Added basic MQTT connection code for the ESP32 with detailed comments

platformio.ini

@@ -12,3 +12,4 @@
 platform = espressif32
 board = esp-wrover-kit
 framework = arduino
+lib_deps = knolleary/PubSubClient@^2.8

src/main.cpp

@@ -1,9 +1,166 @@
 #include <Arduino.h>
+#include <WiFi.h>
+#include <PubSubClient.h>
 
+// Set the PIN of the output LED
+#define O_LED 13
+
+// WiFi name
+const char* ssid = "........";
+// WiFi password
+const char* password = "........";
+// MQTT broker URI
+const char* mqtt_broker = "........";
+
+// Create a new WiFi client
+WiFiClient espClient;
+// Create a new MQTT client
+PubSubClient client(espClient);
+// Create a variable to store the incoming message
+String lastMsg;
+// Set the size of the message buffer (where the message is stored before it goes into the variable)
+#define MSG_BUFFER_SIZE	(50)
+// Create an array (a list) of characters for the incomming message, the length of the array is the aforementioned buffersize
+char msg[MSG_BUFFER_SIZE];
+
+// Create a function to handle setting up of the WiFi
+// This function is of the type "void" which means that it doesn't have a return value
+void setup_wifi() {
+  // Wait 10 milliseconds
+  delay(10);
+  // Write a message to the serial monitor so we know that it is connection to the WiFi
+  Serial.println();
+  Serial.print("Connecting to ");
+  Serial.println(ssid);
+
+  // Set a standard WiFi mode
+  WiFi.mode(WIFI_STA);
+  // Connect to the WiFi using the given SSID (WiFi name) and password
+  WiFi.begin(ssid, password);
+
+  // While the status of the WiFi is not connected
+  while (WiFi.status() != WL_CONNECTED) {
+    // Wait 500 milliseconds
+    delay(500);
+    // Wrint a "." to the serial monitor
+    Serial.print(".");
+  }
+
+  // If the code gets to here that means you have successfully connected to the WiFi network
+
+  // For proper pseudo-random number generation you need a different seed (base of the number generation) each time, set that here
+  randomSeed(micros());
+
+  // Print a status update to the serial monitor with the IP the EPS32 received
+  Serial.println("");
+  Serial.println("WiFi connected");
+  Serial.println("IP address: ");
+  Serial.println(WiFi.localIP());
+}
+
+// Create a function for the MQTT library to run when it receives a message.
+// This function takes 3 variables:
+//    1. The topic the message arrived in
+//    2. The payload aka the actual message content
+//    3. The length of the message
+// The "*" symbol at the end of the variable types means that it is an array with an undefined length
+// This function is of the type "void" which means that it doesn't have a return value
+void callback(char* topic, byte* payload, unsigned int length) {
+  // Print to the serial monitor that you have received a message and which topic it came from
+  Serial.print("Message arrived [");
+  Serial.print(topic);
+  Serial.print("] ");
+
+  // For each character in the message payload (content) print it to the serial monitor and add it to the variable called 
+  for (int i = 0; i < length; i++) {
+    // Print the currect character in the payload to the serial monitor.
+    Serial.print((char)payload[i]);
+    // Add the current character in the payload to the message string
+    lastMsg += (char)payload[i];
+  }
+  // Print an empty line to the serial monitor once the writing of the entire payload to the serial monitor has completed
+  Serial.println();
+
+  // If the first character in the payload (message) is a "1" turn the output LED on
+  if ((char)payload[0] == '1') {
+    // Write the pin that the output LED is connected to HIGH (on)
+    digitalWrite(O_LED, HIGH);
+  } else {
+    // Write the pin that the output LED is connected to LOW (off)
+    digitalWrite(O_LED, LOW);
+  }
+}
+
+// Create a function to reconnect to the MQTT broker in case the connection drops
+// This function is of the type "void" which means that it doesn't have a return value
+void reconnect() {
+  // Whole the MQTT client is NOT connected
+  while (!client.connected()) {
+    // Write the current status to the serial monitor
+    Serial.print("Attempting MQTT connection...");
+    // Create a random client ID (the name of the MQTT client) starting with "ESP32Client-"
+    String clientId = "ESP32Client-";
+    // Add some random hex characters at the end of the client ID string
+    clientId += String(random(0xffff), HEX);
+
+    // Attempt to connect to the MQTT broker
+
+    // If the client connected successfully using the aforementioned clientID
+    if (client.connect(clientId.c_str())) {
+      // Print that you have connected to the serial monitor
+      Serial.println("connected");
+
+      // When it has connected to the MQTT broker publish a message announcing the connection
+      // This is not required, but is considered good practice
+      // The publish function takes two arguments:
+      //    1. The topic to which the payload should be delivered
+      //    2. The payload itself
+      client.publish("ClientAnnouncements", "Hello, world!");
+
+      // Subscribe to the desired topic(s)
+      // The subscribe function takes one argument
+      //    1. The topic to subscribe to
+      // Subscribing to a topic means the client will receieve all messages published in said topic
+      client.subscribe("inTopic");
+
+    // If the MQTT client coulld not successfully connect to the broker
+    } else {
+      // Print a status update to the serial monitor with the reason it failed and the current connection state
+      Serial.print("failed, rc=");
+      Serial.print(client.state());
+      Serial.println(" try again in 5 seconds");
+      // Wait 5 seconds before retrying
+      delay(5000);
+    }
+  }
+}
+
+// Create the "setup" function, this is automatically called once upon boot
+// The setup function is as the name implies usually used for one time setup upon boot
+// This function is of the type "void" which means that it doesn't have a return value
 void setup() {
-  // put your setup code here, to run once:
+  // Set the LED pin as an output
+  pinMode(O_LED, OUTPUT);
+  // Start serial communication at a baud rate (speed) of 115200Hz
+  Serial.begin(115200);
+  // Run the setup WiFi function to connect to the WiFi network
+  setup_wifi();
+  // Set the client's MQTT broker (server) to the one we specified earlier using the standard unencrypted MQTT communication port (1883)
+  // The "setServer" function takes two arguments
+  //    1. The URI of the MQTT broker
+  //    2. The port the communication happens on
+  client.setServer(mqtt_broker, 1883);
+  // Tell the MQTT client which function to call when it receives a message
+  client.setCallback(callback);
 }
 
+// Create the loop function, this runst continually and loops forever until power loss
+// This function is of the type "void" which means that it doesn't have a return value
 void loop() {
-  // put your main code here, to run repeatedly:
+  // If the MQTT client is not connected use the reconnect function
+  if (!client.connected()) {
+    reconnect();
+  }
+  // Loop the MQTT client to make sure it doesn't disconnect 
+  client.loop();
 }