Dart getter返回枚举name属性时与setter类型不兼容报错问题

实现parity（奇偶校验）相关逻辑时，通过enum.enum_entry.name属性提取枚举项对应的字符串值返回，触发类型不兼容报错。

相关代码 #

枚举定义与getter实现 #

enum SerialPortParity {
  none,
  even,
  odd,
  mark,
  space,
}

int _parity = UsbPort.PARITY_NONE;
String get parity {
  switch (_parity) {
    case UsbPort.PARITY_NONE:
      return SerialPortParity.none.name;
    case UsbPort.PARITY_EVEN:
      return SerialPortParity.even.name;
    case UsbPort.PARITY_ODD:
      return SerialPortParity.odd.name;
    case UsbPort.PARITY_MARK:
      return SerialPortParity.mark.name;
    case UsbPort.PARITY_SPACE:
      return SerialPortParity.space.name;
    default:
      return SerialPortParity.none.name;
  }
}

报错信息 #

The return type of getter 'parity' is 'String' which isn't a subtype of the type 'SerialPortParity' of its setter 'parity'.
Try changing the types so that they are compatible.

对应setter实现 #

set parity(SerialPortParity parity) {
  switch (parity) {
    case SerialPortParity.none:
      _parity = UsbPort.PARITY_NONE;
      break;
    case SerialPortParity.even:
      _parity = UsbPort.PARITY_EVEN;
      break;
    case SerialPortParity.odd:
      _parity = UsbPort.PARITY_ODD;
      break;
    case SerialPortParity.mark:
      _parity = UsbPort.PARITY_MARK;
      break;
    case SerialPortParity.space:
      _parity = UsbPort.PARITY_SPACE;
      break;
    default:
      // 无操作
      break;
  }
}

Dart语法规定，同名getter和setter必须类型兼容：如果setter的入参类型为SerialPortParity，对应getter的返回值类型必须是SerialPortParity或其子类。当前实现中getter返回值定义为String，和setter入参类型不匹配，因此编译器抛出错误。

根据实际业务需求二选一即可，优先推荐第一种方案：

• 方案1：对外暴露枚举类型，单独提供字符串名称获取能力
  将getter返回类型修改为SerialPortParity，直接返回枚举值而非.name字符串；如果业务场景需要用到字符串格式的枚举名称，单独新增一个getter获取即可。该方案保留了枚举的类型检查能力，避免非法入参导致运行时异常，是Dart枚举使用的标准实践。
  修正后的getter代码：

  SerialPortParity get parity {
    switch (_parity) {
      case UsbPort.PARITY_NONE:
        return SerialPortParity.none;
      case UsbPort.PARITY_EVEN:
        return SerialPortParity.even;
      case UsbPort.PARITY_ODD:
        return SerialPortParity.odd;
      case UsbPort.PARITY_MARK:
        return SerialPortParity.mark;
      case UsbPort.PARITY_SPACE:
        return SerialPortParity.space;
      default:
        return SerialPortParity.none;
    }
  }
  
  // 需要字符串名称时单独调用该属性
  String get parityName => parity.name;
  

• 方案2：parity属性直接对外暴露字符串类型
  将setter的入参类型修改为String，在setter内部完成字符串到枚举值、再到底层USB端口常量的转换。该方案类型安全性较差，传入非法字符串时会触发运行时错误，非必要不选择。
  参考实现：

  set parity(String parityName) {
    final parity = SerialPortParity.values.byName(parityName);
    switch (parity) {
      case SerialPortParity.none:
        _parity = UsbPort.PARITY_NONE;
        break;
      case SerialPortParity.even:
        _parity = UsbPort.PARITY_EVEN;
        break;
      case SerialPortParity.odd:
        _parity = UsbPort.PARITY_ODD;
        break;
      case SerialPortParity.mark:
        _parity = UsbPort.PARITY_MARK;
        break;
      case SerialPortParity.space:
        _parity = UsbPort.PARITY_SPACE;
        break;
    }
  }
  

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