enums - Dart枚举编译器错误

Question

我正在尝试在Dart中定义以下“枚举”:

class Place {
    static const SigninPlace = const Place._("", new Map<String,String> { "fizz": "buzz"});
    static const SignoutPlace = const Place._("", null);

    static get values => [SigninPlace, SignoutPlace];

    final String name;
    final Map<String,String> params;

    const Place._(this.name, this.params);
}

但是在 SigninPlace声明中出现编译器错误:

Arguments of a constant creation must be constant expressions

为什么？该怎么解决？ (提示:每个 Place的 <String,String>映射不会改变，并且在启动时就已知)。

Answer 1

当您定义const时，其所有成员都必须是常量表达式。在您的情况下，new Map<String,String> { "fizz": "buzz"}不是常量表达式。您必须使用const <String,String>{ "fizz": "buzz"}创建一个常量Map。

class Place {
  static const SigninPlace = const Place._("", 
      const <String,String>{ "fizz": "buzz"});
  static const SignoutPlace = const Place._("", null);

  static get values => [SigninPlace, SignoutPlace];

  final String name;
  final Map<String,String> params;

  const Place._(this.name, this.params);
}