ESP32 MQTT Sensor Publisher with PubSubClient, DHT22 and Last Will

#include <WiFi.h>
#include <PubSubClient.h>
#include <DHT.h>
#include <ArduinoJson.h>

// ---------- user settings ----------
const char* WIFI_SSID   = "YourWiFi";
const char* WIFI_PASS   = "YourWiFiPassword";
const char* MQTT_HOST   = "192.168.1.50";   // broker IP
const uint16_t MQTT_PORT = 1883;
const char* MQTT_USER   = "iotuser";
const char* MQTT_PASS   = "secret";
const char* NODE        = "home/node1";     // base topic
// -----------------------------------

DHT dht(4, DHT22);
WiFiClient net;
PubSubClient mqtt(net);

String tTelemetry = String(NODE) + "/telemetry";
String tStatus    = String(NODE) + "/status";
String tInterval  = String(NODE) + "/interval/set";

uint32_t intervalMs = 15000;
uint32_t lastSend = 0, lastTry = 0;

// Called by mqtt.loop() whenever a subscribed message arrives
void onMessage(char* topic, byte* payload, unsigned int len) {
  String msg;
  for (unsigned int i = 0; i < len; i++) msg += (char)payload[i];
  if (tInterval == topic) {
    long s = msg.toInt();
    if (s >= 5 && s <= 3600) {
      intervalMs = s * 1000UL;
      Serial.println("Interval set to " + msg + " s");
    }
  }
}

void connectWiFi() {
  WiFi.mode(WIFI_STA);
  WiFi.begin(WIFI_SSID, WIFI_PASS);
  Serial.print("WiFi");
  while (WiFi.status() != WL_CONNECTED) { delay(500); Serial.print("."); }
  Serial.println(" connected, IP " + WiFi.localIP().toString());
}

// Try once to connect; returns true on success. Never blocks for long.
bool connectMqtt() {
  String clientId = "esp32-" + String((uint32_t)ESP.getEfuseMac(), HEX);
  // connect(id, user, pass, willTopic, willQos, willRetain, willMessage)
  if (!mqtt.connect(clientId.c_str(), MQTT_USER, MQTT_PASS,
                    tStatus.c_str(), 1, true, "offline")) {
    Serial.print("MQTT failed, state=");
    Serial.println(mqtt.state());
    return false;
  }
  mqtt.publish(tStatus.c_str(), "online", true);   // retained
  mqtt.subscribe(tInterval.c_str(), 1);
  Serial.println("MQTT connected");
  return true;
}

void publishReading() {
  float t = dht.readTemperature();
  float h = dht.readHumidity();
  if (isnan(t) || isnan(h)) { Serial.println("DHT read failed"); return; }

  JsonDocument doc;
  doc["temp_c"]  = round(t * 10) / 10.0;
  doc["hum_pct"] = round(h * 10) / 10.0;
  doc["rssi"]    = WiFi.RSSI();
  char buf[128];
  size_t n = serializeJson(doc, buf);
  mqtt.publish(tTelemetry.c_str(), (const uint8_t*)buf, n, false);
  Serial.println(buf);
}

void setup() {
  Serial.begin(115200);
  dht.begin();
  connectWiFi();
  mqtt.setServer(MQTT_HOST, MQTT_PORT);
  mqtt.setCallback(onMessage);
  mqtt.setKeepAlive(15);       // seconds
}

void loop() {
  if (WiFi.status() != WL_CONNECTED) {   // Wi-Fi lost: let it reconnect
    WiFi.reconnect();
    delay(1000);
    return;
  }
  if (!mqtt.connected()) {
    // retry every 5 s without blocking the rest of the loop
    if (millis() - lastTry > 5000) { lastTry = millis(); connectMqtt(); }
    return;
  }
  mqtt.loop();                 // keeps the connection alive, runs callbacks

  if (millis() - lastSend >= intervalMs) {
    lastSend = millis();
    publishReading();
  }
}