为什么在类之外声明枚举？

Question

我正在查看 flutter 的单选按钮官方文档，注意到枚举已在 class 之外声明。为什么我们不能在 class 中声明它们？如果可以，在 class 内部和外部声明它们有什么区别？

示例代码如下：

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  static const String _title = 'Flutter Code Sample';

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: _title,
      home: Scaffold(
        appBar: AppBar(title: const Text(_title)),
        body: const Center(
          child: MyStatefulWidget(),
        ),
      ),
    );
  }
}

enum SingingCharacter { lafayette, jefferson }


class MyStatefulWidget extends StatefulWidget {
  const MyStatefulWidget({Key? key}) : super(key: key);

  @override
  State<MyStatefulWidget> createState() => _MyStatefulWidgetState();
}


class _MyStatefulWidgetState extends State<MyStatefulWidget> {
  SingingCharacter? _character = SingingCharacter.lafayette;

  @override
  Widget build(BuildContext context) {
    return Column(
      children: <Widget>[
        ListTile(
          title: const Text('Lafayette'),
          leading: Radio<SingingCharacter>(
            value: SingingCharacter.lafayette,
            groupValue: _character,
            onChanged: (SingingCharacter? value) {
              setState(() {
                _character = value;
              });
            },
          ),
        ),
        ListTile(
          title: const Text('Thomas Jefferson'),
          leading: Radio<SingingCharacter>(
            value: SingingCharacter.jefferson,
            groupValue: _character,
            onChanged: (SingingCharacter? value) {
              setState(() {
                _character = value;
              });
            },
          ),
        ),
      ],
    );
  }
}

谁能解释一下？任何帮助将不胜感激。

Answer 1

因为枚举是一种特殊的class，与此相关language tour

Enumerated types, often called enumerations or enums, are a special kind of class used to represent a fixed number of constant values.

而且你不能在另一个 class

中创建一个 class

另外，您可以为单选按钮使用地图

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