import 'dart:async';
//import 'dart:convert';

import 'package:flutter/foundation.dart';
import 'package:flutter/material.dart';
import 'package:flutter_bluetooth_serial/flutter_bluetooth_serial.dart';

void main()=> runApp(const MyApp());

class MyApp extends StatefulWidget {
  const MyApp({super.key});

  @override
  // ignore: library_private_types_in_public_api
  _MyAppState createState()=> _MyAppState();
}

class _MyAppState extends State<MyApp> {
  List<BluetoothDevice> _devices=[];
  late BluetoothConnection connection;
  String adr="00:21:07:00:50:69"; // my bluetooth device MAC Adres

  @override
  void initState() {
    super.initState();
    _loadDevices();
  }

  Future<void> _loadDevices() async {
    List<BluetoothDevice> devices=await FlutterBluetoothSerial.instance.getBondedDevices();

    setState(() {   _devices=devices;    });
  }
//----------------------------
  Future<void> sendData(String data)  async {

    data=data.trim();
    try {
      List<int> list=data.codeUnits;
      Uint8List bytes=Uint8List.fromList(list);
      connection.output.add(bytes);
      await connection.output.allSent;
      if (kDebugMode) {
        // print('Data sent successfully');
      }
    } catch (e) {
      //print(e.toString());
    }
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
        home: Scaffold(
          appBar: AppBar(
            title: const Text("Bluetooth Single LED Control"),
          ),
          body: Center(
            child: Column(
              mainAxisAlignment: MainAxisAlignment.center,
              children: [
                const Text("MAC Adress: 00:21:07:00:50:69"),

                ElevatedButton(child:Text("Connect"),onPressed: () {
                  connect(adr);

                },),
                SizedBox(height: 30.0,),

                ElevatedButton(child:Text(" OPEN "),onPressed: () {
                  sendData("on");

                },),

                SizedBox(height: 10.0,),
                ElevatedButton(child:Text("CLOSE"),onPressed: () {
                  sendData("off");

                },),
              ],
            ),
          ),

        )

    );
  }

  Future connect(String address) async {
    try {
      connection=await BluetoothConnection.toAddress(address);
      sendData('111');
      //durum="Connected to the device";
      connection.input!.listen((Uint8List data) {
        //Data entry point
        // durum=ascii.decode(data);
      });

    } catch (exception) {
      // durum="Cannot connect, exception occured";
    }
  }

// --------------**************data gonder
//Future send(Uint8List data) async {
//connection.output.add(data);
//await connection.output.allSent;
}
//------------*********** data gonder end

Arduino codes are below:

#include <SoftwareSerial.h>

SoftwareSerial mySerial(10, 11); // RX, TX
String rec_data="off";

void setup() {
   pinMode(9, OUTPUT);
  Serial.begin(9600);
  mySerial.begin(9600);   // BlueTooth Data baud,set the data rate for the SoftwareSerial port
}

void loop() { // run over and over
  if (mySerial.available()) {
    
    rec_data=mySerial.readString();
    
    if(rec_data=="on"){digitalWrite(9, HIGH);}
    if(rec_data=="off"){digitalWrite(9, LOW);}

    
    
    Serial.println(rec_data);
    delay(100);
  }
  
}