Enum constructor support

[keertip]

@pedromassango commented on Wed Sep 11 2019

I'm coming from Android development with Kotlin where we have support for constructors and properties while in Dart we don't.

In my current Flutter project I have the following code to handle my EventStatus Enum:

class EnumValues<T> {
  Map<String, T> map;
  Map<T, String> reverseMap;

  EnumValues(this.map);

  Map<T, String> get reverse {
    if (reverseMap == null) {
      reverseMap = map.map((k, v) => new MapEntry(v, k));
    }
    return reverseMap;
  }
}

class EventStatusConverter {
  static final _statusValues = new EnumValues<EventStatus>({
    'UPCOMING': EventStatus.UPCOMING,
    'CURRENT': EventStatus.PUBLISHED,
  });

  static EventStatus decode(String status) => _statusValues.map[status];

  static String encode(EventStatus status) => _statusValues.reverse[status];
}

You can see that it is very dificult and we have to write alot of boilerplate code to handle Enums in Dart. We can add support for constructors and params in Enum to prevent boilerplate code. For example we can have something like this:

enum EventStatus {
  const MyEnum(this.value);

  final String value;

  UPCOMING("UPCOMING"), CURRENT("CURRENT");
}

The code above compile in Java. And the code bellow compile in Kotlin:

enum class Color(val rgb: Int) {
        RED(0xFF0000),
        GREEN(0x00FF00),
        BLUE(0x0000FF)
}

What I am proposing is to have a more concise and easy way to deal with Enum in Dart.

[munificent]

I think #158 is essentially a duplicate of this.