Bi-direction Bluetooth Classic comm. between ESP32 (Arduino framework)

Last post show my exercise of "ESP-32S as Bluetooth classic Server, bi-direction communication with Raspberry Pi/Python". Here is the ESP32 implementation for the Client side, to connect to the ESP32 server in last post.

NodeMCU ESP-32S act as a server (in last post):
It echo what received from bluetooth back to sender, and display on SPI ST7735 display.

ESP32-DevKitC-V4 as client (this post):
Connect to server, forward data from serial,  to Bluetooth. Display data from Bluetooth on I2C SSD1306 OLED display.

The code start from ESP32 example of SerialToSerialBTM. But I found that the code SerialBT.connect() always return "true", and prompt "Connected Succesfully!", no matter the device name and MAC address, even no server exist. 

To solve it, I implement my bluetooth callback function to double check if ESP_SPP_OPEN_EVT raised. I don't know is it a long time solution, anyway it work in this exercise.

SPPClient_ESP32_ssd1306_20201231b.ino

/*
 * SPP Client on ESP32
 * Display on  SSD1306
 */

#include "ssd1306.h"
#include "ssd1306_console.h"
#include "BluetoothSerial.h"

Ssd1306Console  console;

BluetoothSerial SerialBT;

String ServerMACadd = "3C:71:BF:0D:DD:6A";
uint8_t ServerMAC[6]  = {0x3C, 0x71, 0xBF, 0x0D, 0xDD, 0x6A};
String ServerName = "ESP32_SPP";
char *pin = "1234"; //<- standard pin would be provided by default
bool connected;
bool isSppOpened = false;

/*
.arduino15/packages/esp32/hardware/esp32/1.0.4/libraries/
BluetoothSerial/src/BluetoothSerial.cpp

 */

void btCallback(esp_spp_cb_event_t event, esp_spp_cb_param_t *param){
  
  switch (event)
    {
    case ESP_SPP_INIT_EVT:
        Serial.println("ESP_SPP_INIT_EVT");
        break;

    case ESP_SPP_SRV_OPEN_EVT://Server connection open
        Serial.println("ESP_SPP_SRV_OPEN_EVT");
        break;

    case ESP_SPP_CLOSE_EVT://Client connection closed
        Serial.println("ESP_SPP_CLOSE_EVT");
        isSppOpened = false;
        break;

    case ESP_SPP_CONG_EVT://connection congestion status changed
        Serial.println("ESP_SPP_CONG_EVT");
        break;

    case ESP_SPP_WRITE_EVT://write operation completed
        Serial.println("ESP_SPP_WRITE_EVT");
        break;

    case ESP_SPP_DATA_IND_EVT://connection received data
        Serial.println("ESP_SPP_DATA_IND_EVT");
        break;

    case ESP_SPP_DISCOVERY_COMP_EVT://discovery complete
        Serial.println("ESP_SPP_DISCOVERY_COMP_EVT");
        break;

    case ESP_SPP_OPEN_EVT://Client connection open
        Serial.println("ESP_SPP_OPEN_EVT");
        isSppOpened = true;
        break;

    case ESP_SPP_START_EVT://server started
        Serial.println("ESP_SPP_START_EVT");
        break;

    case ESP_SPP_CL_INIT_EVT://client initiated a connection
        Serial.println("ESP_SPP_CL_INIT_EVT");
        break;

    default:
        Serial.println("unknown event!");
        break;
    }
}

static void startupScreen()
{
    ssd1306_setFixedFont(ssd1306xled_font6x8);
    ssd1306_clearScreen();
    ssd1306_printFixed(0, 0, "arduiino-er.blogspot.com", STYLE_BOLD);
    ssd1306_printFixed(0, 24, "ESP32 SPP Client", STYLE_NORMAL);
}

void setup()
{
    Serial.begin(115200);
    Serial.println("\n------ begin ----------------\n");
    
    ssd1306_128x64_i2c_init();
    ssd1306_clearScreen();
    startupScreen();
    delay(500);

    Serial.println("- to connect -");
    ssd1306_printFixed(0, 32, "...to connect", STYLE_NORMAL);
    
    SerialBT.begin("ESP32_Client", true);
    SerialBT.register_callback(btCallback);
    //connected = SerialBT.connect(ServerName);
    connected = SerialBT.connect(ServerMAC);

    /*
     * In my trial, 
     * SerialBT.connect() always return  true, even no server exist.
     * To solve it, I implemented bluetooth event callback function,
     * double varify if ESP_SPP_OPEN_EVT raised.
     */
    
    if(connected) {
      Serial.println("SerialBT.connect() == true");
    } else {
      Serial.println("Failed to connect! Reset to re-try");
      Serial.println("SerialBT.connect() == false");
      ssd1306_printFixed(0, 32, 
          "Failed to connect! Reset to re-try", STYLE_NORMAL);
      while(true){
      }
    }

    //may be there are some delay to call callback function,
    //delay before check
    delay(500);
    if(isSppOpened == false){
      Serial.println("isSppOpened == false");
      Serial.println("Reset to re-try");
      ssd1306_printFixed(0, 32, 
          "SPP_OPEN not raised! Reset to re-try", STYLE_NORMAL);
      while(true){
      }
    }
    
    Serial.println("isSppOpened == true");
    Serial.println("CONNECTED");

    ssd1306_clearScreen();
    ssd1306_setFixedFont(ssd1306xled_font6x8);
    console.println("CONNECTED:");
}

void loop()
{
    if(!isSppOpened){
      Serial.println("isSppOpened == false : DISCONNECTED");
      Serial.println("Reset to re-connect");
      console.println("DISCONNECTED");
      console.println("Reset to re-connect");
      while(true){
      }
    }
    if (Serial.available()) {
      SerialBT.write(Serial.read());
    }
    if (SerialBT.available()) {
      char c = SerialBT.read();
      //Serial.write(c);
      console.print(c);
    }
    delay(20);
}

For the setup of SPI ST7735 IPS used on server side, refer to the post "ESP32 display with 0.96" 80x160 SPI ST7735 IPS Display, using TFT_eSPI lib".

For the setup of I2C SSD1306 OLED used on client side, refer to the post "I2C SSD1306 OLED@ESP32 (ESP32-DevKitC-V4), using SSD1306 lib".