How to execute write method in function that changes value in slidebar? - flutter

Question

When the slide bar value changes, it tries to write a packet.
I am using the example,
I made a function and tried it, but a null error occurs. (discoverService, characteristic.., etc.)
How can I solve this?
I'm using flutter_blue
In the flutter_blue example, write works fine.
I'm flutter beginer.
I need your help. thank you.
https://pub.dev/packages/flutter_blue
onWriteData
var data = [0x80, 0x80, 0xF0, 0x7D, 0x05, 0x00, 0x0E, 0x01, 60, 100, 00,0x80, 0xF7];
FlutterBlue flutterBlue = FlutterBlue.instance;
BluetoothDevice device;
BluetoothService service;
onWriteData() async {
List<BluetoothService> services = await device.discoverServices();
services.forEach((service) {
});
var characteristics = service.characteristics;
for(BluetoothCharacteristic c in characteristics) {
List<int> value = await c.read();
print(value);
await c.write(data);
}
}
slideBar class
class slidecontrollerr extends State<slidecontroller> {
static double volumeValue = 60;
void onVolumeChanged(double value) async { **// I want to execute the write function here**
setState(() {
volumeValue = value;
});
onWriteData(); **//error**
}
#override
Widget build(BuildContext context) {
return Scaffold(
.
.
.
[Error Code][1]
E/flutter ( 5640): [ERROR:flutter/lib/ui/ui_dart_state.cc(199)] Unhandled Exception: NoSuchMethodError: The method 'discoverServices' was called on null.
E/flutter ( 5640): Receiver: null
E/flutter ( 5640): Tried calling: discoverServices()
E/flutter ( 5640): #0 Object.noSuchMethod (dart:core-patch/object_patch.dart:54:5)
E/flutter ( 5640): #1 onWriteData (package:auto_humidity/main.dart:467:48)
E/flutter ( 5640): #2 slidecontrollerr.onVolumeChanged (package:auto_humidity/main.dart:1489:5)
E/flutter ( 5640): #3 RenderRadialAxisParent.createPointerValueChangedArgs (package:syncfusion_flutter_gauges/src/radial_gauge/axis/radial_axis_parent_widget.dart:584:43)
E/flutter ( 5640): #4 RenderRadialAxisParent._setCurrentPointerValue (package:syncfusion_flutter_gauges/src/radial_gauge/axis/radial_axis_parent_widget.dart:548:5)
E/flutter ( 5640): #5 RenderRadialAxisParent._updateDragValue (package:syncfusion_flutter_gauges/src/radial_gauge/axis/radial_axis_parent_widget.dart:527:7)
E/flutter ( 5640): #6 RenderRadialAxisParent._updatePointerValue (package:syncfusion_flutter_gauges/src/radial_gauge/axis/radial_axis_parent_widget.dart:452:7)
E/flutter ( 5640): #7 RenderRadialAxisParent._handleDragUpdate (package:syncfusion_flutter_gauges/src/radial_gauge/axis/radial_axis_parent_widget.dart:339:5)
E/flutter ( 5640): #8 DragGestureRecognizer._checkUpdate.<anonymous closure> (package:flutter/src/gestures/monodrag.dart:436:55)
E/flutter ( 5640): #9 GestureRecognizer.invokeCallback (package:flutter/src/gestures/recognizer.dart:182:24)
E/flutter ( 5640): #10 DragGestureRecognizer._checkUpdate (package:flutter/src/gestures/monodrag.dart:436:7)
E/flutter ( 5640): #11 DragGestureRecognizer.handleEvent (package:flutter/src/gestures/monodrag.dart:289:9)
E/flutter ( 5640): #12 PointerRouter._dispatch (package:flutter/src/gestures/pointer_router.dart:93:12)
E/flutter ( 5640): #13 PointerRouter._dispatchEventToRoutes.<anonymous closure> (package:flutter/src/gestures/pointer_router.dart:138:9)
E/flutter ( 5640): #14 _LinkedHashMapMixin.forEach (dart:collection-patch/compact_hash.dart:397:8)
E/flutter ( 5640): #15 PointerRouter._dispatchEventToRoutes (package:flutter/src/gestures/pointer_router.dart:136:18)
E/flutter ( 5640): #16 PointerRouter.route (package:flutter/src/gestures/pointer_router.dart:122:7)
E/flutter ( 5640): #17 GestureBinding.handleEvent (package:flutter/src/gestures/binding.dart:439:19)
E/flutter ( 5640): #18 GestureBinding.dispatchEvent (package:flutter/src/gestures/binding.dart:419:22)
E/flutter ( 5640): #19 RendererBinding.dispatchEvent (package:flutter/src/rendering/binding.dart:287:11)
E/flutter ( 5640): #20 GestureBinding._handlePointerEventImmediately (package:flutter/src/gestures/binding.dart:374:7)
E/flutter ( 5640): #21 GestureBinding.handlePointerEvent (package:flutter/src/gestures/binding.dart:338:5)
E/flutter ( 5640): #22 GestureBinding._flushPointerEventQueue (package:flutter/src/gestures/binding.dart:296:7)
E/flutter ( 5640): #23 GestureBinding._handlePointerDataPacket (package:flutter/src/gestures/binding.dart:279:7)
E/flutter ( 5640): #24 _rootRunUnary (dart:async/zone.dart:1370:13)
E/flutter ( 5640): #25 _CustomZone.runUnary (dart:async/zone.dart:1265:19)
E/flutter ( 5640): #26 _CustomZone.runUnaryGuarded (dart:async/zone.dart:1170:7)
E/flutter ( 5640): #27 _invoke1 (dart:ui/hooks.dart:182:10)
E/flutter ( 5640): #28 PlatformDispatcher._dispatchPointerDataPacket (dart:ui/platform_dispatcher.dart:282:7)
E/flutter ( 5640): #29 _dispatchPointerDataPacket (dart:ui/hooks.dart:96:31)

Answer 1

You need to connect to a specific service and characteristics of your BLE device
Here is a piece of code that I am using
final String serviceUUID = "0000ffe0-0000-1000-8000-00805f9b34fb";
final String characteristicWriteUUID = "0000fff1-0000-1000-8000-00805f9b34fb";
final String characteristicReadUUID = "0000fff2-0000-1000-8000-00805f9b34fb";
These services and characteristics must be set in the module itself by default or by you manually
discoverServices(BluetoothDevice device) async {
if (device == null) return;
List<BluetoothService> services = await device.discoverServices();
services.forEach((service) {
// do something with service
if (service.uuid.toString().toLowerCase() == serviceUUID) {
service.characteristics.forEach((characteristic) {
if (characteristic.uuid.toString().toLowerCase() == characteristicReadUUID) {
targetReadCharacteristic = characteristic;
_readData(targetReadCharacteristic);
}
if (characteristic.uuid.toString().toLowerCase() == characteristicWriteUUID) {
targetWriteCharacteristic = characteristic;
writeData(targetWriteCharacteristic, [0x49, 0x44, 0x02]);
}
});
}
});
}
Future<void> writeData(characteristic, data) async{
if (characteristic == null) return;
try {
List<int> bytes = data;
await characteristic.write(bytes, withoutResponse: true );
print("data: Send: $data");
} catch (e) {
print('Data Error ${e.message} | $data');
}
}
_readData(characteristic) async {
if (!characteristic.isNotifying) {
await characteristic.setNotifyValue(true);
}
readSubScription = characteristic.value.listen((value) {
List<int> readData = new List.from(value);
if(readData.isNotEmpty && readData != []){
print('BLE read data: $readData');
}
});
}