Problem with BLE module : Exception: Another scan is already in progress - flutter

Question

I'm trying to make possible the following actions:
I run my app and when I press the "Connect" button I connect to my BLE module.
A second button allows me to disconnect from it.
The problem is that if I connect to the BLE module, and then press the disconnect button, it disconnect but I will no longer be able to connect to the BLE anymore (and I have to restart the app).
There is my code:
import 'dart:async';
import 'dart:convert' show utf8;
import 'package:flutter/material.dart';
import 'package:flutter/services.dart';
import 'package:flutter_blue/flutter_blue.dart';
void main() {
SystemChrome.setPreferredOrientations([
DeviceOrientation.landscapeRight,
DeviceOrientation.landscapeLeft
]);
runApp(MainScreen());
}
class MainScreen extends StatelessWidget {
#override
Widget build(BuildContext context) {
return MaterialApp(
title: 'BLUETOOTH',
debugShowCheckedModeBanner: false,
home: Home(),
theme: ThemeData.dark(),
);
}
}
class Home extends StatefulWidget {
#override
_HomeState createState() => _HomeState();
}
class _HomeState extends State<Home> {
String connectionText = "";
final String SERVICE_UUID = "4fafc201-1fb5-459e-8fcc-c5c9c331914b";
final String CHARACTERISTIC_UUID = "beb5483e-36e1-4688-b7f5-ea07361b26a8";
final String TARGET_DEVICE_NAME = "MLT-BT05";
FlutterBlue flutterBlue = FlutterBlue.instance; // * Instance de FlutterBlue
StreamSubscription<ScanResult> scanSubScription; // * StreamSubscription
BluetoothDevice targetDevice; // * Device
BluetoothCharacteristic targetCharacteristic; // * Characteristiques
#override
void initState() {
super.initState();
// startScan();
}
startScan() {
setState(() {
connectionText = "Start Scanning";
});
scanSubScription = flutterBlue.scan().listen((scanResult) {
print(scanResult.device.name.toString()); // ! TEST
if (scanResult.device.name == TARGET_DEVICE_NAME) {
print('DEVICE found');
stopScan();
setState(() {
connectionText = "Found Target Device";
});
targetDevice = scanResult.device;
connectToDevice();
}
}, onDone: () => stopScan());
}
stopScan() {
scanSubScription?.cancel();
scanSubScription = null;
}
connectToDevice() async {
if (targetDevice == null) return;
setState(() {
connectionText = "Device Connecting";
});
await targetDevice.connect();
print('DEVICE CONNECTED');
setState(() {
connectionText = "Device Connected";
});
discoverServices();
}
disconnectFromDevice() {
if (targetDevice == null) return;
targetDevice.disconnect();
setState(() {
connectionText = "Device Disconnected";
});
}
discoverServices() async {
if (targetDevice == null) return;
List<BluetoothService> services = await targetDevice.discoverServices();
services.forEach((service) {
// do something with service
if (service.uuid.toString() == SERVICE_UUID) {
service.characteristics.forEach((characteristic) {
if (characteristic.uuid.toString() == CHARACTERISTIC_UUID) {
targetCharacteristic = characteristic;
writeData("Hi there, ESP32!!");
setState(() {
connectionText = "All Ready with ${targetDevice.name}";
});
}
});
}
});
}
writeData(String data) {
if (targetCharacteristic == null) return;
List<int> bytes = utf8.encode(data);
targetCharacteristic.write(bytes);
}
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text(connectionText),
centerTitle: true,
),
body: Center(
child: Column(
children: <Widget>[
FlatButton(
child: Text("Connect"),
color: Colors.grey[700],
onPressed: () {
startScan();
},
),
FlatButton(
child: Text("Disconnect"),
color: Colors.grey[700],
onPressed: () {
disconnectFromDevice();
},
),
],
),
),
);
}
}
The error tells me that another scan is already in progress even though the scanSubScription is canceled and set to null once the device is connected.
The error:
I/flutter (13433): [TV] Samsung 7 Series (55)
I/flutter (13433): MLT-BT05
I/flutter (13433): DEVICE found
I/flutter (13433): DEVICE CONNECTED
E/flutter (13433): [ERROR:flutter/lib/ui/ui_dart_state.cc(157)] Unhandled Exception: Exception: Another scan is already in progress.
E/flutter (13433): #0 FlutterBlue.scan (package:flutter_blue/src/flutter_blue.dart:81:7)
E/flutter (13433): <asynchronous suspension>
E/flutter (13433): #1 _HomeState.startScan (package:bluetoothv8/main.dart:53:36)
E/flutter (13433): #2 _HomeState.build.<anonymous closure> (package:bluetoothv8/main.dart:140:17)
E/flutter (13433): #3 _InkResponseState._handleTap (package:flutter/src/material/ink_well.dart:706:14)
E/flutter (13433): #4 _InkResponseState.build.<anonymous closure> (package:flutter/src/material/ink_well.dart:789:36)
E/flutter (13433): #5 GestureRecognizer.invokeCallback (package:flutter/src/gestures/recognizer.dart:182:24)
E/flutter (13433): #6 TapGestureRecognizer.handleTapUp (package:flutter/src/gestures/tap.dart:486:11)
E/flutter (13433): #7 BaseTapGestureRecognizer._checkUp (package:flutter/src/gestures/tap.dart:264:5)
E/flutter (13433): #8 BaseTapGestureRecognizer.handlePrimaryPointer (package:flutter/src/gestures/tap.dart:199:7)
E/flutter (13433): #9 PrimaryPointerGestureRecognizer.handleEvent (package:flutter/src/gestures/recognizer.dart:467:9)
E/flutter (13433): #10 PointerRouter._dispatch (package:flutter/src/gestures/pointer_router.dart:76:12)
E/flutter (13433): #11 PointerRouter._dispatchEventToRoutes.<anonymous closure> (package:flutter/src/gestures/pointer_router.dart:117:9)
E/flutter (13433): #12 _LinkedHashMapMixin.forEach (dart:collection-patch/compact_hash.dart:379:8)
E/flutter (13433): #13 PointerRouter._dispatchEventToRoutes (package:flutter/src/gestures/pointer_router.dart:115:18)
E/flutter (13433): #14 PointerRouter.route (package:flutter/src/gestures/pointer_router.dart:101:7)
E/flutter (13433): #15 GestureBinding.handleEvent (package:flutter/src/gestures/binding.dart:218:19)
E/flutter (13433): #16 GestureBinding.dispatchEvent (package:flutter/src/gestures/binding.dart:198:22)
E/flutter (13433): #17 GestureBinding._handlePointerEvent (package:flutter/src/gestures/binding.dart:156:7)
E/flutter (13433): #18 GestureBinding._flushPointerEventQueue (package:flutter/src/gestures/binding.dart:102:7)
E/flutter (13433): #19 GestureBinding._handlePointerDataPacket (package:flutter/src/gestures/binding.dart:86:7)
E/flutter (13433): #20 _rootRunUnary (dart:async/zone.dart:1138:13)
E/flutter (13433): #21 _CustomZone.runUnary (dart:async/zone.dart:1031:19)
E/flutter (13433): #22 _CustomZone.runUnaryGuarded (dart:async/zone.dart:933:7)
E/flutter (13433): #23 _invoke1 (dart:ui/hooks.dart:273:10)
E/flutter (13433): #24 _dispatchPointerDataPacket (dart:ui/hooks.dart:182:5)

Answer 1

Instead of:
stopScan() {
scanSubScription?.cancel();
scanSubScription = null;
}
write:
stopScan() {
flutterBlue.stopScan();
scanSubScription?.cancel();
scanSubScription = null;
}

Answer 2

please use like this
bluetoothInstance.stopScan().then((value) {
scanSubscription.cancel();
scanBLE(); //if you need, you can start scan again
});