Enum from String

Question

I have an Enum and a function to create it from a String because i couldn\'t find a built in way to do it

Answer 1

Collin Jackson's solution didn't work for me because Dart stringifies enums into EnumName.value rather than just value (for instance, Fruit.apple), and I was trying to convert the string value like apple rather than converting Fruit.apple from the get-go.

With that in mind, this is my solution for the enum from string problem

enum Fruit {apple, banana}

Fruit getFruitFromString(String fruit) {
  fruit = 'Fruit.$fruit';
  return Fruit.values.firstWhere((f)=> f.toString() == fruit, orElse: () => null);
}

Answer 2

There are a couple of enums packages which allowed me to get just the enum string rather than the type.value string (Apple, not Fruit.Apple).

https://pub.dartlang.org/packages/built_value (this is more up to date)

https://pub.dartlang.org/packages/enums

void main() {
  print(MyEnum.nr1.index);            // prints 0
  print(MyEnum.nr1.toString());       // prints nr1
  print(MyEnum.valueOf("nr1").index); // prints 0
  print(MyEnum.values[1].toString())  // prints nr2
  print(MyEnum.values.last.index)     // prints 2
  print(MyEnum.values.last.myValue);  // prints 15
}  

Answer 3

another variant, how it might be solved:

enum MyEnum {
  value1,
  value2,
}

extension MyEnumX on MyEnum {
  String get asString {
    switch (this) {
      case MyEnum.value1:
        return _keyValue1;
      case MyEnum.value2:
        return _keyValue2;
    }
    throw Exception("unsupported type");
  }

  MyEnum fromString(String string) {
    switch (string) {
      case _keyValue1:
        return MyEnum.value1;
      case _keyValue2:
        return MyEnum.value2;
    }
    throw Exception("unsupported type");
  }
}

const String _keyValue1 = "value1";
const String _keyValue2 = "value2";

void main() {
    String string = MyEnum.value1.asString;
    MyEnum myEnum = MyEnum.value1.fromString(string);
}

Answer 4

I had the same problem with building objects from JSON. In JSON values are strings, but I wanted enum to validate if the value is correct. I wrote this helper which works with any enum, not a specified one:

class _EnumHelper {


 var cache = {};

  dynamic str2enum(e, s) {
    var o = {};
    if (!cache.containsKey(e)){
      for (dynamic i in e) {
        o[i.toString().split(".").last] = i;
      }
      cache[e] = o;
    } else {
      o = cache[e];
    }
    return o[s];
  }
}

_EnumHelper enumHelper = _EnumHelper();

Usage:

enumHelper.str2enum(Category.values, json['category']);

PS. I did not use types on purpose here. enum is not type in Dart and treating it as one makes things complicated. Class is used solely for caching purposes.

Answer 5

I use this function, I think it's simple and doesn't need any kind of 'hack':

T enumFromString<T>(List<T> values, String value) {
    return values.firstWhere((v) => v.toString().split('.')[1] == value,
                             orElse: () => null);
}

You can use it like this:

enum Test {
    value1,
    value2,
}

var test = enumFromString(Test.value, 'value2') // Test.value2

Answer 6

Mirrors aren't always available, but fortunately you don't need them. This is reasonably compact and should do what you want.

enum Fruit { apple, banana }

// Convert to string
String str = Fruit.banana.toString();

// Convert to enum
Fruit f = Fruit.values.firstWhere((e) => e.toString() == str);

assert(f == Fruit.banana);  // it worked

Fix: As mentioned by @frostymarvelous in the comments section, this is correct implementation:

Fruit f = Fruit.values.firstWhere((e) => e.toString() == 'Fruit.' + str);