Bluetooth LE with the ESP32: the phone without a network

No router, no network: the phone talks to the board over Bluetooth LE, the same one used by fitness bands and home sensors. The phone reads the temperature and receives updates on its own; writing 1 or 0 turns the LED on and off.

// Bluetooth LE server: the phone reads the temperature (and receives updates) and turns on the LED by writing 1 or 0.
#include <BLEDevice.h>
#include <BLEServer.h>
#include <BLE2902.h>
#include <Adafruit_BME280.h>

// 128-bit UUIDs made up for this project (generated at random, as the Bluetooth SIG expects for custom services)
#define SERVICE_UUID      "cb5a991b-bf61-4bee-af4b-976ee137c7e6"
#define TEMPERATURE_UUID  "a3cc1db8-b2b8-4886-af86-36deee2fdd20"
#define LED_UUID          "1171cd6c-7e71-4867-b3e1-2a8ac3b36cd4"

const int LED = 2;
Adafruit_BME280 bme;
BLECharacteristic* temperature;
bool connected = false;

class Connection : public BLEServerCallbacks {
  void onConnect(BLEServer* s) { connected = true; }
  void onDisconnect(BLEServer* s) {
    connected = false;
    s->startAdvertising();                 // becomes visible again for the next phone
  }
};

class LedCommand : public BLECharacteristicCallbacks {
  void onWrite(BLECharacteristic* c) {
    auto value = c->getValue();            // std::string in package 2.x, String in 3.x: auto works for both
    if (value.length() > 0) digitalWrite(LED, value[0] == '1' ? HIGH : LOW);
  }
};

void setup() {
  Serial.begin(115200);
  pinMode(LED, OUTPUT);
  if (!bme.begin(0x77)) Serial.println("BME280 not found");

  BLEDevice::init("FireBeetle-Meteo");                   // the name you see on the phone
  BLEServer* server = BLEDevice::createServer();
  server->setCallbacks(new Connection());
  BLEService* service = server->createService(SERVICE_UUID);

  temperature = service->createCharacteristic(TEMPERATURE_UUID,
                  BLECharacteristic::PROPERTY_READ | BLECharacteristic::PROPERTY_NOTIFY);
  temperature->addDescriptor(new BLE2902());             // the phone needs it to turn on notifications
  BLECharacteristic* led = service->createCharacteristic(LED_UUID, BLECharacteristic::PROPERTY_WRITE);
  led->setCallbacks(new LedCommand());

  service->start();
  BLEDevice::getAdvertising()->addServiceUUID(SERVICE_UUID);
  BLEDevice::startAdvertising();
  Serial.println("Ready: look for FireBeetle-Meteo with nRF Connect");
}

void loop() {
  char text[8];
  snprintf(text, sizeof(text), "%.1f", bme.readTemperature());
  temperature->setValue((uint8_t*)text, strlen(text));   // the value the phone reads
  if (connected) temperature->notify();                  // and sends it to whoever turned on notifications
  delay(2000);
}