Enums

An enum defines a type with a fixed set of possible values. Use one whenever a value must be one of a known list of options.

Example

enum Direction { north, south, east, west }

String arrow(Direction d) => switch (d) {
      Direction.north => '↑',
      Direction.south => '↓',
      Direction.east => '→',
      Direction.west => '←',
    };

void main() {
  const heading = Direction.east;
  print(heading);
  print(heading.name);
  print(heading.index);
  print(arrow(heading));
  print(Direction.values);
  print(Direction.values.byName('west'));
}

Expected output

Direction.east
east
2
→
[Direction.north, Direction.south, Direction.east, Direction.west]
Direction.west

• The switch needs no _ case because it covers every value. Add a new direction later, and the compiler flags every switch that doesn't handle it.
• name is the value's name as text; index its position.
• values lists every value; values.byName('west') converts text back to an enum value.

Enhanced enums: fields and methods

Enum values can carry data:

enum Size {
  small(250, 2.5),
  medium(350, 3.0),
  large(450, 3.5);

  final int ml;
  final double price;
  const Size(this.ml, this.price);

  String get label => '${name} (${ml}ml): ${price.toStringAsFixed(2)} EUR';
}

void main() {
  for (final size in Size.values) {
    print(size.label);
  }
}

small (250ml): 2.50 EUR
medium (350ml): 3.00 EUR
large (450ml): 3.50 EUR

Note the semicolon after the last value, and the const constructor.

Common mistakes

• Using strings like 'small' for a fixed set of options. A typo in a string is a silent bug; a typo in an enum value is a compile error.
• Adding a _ / default case to a switch over your own enum — it hides the error you'd otherwise get when a new value is added.