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

I think my approach is slightly different, but might be more convenient in some cases. Finally, we have parse and tryParse for enum types:

import 'dart:mirrors';

class Enum {
  static T parse<T>(String value) {
    final T result = (reflectType(T) as ClassMirror).getField(#values)
        .reflectee.firstWhere((v)=>v.toString().split('.').last.toLowerCase() == value.toLowerCase()) as T;
    return result;
  }

  static T tryParse<T>(String value, { T defaultValue }) {
    T result = defaultValue;
    try {
      result = parse<T>(value);
    } catch(e){
      print(e);
    }
    return result;
  }
}

EDIT: this approach is NOT working in the Flutter applications, by default mirrors are blocked in the Flutter because it causes the generated packages to be very large.

Answer 2

Using mirrors you could force some behaviour. I had two ideas in mind. Unfortunately Dart does not support typed functions:

import 'dart:mirrors';

enum Visibility {VISIBLE, COLLAPSED, HIDDEN}

class EnumFromString<T> {
  T get(String value) {
    return (reflectType(T) as ClassMirror).getField(#values).reflectee.firstWhere((e)=>e.toString().split('.')[1].toUpperCase()==value.toUpperCase());
  }
}

dynamic enumFromString(String value, t) {
  return (reflectType(t) as ClassMirror).getField(#values).reflectee.firstWhere((e)=>e.toString().split('.')[1].toUpperCase()==value.toUpperCase());
}

void main() {
  var converter = new EnumFromString<Visibility>();

  Visibility x = converter.get('COLLAPSED');
  print(x);

  Visibility y = enumFromString('HIDDEN', Visibility);
  print(y);
}

Outputs:

Visibility.COLLAPSED
Visibility.HIDDEN

Answer 3

I improved Collin Jackson's answer using Dart 2.7 Extension Methods to make it more elegant.

enum Fruit { apple, banana }

extension EnumParser on String {
  Fruit toFruit() {
    return Fruit.values.firstWhere(
        (e) => e.toString().toLowerCase() == 'fruit.$this'.toLowerCase(),
        orElse: () => null); //return null if not found
  }
}

main() {
  Fruit apple = 'apple'.toFruit();
  assert(apple == Fruit.apple); //true
}

Answer 4

Here is an alternative way to @mbartn's approach using extensions, extending the enum itself instead of String.

Faster, but more tedious

// We're adding a 'from' entry just to avoid having to use Fruit.apple['banana'],
// which looks confusing.
enum Fruit { from, apple, banana }

extension FruitIndex on Fruit {
  // Overload the [] getter to get the name of the fruit.
  operator[](String key) => (name){
    switch(name) {
      case 'banana': return Fruit.banana;
      case 'apple':  return Fruit.apple;
      default:       throw RangeError("enum Fruit contains no value '$name'");
    }
  }(key);
}

void main() {
  Fruit f = Fruit.from["banana"];
  print("$f is ${f.runtimeType}"); // Outputs: Fruit.banana is Fruit
}

Less tedius, but slower

If O(n) performance is acceptable you could also incorporate @Collin Jackson's answer:

// We're adding a 'from' entry just to avoid having to use Fruit.apple['banana']
// which looks confusing.
enum Fruit { from, apple, banana }

extension FruitIndex on Fruit {
  // Overload the [] getter to get the name of the fruit.
  operator[](String key) =>
    Fruit.values.firstWhere((e) => e.toString() == 'Fruit.' + key);
}

void main() {
  Fruit f = Fruit.from["banana"];
  print("$f is ${f.runtimeType}"); // Outputs: Fruit.banana is Fruit
}

Answer 5

I had the same problem in one of my projects and existing solutions were not very clean and it didn't support advanced features like json serialization/deserialization.

Flutter natively doesn't currently support enum with values, however, I managed to develop a helper package Vnum using class and reflectors implementation to overcome this issue.

Address to the repository:

https://github.com/AmirKamali/Flutter_Vnum

To answer your problem using Vnum, you could implement your code as below:

@VnumDefinition
class Visibility extends Vnum<String> {
  static const VISIBLE = const Visibility.define("VISIBLE");
  static const COLLAPSED = const Visibility.define("COLLAPSED");
  static const HIDDEN = const Visibility.define("HIDDEN");

  const Visibility.define(String fromValue) : super.define(fromValue);
  factory Visibility(String value) => Vnum.fromValue(value,Visibility);
}

You can use it like :

var visibility = Visibility('COLLAPSED');
print(visibility.value);

There's more documentation in the github repo, hope it helps you out.

Answer 6

    enum HttpMethod { Connect, Delete, Get, Head, Options, Patch, Post, Put, Trace }

    HttpMethod httpMethodFromString({@required String httpMethodName}) {
    assert(httpMethodName != null);

    if (httpMethodName is! String || httpMethodName.isEmpty) {
      return null;
    }

    return HttpMethod.values.firstWhere(
      (e) => e.toString() == httpMethodName,
      orElse: () => null,
    );
  }