Why enums can't be used in const constructors in Flutter?

Question

can someone tell me if there's a way to use enhanced enums in const constructors? I tried both named constructors and factory constructors, with no luck:

enum MyEnum {
  first('first-string'),
  second('second-string');

  final String string;

  const MyEnum(this.string);
}

class MyClass {
  final String input;

  const MyClass({required this.input});

  const MyClass.first({
    this.input = MyEnum.first.string, //error 
  });

  factory MyClass.second() {
    return const MyClass(input: MyEnum.second.string); //error
  }
}

Named constructor gives error: The default value of an optional parameter must be constant Factory constructor gives error: A value of type 'Null' can't be assigned to a parameter of type 'String' in a const constructor. Try using a subtype, or removing the keyword 'const'

Right now the only solution I found was to replace enum with a class containing static const params, like this:

class MyEnumClass {
  static const String first = 'first-string';
  static const String second = 'second-string';
}

Answer 1

You can use enums if you use the enum type itself for the member.

enum MyEnum {
  first('first-string'),
  second('second-string');

  final String string;

  const MyEnum(this.string);
}

class MyClass {
  final MyEnum _input;

  String get input => _input.string;

  const MyClass({required MyEnum input}) : _input = input;

  const MyClass.first({
    MyEnum input = MyEnum.first,
  }) : _input = input;

  factory MyClass.second() {
    return const MyClass(input: MyEnum.second);
  }
}

In general, getters are equivalent to function calls and aren't constant expressions. (There are a some exceptions, and maybe enum members should be too.)