribed message is received.We're going to shred the incoming message.We should use this command in MQTT connection and reconnection blocks.
client.setCallback(callback);Let's define our variables
String Topik; //Our variable that we will use for comparison by keeping the tags/topic information from mqtt
String data_value; // our variable to hold the message information
char message_store[100]; // variable to save the incoming message as a character Our callback function will be as follows.
void callback(char* topic, byte* payload, unsigned int length) { // Let's get the incoming topic-tag information and message
int i = 0;
Serial.println("message received");
Serial.println(topic); // Let's print the incoming topic-tag information to the serial port
Topik = String (topic); // write it to our variable to use the incoming topic information
Serial.print("incomming message:");
for (i = 0; i < length; i++) {
Serial.print((char)payload[i]); // write the incoming message to the serial port
message_store[i] = ((char)payload[i]); // store the incoming characters as characters in the message store
}
message_store[i] = '\0'; // find the end of the message
// Now let's find our data from the incoming message
if (Topik == "devicename/topic"){
data_value = (message_store);
}
}
Now let's make the system that will send the value of 500 with the number topic i to the device named nodemcu1.
Let Nodemcu send the data it receives back to our system with the name of numberresend.
Subscirbe message
Device name : nodemcu1
Topic : number
client.subscribe("nodemcu1/number");Publish message
Device name : nodemcu1
Topic : numberresend
client.publish("nodemcu1/numberresend", String(data_value).c_str());We can make our system more stable by using milis command instead of delay command.Below is the program block that will send a message every two seconds.
unsigned long previousMillis;
unsigned long currentMillis = millis();
if (currentMillis - previousMillis >= 2000) {
client.publish("nodemcu1/numberresend", String(data_value).c_str());// send our message in the topic with the device name
Serial.println("message send");
previousMillis = currentMillis;
}Our main program has been created.You can expand your system by adding as many variables as you want in this program.
#include <ESP8266WiFi.h> //The library we will use for the wifi connection
#include <PubSubClient.h> // The library we will use for the mqtt connection
//mqtt connection library settings
WiFiClient espClient;
PubSubClient client(espClient);
//
//settings for internet and mqtt connection
const char* ssid = ""; // wifi SSID
const char* password = ""; // wifi password
const char* mqttServer = ""; // mqtt server ip to be sent by smartespiot by mail
const int mqttPort = ; // mqtt port to be sent by smartespiot by mail
const char* mqttUser = ""; // mqtt username to be sent by smartespiot by mail
const char* mqttPassword = ""; // mqtt pass to be sent by smartespiot by mail
//****
String Topik; //Our variable that we will use for comparison by keeping the tags/topic information from mqtt
String data_value; // our variable to hold the message information
char message_store[100]; // variable to save the incoming message as a character
//***//
int wificounter = 0; // for counting the number of trying connections
//**//
void setup() {
Serial.begin(115200); // start serial communication at 115200 speed
WiFi.begin(ssid, password); // start the wifi connection process
while (WiFi.status() != WL_CONNECTED) { //
delay(5000); //wait 5000ms
Serial.println("Connecting to wifi..."); // wait until the wifi process connects
//// trying 10 times connection after reset to esp
wificounter = wificounter + 1;
if (wificounter == 10) {
ESP.reset();
}
/////
}
Serial.println("Wifi connection ready :)"); // print the connection made information to the serial port
client.setServer(mqttServer, mqttPort);//start connecting to the mqtt server from the specified port
/////*****
client.setCallback(callback);
////******
while (!client.connected()) { //wait until we connect to mqtt
Serial.println("Connecting to MQTT server...");
if (client.connect("nodemcu1", mqttUser, mqttPassword )) {//login to mqtt server with username and password with smartespIOT name
Serial.println("MQTT connection established :) ");
} else {
Serial.print("Unable to connect... ");// If the connection cannot be established, let's give a warning
Serial.print(client.state()); // print the problem description
// We can understand the problem according to the information returned from the clients state.
//-4 : MQTT_CONNECTION_TIMEOUT - the server did not respond in the ongoing time
//-3 : MQTT_CONNECTION_LOST - network connection lost
//-2 : MQTT_CONNECT_FAILED - network connection failed
//-1 : MQTT_DISCONNECTED - disconnected
//0 : MQTT_CONNECTED - connection established
//1 : MQTT_CONNECT_BAD_PROTOCOL - The server does not support the requested MQTT version
//2 : MQTT_CONNECT_BAD_CLIENT_ID - The server did not accept your information
//3 : MQTT_CONNECT_UNAVAILABLE - The server did not accept the connectionetmed
//4 : MQTT_CONNECT_BAD_CREDENTIALS - username password not accepted
//5 : MQTT_CONNECT_UNAUTHORIZED - you are not authorized to connect
delay(2000); // wait two seconds and try connecting again
////
ESP.reset(); // unable to connect esp reset
///
}
}
////*****
client.subscribe("nodemcu1/number");
////*****
}
////reconnect funtion //
void reconnect() {
WiFi.disconnect();
delay(100);
WiFi.begin(ssid, password); // start the wifi connection process
while (WiFi.status() != WL_CONNECTED) { //
delay(5000); //wait 5000ms
Serial.println("Connecting to wifi..."); // wait until the wifi process connects
//// trying 10 times connection after reset to esp
wificounter = wificounter + 1;
if (wificounter == 10) {
ESP.reset();
}
/////
}
Serial.println("Wifi connection ready :)"); // print the connection made information to the serial port
client.setServer(mqttServer, mqttPort);//start connecting to the mqtt server from the specified port
/////*****
client.setCallback(callback);
////******
while (!client.connected()) { //wait until we connect to mqtt
Serial.println("Connecting to MQTT server...");
if (client.connect("SmartEspIOT", mqttUser, mqttPassword )) {//login to mqtt server with username and password with smartespIOT name
Serial.println("MQTT connection established :) ");
} else {
Serial.print("Unable to connect... ");// If the connection cannot be established, let's give a warning
Serial.print(client.state()); // print the problem description
// We can understand the problem according to the information returned from the clients state.
//-4 : MQTT_CONNECTION_TIMEOUT - the server did not respond in the ongoing time
//-3 : MQTT_CONNECTION_LOST - network connection lost
//-2 : MQTT_CONNECT_FAILED - network connection failed
//-1 : MQTT_DISCONNECTED - disconnected
//0 : MQTT_CONNECTED - connection established
//1 : MQTT_CONNECT_BAD_PROTOCOL - The server does not support the requested MQTT version
//2 : MQTT_CONNECT_BAD_CLIENT_ID - The server did not accept your information
//3 : MQTT_CONNECT_UNAVAILABLE - The server did not accept the connectionetmed
//4 : MQTT_CONNECT_BAD_CREDENTIALS - username password not accepted
//5 : MQTT_CONNECT_UNAUTHORIZED - you are not authorized to connect
delay(2000); // wait two seconds and try connecting again
////
ESP.reset(); // unable to connect esp reset
///
}
}
////*****
client.subscribe("nodemcu1/number");
////*****
}
////////////////////////////////////////
///****
void callback(char* topic, byte* payload, unsigned int length) { // Let's get the incoming topic-tag information and message
int i = 0;
Serial.println("message received");
Serial.println(topic); // Let's print the incoming topic-tag information to the serial port
Topik = String (topic); // write it to our variable to use the incoming topic information
Serial.print("incomming message:");
for (i = 0; i < length; i++) {
Serial.print((char)payload[i]); // write the incoming message to the serial port
message_store[i] = ((char)payload[i]); // store the incoming characters as characters in the message store
}
message_store[i] = '\0'; // find the end of the message
// Now let's find our data from the incoming message
if (Topik == "nodemcu1/number"){
data_value = (message_store);
}
}
/////*******
void loop() {
client.loop(); // mrun the mqtt data processing function
///// test client connected and start reconnect ///
if (!client.connected()) {
reconnect();
}
/////
unsigned long previousMillis;
unsigned long currentMillis = millis();
if (currentMillis - previousMillis >= 2000) {
client.publish("nodemcu1/numberresend", String(data_value).c_str());// send our message in the topic with the device name
Serial.println("message send");
previousMillis = currentMillis;
}
}