[Feature] Support multiple service/characteristic instances with same UUID

lib/flutter_blue_plus.dart

@@ -5,16 +5,17 @@
 library flutter_blue_plus;
 
 import 'dart:async';
-import 'dart:io';
 import 'dart:convert';
+import 'dart:io';
 
 import 'package:flutter/services.dart';
 
+part 'src/bluetooth_attribute.dart';
 part 'src/bluetooth_characteristic.dart';
 part 'src/bluetooth_descriptor.dart';
 part 'src/bluetooth_device.dart';
-part 'src/bluetooth_msgs.dart';
 part 'src/bluetooth_events.dart';
+part 'src/bluetooth_msgs.dart';
 part 'src/bluetooth_service.dart';
 part 'src/bluetooth_utils.dart';
 part 'src/flutter_blue_plus.dart';

lib/src/bluetooth_attribute.dart

@@ -0,0 +1,77 @@
+// Copyright 2017-2023, Charles Weinberger & Paul DeMarco.
+// All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+part of flutter_blue_plus;
+
+abstract class BluetoothAttribute {
+  final BluetoothDevice device;
+  final Guid uuid;
+  final int? index;
+
+  BluetoothAttribute({
+    required this.device,
+    required this.uuid,
+    this.index,
+  });
+
+  DeviceIdentifier get remoteId => device.remoteId;
+
+  @Deprecated('Use remoteId instead')
+  DeviceIdentifier get deviceId => remoteId;
+
+  BluetoothAttribute? get _parentAttribute => null;
+
+  String get identifier => "$uuid:$index";
+
+  String get identifierPath =>
+      _parentAttribute != null ? "${_parentAttribute!.identifierPath}/$identifier" : identifier;
+}
+
+abstract class BluetoothValueAttribute extends BluetoothAttribute {
+  List<int> _lastValue = [];
+
+  BluetoothValueAttribute({
+    required BluetoothDevice device,
+    required Guid uuid,
+    int? index,
+  }) : super(device: device, uuid: uuid, index: index);
+
+  /// this variable is updated:
+  ///   - anytime `read()` is called
+  ///   - anytime `write()` is called
+  ///   - anytime a notification arrives (characteristics, if subscribed)
+  ///   - when the device is disconnected it is cleared
+  List<int> get lastValue => _lastValue;
+
+  /// this stream emits values:
+  ///   - anytime `read()` is called
+  ///   - anytime `write()` is called
+  ///   - anytime a notification arrives (characteristics, if subscribed)
+  ///   - and when first listened to, it re-emits the last value for convenience
+  Stream<List<int>> get lastValueStream => FlutterBluePlus._methodStream.stream
+      .where((e) =>
+          e is OnCharacteristicReceivedEvent ||
+          e is OnCharacteristicWrittenEvent ||
+          e is OnDescriptorReadEvent ||
+          e is OnDescriptorWrittenEvent)
+      .map((e) => e as GetAttributeValueMixin)
+      .where((e) => e.attribute == this)
+      .map((e) => e.value)
+      .newStreamWithInitialValue(lastValue);
+
+  /// this stream emits values:
+  ///   - anytime `read()` is called
+  ///   - anytime a notification arrives (if subscribed)
+  Stream<List<int>> get onValueReceived => FlutterBluePlus._methodStream.stream
+      .where((e) => e is OnCharacteristicReceivedEvent || e is OnDescriptorReadEvent)
+      .map((e) => e as GetAttributeValueMixin)
+      .where((e) => e.attribute == this)
+      .map((e) => e.value);
+
+  @Deprecated('Use onValueReceived instead')
+  Stream<List<int>> get value => onValueReceived;
+
+  @Deprecated('Use onValueReceived instead')
+  Stream<List<int>> get onValueChangedStream => onValueReceived;
+}