Dart Lesson 52 of 102 3 min read

Enums in Dart: Simple and Enhanced Enumerations

Learn enums in Dart: define a fixed set of values, use them in switch, and add fields, methods and constructors with enhanced enums.

An enum (enumeration) is a type with a small, fixed set of possible values. Use one whenever a variable should only ever be one of a few named options: the days of the week, the state of an order, the size of a T-shirt.

Why not strings? #

With String status = 'shiped'; the typo compiles and fails silently. With an enum, a misspelt value is a compile error and autocomplete lists every option.

A simple enum #

enum OrderStatus { pending, paid, shipped, delivered, cancelled }

void main() {
  var status = OrderStatus.shipped;

  print(status);
  print(status.name);   // the name as a String
  print(status.index);  // position, starting at 0
  print(status == OrderStatus.shipped);
}

OrderStatus.shipped
shipped
2
true

All the values #

enum Size { small, medium, large }

void main() {
  print(Size.values);

  for (final size in Size.values) {
    print('${size.index}: ${size.name}');
  }

  // From a String back to an enum value.
  var chosen = Size.values.byName('medium');
  print(chosen);
}

[Size.small, Size.medium, Size.large]
0: small
1: medium
2: large
Size.medium

byName throws if the name does not exist. For untrusted text, search safely with Size.values.where((s) => s.name == text).firstOrNull.

Enums and switch #

Dart checks that a switch on an enum handles every value. If you add a new value later, every switch that forgot it stops compiling, which is exactly what you want.

enum Light { red, yellow, green }

String action(Light light) => switch (light) {
      Light.red => 'Stop',
      Light.yellow => 'Slow down',
      Light.green => 'Go',
    };

void main() {
  print(action(Light.yellow));
}

Slow down

Enhanced enums: fields, constructors and methods #

An enum can carry data and behaviour, like a small class. Each value calls the constructor with its own arguments.

enum Planet {
  mercury(diameterKm: 4879, moons: 0),
  earth(diameterKm: 12742, moons: 1),
  jupiter(diameterKm: 139820, moons: 95);

  const Planet({required this.diameterKm, required this.moons});

  final int diameterKm;
  final int moons;

  bool get hasMoons => moons > 0;

  String describe() => '$name: $diameterKm km across, $moons moon(s)';
}

void main() {
  print(Planet.earth.describe());
  print(Planet.mercury.hasMoons);

  var biggest = Planet.values.reduce(
    (a, b) => a.diameterKm > b.diameterKm ? a : b,
  );
  print(biggest.name);
}

earth: 12742 km across, 1 moon(s)
false
jupiter

The rules for enhanced enums:

• List the values first, and end the list with a semicolon.
• The constructor must be const, and all fields must be final.
• You cannot create new values at run time.

A practical example #

enum Plan {
  free('Free', 0, 1),
  pro('Pro', 499, 10),
  team('Team', 1999, 100);

  const Plan(this.label, this.pricePerMonth, this.maxProjects);

  final String label;
  final int pricePerMonth;
  final int maxProjects;

  bool canCreate(int currentProjects) => currentProjects < maxProjects;
}

void main() {
  var plan = Plan.free;
  print('${plan.label}: Rs. ${plan.pricePerMonth}/month');
  print(plan.canCreate(1));
  print(Plan.pro.canCreate(1));
}

Free: Rs. 0/month
false
true

Enums can implement interfaces and use mixins #

abstract class HasCode {
  String get code;
}

enum Currency implements HasCode {
  npr('NPR'),
  usd('USD');

  const Currency(this.code);

  @override
  final String code;
}

void main() => print(Currency.npr.code);

NPR

Tips #

• Name the enum in UpperCamelCase and its values in lowerCamelCase.
• Do not store index in a database or file. Reordering the values would change every number. Store name instead.
• An enum is ideal for a closed list of simple options. When each option needs its own different data, use a sealed class.

Try it yourself #

Create a Weekday enum with a field isWeekend. Add a getter shortName that returns the first three letters in upper case. Loop over all values and print only the weekdays that are not weekend days.