//Reference 
//https://github.com/esp8266/Arduino/issues/3046


#define PIN_SCL 22
#define PIN_SDA 21
// IO22 = SCL = (Above TX/RX) = Green
// IO21 = SDA = (Below TX/RX) = Yellow
String c;
//Receiver
#include <Wire.h>

void setup() {
  Wire.setClock(4000);
  Wire.begin(PIN_SDA, PIN_SCL);       // join i2c bus (address optional for master)
  Serial.begin(9600);  // start serial for output
  Serial.print("ESP32 Start communication") ;
}

void loop() {
  Wire.requestFrom(8, 6);    // request 6 bytes from slave device #8
  while (Wire.available()) { // slave may send less than requested
    //Serial.print("WireAvailable: "+ String(Wire.available()));
    //Serial.print("ESP32 : Message received");
    //int c = Wire.read();    // receive a byte as character
    Serial.print(Wire.available());
    c = (String) Wire.read();    // receive a byte as character
    Serial.print(c);         // print the character

    if(Wire.available() == 0 ){
      Serial.println("Message transmitted");
      
       break;
    }
  } 
}


//Sender
#include <Wire.h>

//ESP8266
#define PIN_SCL 14 //5
#define PIN_SDA 2 //4
// D1 = GPIO5 = SCL = GREEN --> not correct
// D2 = GPIO4 = SDA = YELLOW --> not correct
//IO14 I2C_SCL = D5 = Green
//IO2 I2C_SDA = D4 = Yellow
// function that executes whenever data is requested by master
// this function is registered as an event, see setup()
void requestEvent() {
    Wire.write("123456"); // respond with message of 6 bytes
    // as expected by master
    Serial.println("ESP86 answers");
}

void setup() {
    Serial.begin(9600);
    Wire.begin(PIN_SDA,PIN_SCL,8);                // join i2c bus with address #8
    Wire.onRequest(requestEvent); // register event
    Serial.println("ESP86 setup");
}

void loop() {
    delay(1);
}