#include "BluetoothSerial.h"
 
#if !defined(CONFIG_BT_ENABLED) || !defined(CONFIG_BLUEDROID_ENABLED)
#error Bluetooth is not enabled! Please run `make menuconfig` to and enable it
#endif
 
BluetoothSerial SerialBT;
 
void setup() {
  Serial.begin(115200);
  SerialBT.begin("ESP32test"); //Bluetooth device name
  Serial.println("The device started, now you can pair it with bluetooth!");
}
 
void loop() {
  bool a = SerialBT.hasClient();
  static bool b = a;
  if (a != b) Serial.printf("\nhasClient: %s\n", (b = a) ? "true" : "false" );
  while (Serial.available()) SerialBT.write(Serial.read());
  while (SerialBT.available()) Serial.write(SerialBT.read());
}

on android use: Arduino bluetooth controller