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

enum Visibility{VISIBLE,COLLAPSED,HIDDEN}

Visibility visibilityFromString(String value){
  return Visibility.values.firstWhere((e)=>
      e.toString().split('.')[1].toUpperCase()==value.toUpperCase());
}

//used as
Visibility x = visibilityFromString('COLLAPSED');

but it seems like i have to rewrite this function for every Enum i have, is there a way to write the same function where it takes the Enum type as parameter? i tried to but i figured out that i can't cast to Enum.

//is something with the following signiture actually possible?
     dynamic enumFromString(Type enumType,String value){

     }

Answer 1

You can add an extension, it will still be accessed from the enum!

usage example: this answer

Answer 2

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 3

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 4

This is all so complicated I made a simple library that gets the job done:

https://pub.dev/packages/enum_to_string

import 'package:enum_to_string:enum_to_string.dart';

enum TestEnum { testValue1 };

convert(){
    String result = EnumToString.parse(TestEnum.testValue1);
    //result = 'testValue1'

    String resultCamelCase = EnumToString.parseCamelCase(TestEnum.testValue1);
    //result = 'Test Value 1'

    final result = EnumToString.fromString(TestEnum.values, "testValue1");
    // TestEnum.testValue1
}

Answer 5

@Collin Jackson has a very good answer IMO. I had used a for-in loop to achieve a similar result prior to finding this question. I am definitely switching to using the firstWhere method.

Expanding on his answer this is what I did to deal with removing the type from the value strings:

enum Fruit { apple, banana }

class EnumUtil {
    static T fromStringEnum<T>(Iterable<T> values, String stringType) {
        return values.firstWhere(
                (f)=> "${f.toString().substring(f.toString().indexOf('.')+1)}".toString()
                    == stringType, orElse: () => null);
    }
}

main() {
    Fruit result = EnumUtil.fromStringEnum(Fruit.values, "apple");
    assert(result == Fruit.apple);
}

Maybe someone will find this useful...

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);