#include <BLEDevice.h>
#include <BLEUtils.h>
#include <BLEServer.h>
// See the following for generating UUIDs:
// https://www.uuidgenerator.net/
#define SERVICE_UUID "4fafc201-1fb5-459e-8fcc-c5c9c331914b"
#define CHARACTERISTIC_UUID "beb5483e-36e1-4688-b7f5-ea07361b26a8"
class MyCallbacks : public BLECharacteristicCallbacks {
 void onWrite(BLECharacteristic *pCharacteristic) {
 String value = pCharacteristic->getValue();
 if (value.length() > 0) {
 Serial.println("*********");
 Serial.print("New value: ");
 for (int i = 0; i < value.length(); i++) {
 Serial.print(value[i]);
 }
 Serial.println();
 Serial.println("*********");
 }
 }
};
void setup() {
 Serial.begin(115200);
 Serial.println("1- Download and install an BLE scanner app in your phone");
 Serial.println("2- Scan for BLE devices in the app");
 Serial.println("3- Connect to MyESP32");
 Serial.println("4- Go to CUSTOM CHARACTERISTIC in CUSTOM SERVICE and write something");
 Serial.println("5- See the magic =)");
 BLEDevice::init("MyESP32");
 BLEServer *pServer = BLEDevice::createServer();
 BLEService *pService = pServer->createService(SERVICE_UUID);
 BLECharacteristic *pCharacteristic =
 pService->createCharacteristic(CHARACTERISTIC_UUID, BLECharacteristic::PROPERTY_READ | BLECharacteristic::PROPERTY_WRITE_NR);
 pCharacteristic->setCallbacks(new MyCallbacks());
 pCharacteristic->setValue("Hello World");
 pService->start();
 BLEAdvertising *pAdvertising = pServer->getAdvertising();
 pAdvertising->start();
}
void loop() {
 // put your main code here, to run repeatedly:
 delay(2000);
}

// Comment: Write value "hi" as TEXT (UTF-8) 
*********
New value: hi
*********
*********
New value: hi
*********