使用Flutter Blue Plus读取ESP32特征时触发PlatformException问题

问题现象 #

使用Flutter Blue Plus库从ESP32读取蓝牙特征时，触发PlatformException，异常信息如下：

PlatformException(read_characteristic_error,unknown reason,may occur if readCharacteristic was called before last read finished, null, null)

用户提供的读取特征代码：

Future<void> readData async{
  BluetoothService? DataService =
  BLEComm().getService(DeviceServiceUUIDs.Service);
  if (null != DataService) {
  await Future.delayed(Duration(milliseconds:50),() async{
  BluetoothCharacteristic? DurationCharacteristic =
  BLEComm().getCharacteristic(
  DeviceServiceUUIDs.duration,
  DataService);
  List<int>? DurationResponse = await BLEComm()
      .readCharacteristic(DurationCharacteristic);
  String Duration = String.fromCharCodes(DurationResponse!);
  var Duration_Data = double.parse(Duration);
  });

依赖的底层BLEComm实现代码：

/// Get a specific service from the list of services.
BluetoothService? getService(String uuid) {
  for (BluetoothService? service in _deviceServices) {
    if (service?.uuid.toString() == uuid) {
      return service;
    }
  }
  return null;
}

/// Get Characteristic object from a service
BluetoothCharacteristic? getCharacteristic(
    String uuid, BluetoothService bleService) {
  var characteristics = bleService.characteristics;
  for (BluetoothCharacteristic characteristic in characteristics) {
    if (characteristic.uuid.toString() == uuid) {
      return characteristic;
    }
  }
  return null;
}

/// read value to a characteristic
/// Throws Exceptions on Non readable characteristic and general exceptions.
Future<List<int>?> readCharacteristic(
    BluetoothCharacteristic? characteristic) async {
  List<int>? value;
  try {
    if (null != characteristic && !characteristic.properties.read) {
      throw ("characteristic ${characteristic.uuid.toString()} is not readable");
    }
    value = await characteristic?.read();
  } catch (ex) {
    throw Exception(ex);
  }
  return value;
}

排查分析 #

从异常提示readCharacteristic was called before last read finished可以确定，核心问题是重复发起特征读取请求，前一次读取操作还未完成就触发了新的请求。结合代码细节，具体问题点包括：

• BLEComm实例重复创建：每次调用BLEComm()都会生成新实例，无法统一管理蓝牙操作状态，容易引发并发读取冲突。
• 无并发控制机制：没有状态标记或锁来限制同一时间仅执行一次读取操作，若readData被多次调用会直接触发重复请求。
• Future.delayed无效：用固定延迟规避时序问题不可靠，无法精准确保前一次读取操作完成。
• 空值处理不严谨：DurationResponse!强制解包会掩盖读取失败的错误，同时可能触发空指针异常。

解决方法 #

1. 改造BLEComm为单例模式 #

确保全局只有一个BLEComm实例，统一管理蓝牙操作状态：

class BLEComm {
  static final BLEComm _instance = BLEComm._internal();
  factory BLEComm() => _instance;
  BLEComm._internal();

  // 原有getService、getCharacteristic方法保持不变
}

2. 添加读取操作的并发控制 #

在BLEComm中加入状态标记，避免同一时间执行多个读取操作：

class BLEComm {
  static final BLEComm _instance = BLEComm._internal();
  factory BLEComm() => _instance;
  BLEComm._internal();

  bool _isReading = false; // 读取状态标记

  Future<List<int>?> readCharacteristic(
      BluetoothCharacteristic? characteristic) async {
    if (_isReading) {
      throw Exception("前一次读取操作尚未完成");
    }
    List<int>? value;
    try {
      _isReading = true;
      if (null != characteristic && !characteristic.properties.read) {
        throw ("characteristic ${characteristic.uuid.toString()} 不支持读取");
      }
      value = await characteristic?.read();
    } catch (ex) {
      throw Exception(ex);
    } finally {
      _isReading = false; // 无论成败都重置状态
    }
    return value;
  }

  // 原有getService、getCharacteristic方法保持不变
}

3. 优化readData方法逻辑 #

移除无效的延迟，完善空值和异常处理：

Future<void> readData() async {
  final bleComm = BLEComm();
  BluetoothService? dataService = bleComm.getService(DeviceServiceUUIDs.Service);
  if (dataService == null) {
    print("未找到目标服务");
    return;
  }

  BluetoothCharacteristic? durationCharacteristic = bleComm.getCharacteristic(
    DeviceServiceUUIDs.duration,
    dataService,
  );
  if (durationCharacteristic == null) {
    print("未找到目标特征");
    return;
  }

  try {
    List<int>? durationResponse = await bleComm.readCharacteristic(durationCharacteristic);
    if (durationResponse == null || durationResponse.isEmpty) {
      print("读取结果为空");
      return;
    }
    String durationStr = String.fromCharCodes(durationResponse);
    double durationData = double.parse(durationStr);
    print("读取到的时长数据：$durationData");
    // 此处添加数据处理逻辑
  } catch (e) {
    print("读取特征失败：$e");
  }
}

4. 检查ESP32端蓝牙逻辑 #

确认ESP32的BLE特征读取处理代码无阻塞，确保每次读取请求都能及时返回响应，避免因硬件端未完成处理导致客户端重复发起请求。

内容的提问来源于stack exchange，提问作者Nns_ninteyFIve