Flutter枚举

Flutter枚举

Enum Extension

在Dart2.7版本，Dart新增了扩展函数，使枚举相关的代码定义、调用更加简洁

通过扩展枚举定义方法

定义一个元素枚举

enum Element { hydrogon, carbon, oxide }

定义一个枚举扩展，通过switch case获取每个元素的相对原子质量和中文描述

extension ElementExtension on Element {
  int get number {
    switch (this) {
      case Element.hydrogon:
        return 1;
      case Element.carbon:
        return 6;
      case Element.oxide:
        return 8;
    }
  }

  String get description {
    switch (this) {
      case Element.hydrogon:
        return '氢气';
      case Element.carbon:
        return '碳';
      case Element.oxide:
        return '氧气';
    }
  }

定义一个静态方法来通过相对原子质量获取

static Element fromNumber(int number) {
    switch (number) {
      case 1:
        return Element.hydrogon;
      case 6:
        return Element.carbon;
      case 8:
        return Element.oxide;
      default:
        return Element.oxide;
    }
  }

增强枚举类型

在Flutter 3.0 发布之后，直接在枚举构造方法内部定义相关属性，枚举值声明处进行声明即可

在pubspec.yaml文件中须将sdk版本设置为大于2.17.0

定义一个动物枚举，并定义成员变量name和weight和构造函数

enum Animal {

  const Animal({required this.name, required this.weight});

  final String name;
  final int weight;
}

创建枚举类型dog和cat

dog(name: 'dog', weight: 20),
cat(name: 'cat', weight: 6);

通过name和weight获取枚举类型

  factory Animal.fromName(String name) {
    return Animal.values.firstWhere((element) => element.name == name);
  }

  factory Animal.fromWeight(int weight) {
    return Animal.values.firstWhere((element) => element.weight == weight);
  }

这里直接构造了两个工厂函数，接收name或者weight参数，通过遍历Animal的values，筛选出和参数相同的枚举类型并返回

也可以通过静态方法的方式

  static Animal fromNameStatic(String name) {
    return Animal.values.firstWhere((element) => element.name == name);
  }

  static Animal fromWeightStatic(int weight) {
    return Animal.values.firstWhere((element) => element.weight == weight);
  }

创建枚举并访问成员变量

const dog = Animal.dog;
log(dog.name);
log(dog.weight.toString());

定义toJson方法将枚举类型的所有信息打印

Map<String, dynamic> toJson() {
    return {'name': name, 'weight': weight};
}

完整代码

enum Animal {
  dog(name: 'dog', weight: 20),
  cat(name: 'cat', weight: 6);

  const Animal({required this.name, required this.weight});

  factory Animal.fromName(String name) {
    return Animal.values.firstWhere((element) => element.name == name);
  }

  factory Animal.fromWeight(int weight) {
    return Animal.values.firstWhere((element) => element.weight == weight);
  }

  static Animal fromNameStatic(String name) {
    return Animal.values.firstWhere((element) => element.name == name);
  }

  static Animal fromWeightStatic(int weight) {
    return Animal.values.firstWhere((element) => element.weight == weight);
  }

  Map<String, dynamic> toJson() {
    return {'name': name, 'weight': weight};
  }

  final String name;
  final int weight;
}

Dart2.7之前

在Dart 2.7 的扩展函数发布之前，一般是新建一个顶级函数/工具类内部函数，使用swtich case来返回枚举值对应的关联数据

String getName(Animal animal) {
  switch (animal) {
    case Animal.dog:
      return 'dog';
    case Animal.cat:
      return 'cat';
  }
}

反之亦然

Animal getType(String name) {
  switch (name) {
    case 'dog':
      return Animal.dog;
    case 'cat':
      return Animal.cat;
    default:
      return Animal.dog;
  }
}