ESP32 BLE: a Bluetooth Low Energy server with notifications

// BLE UART lamp: a Bluetooth LE server with the Nordic UART service.
//
// Simulated Bluetooth: a phone in Mokxi, not a real radio. Press Run, open
// the Bluetooth tab and connect to "ESP32 BLE Lamp", like nRF Connect on a
// phone: write on or off to the RX characteristic, and turn on notifications
// on TX to hear back. The same sketch runs on a real ESP32 (Arduino-ESP32 3.x).

#include <BLEDevice.h>
#include <BLEServer.h>
#include <BLEUtils.h>
#include <BLE2902.h>

#define SERVICE_UUID "6E400001-B5A3-F393-E0A9-E50E24DCCA9E"
#define RX_UUID "6E400002-B5A3-F393-E0A9-E50E24DCCA9E"
#define TX_UUID "6E400003-B5A3-F393-E0A9-E50E24DCCA9E"

const int kLed = 4; // an LED and a resistor to ground
BLEServer *server = nullptr;
BLECharacteristic *tx = nullptr;
bool connected = false;
bool wasConnected = false;

void say(const String &text)
{
    tx->setValue(text);
    tx->notify();
}

class ServerCallbacks : public BLEServerCallbacks {
    void onConnect(BLEServer *) override
    {
        connected = true;
        Serial.println("A phone connected.");
    }
    void onDisconnect(BLEServer *) override
    {
        connected = false;
        Serial.println("The phone disconnected.");
    }
};

class RxCallbacks : public BLECharacteristicCallbacks {
    void onWrite(BLECharacteristic *c) override
    {
        String value = c->getValue();
        value.trim();
        value.toLowerCase();
        Serial.print("Phone wrote: ");
        Serial.println(value);
        if (value == "on") {
            digitalWrite(kLed, HIGH);
            say("lamp on");
        } else if (value == "off") {
            digitalWrite(kLed, LOW);
            say("lamp off");
        } else {
            say("send on or off");
        }
    }
};

void setup()
{
    Serial.begin(115200);
    pinMode(kLed, OUTPUT);
    BLEDevice::init("ESP32 BLE Lamp");
    server = BLEDevice::createServer();
    server->setCallbacks(new ServerCallbacks());
    BLEService *service = server->createService(SERVICE_UUID);
    tx = service->createCharacteristic(TX_UUID, BLECharacteristic::PROPERTY_NOTIFY);
    tx->addDescriptor(new BLE2902());
    BLECharacteristic *rx = service->createCharacteristic(RX_UUID, BLECharacteristic::PROPERTY_WRITE);
    rx->setCallbacks(new RxCallbacks());
    service->start();
    BLEAdvertising *advertising = BLEDevice::getAdvertising();
    advertising->addServiceUUID(SERVICE_UUID);
    BLEDevice::startAdvertising();
    Serial.println("Advertising as ESP32 BLE Lamp: connect from the Bluetooth tab.");
}

void loop()
{
    // The board stops advertising when a phone connects. Start again once it
    // has gone, or nothing can find the board a second time.
    if (!connected && wasConnected) {
        delay(500);
        server->startAdvertising();
        Serial.println("Advertising again.");
    }
    wasConnected = connected;
    delay(20);
}