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A Map with compile-time check that every enum constant has an entry in it.

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A Map with the compile-time check that every enum constant has an entry in it.

Overview #

For an enum like this

enum Fruit { apple, orange, banana }

... you can generate the following class:

class FruitMap<V> extends EnumMap<Fruit, V> { // implements Map<Fruit, V>
    required this.apple,
    required this.orange,
    required this.banana,
  // ...

Which means that the app will only build if you set all the values in the constructor. And then you can treat this object like an ordinary map.

Usage #

Add these packages to your pubspec.yaml:

  enum_map: ^0.1.0

  enum_map_gen: ^0.1.0

Run dart pub get

Given that your enum is in fruit.dart, add:

  1. part 'fruit.g.dart; directive.
  2. @enumMap or @unmodifiableEnumMap annotation, or both.
import 'package:enum_map/enum_map.dat';

part 'fruit.g.dart';   // Add '.g' to the current file name before '.dart' extension.

enum Fruit { apple, orange, banana }

Then run this in your project root:

$ dart run build_runner build

This creates a file next to your enum file. If it was lib/fruit.dart, then the generated file is lib/fruit.g.dart.

This is how you use it:

import 'fruit.dart';

void main() {
  final modifiableMap = FruitMap<String>(apple: 'a', orange: 'o', banana: 'b');
  print(modifiableMap);  // prints: {Fruit.apple: a, Fruit.orange: o, Fruit.banana: b}
  print(modifiableMap[Fruit.apple]);      // prints: a
  print(modifiableMap.get(Fruit.apple));  // prints: a

  const unmodifiableMap = UnmodifiableFruitMap<String>(apple: 'a', orange: 'o', banana: 'b');
  print(unmodifiableMap);  // prints: {Fruit.apple: a, Fruit.orange: o, Fruit.banana: b}
  print(unmodifiableMap[Fruit.apple]);      // prints: a
  print(unmodifiableMap.get(Fruit.apple));  // prints: a

The generated classes contain:

  • All 21 of the public members of the Map interface.
  • V get(Fruit key) -- the non-nullable getter that guarantees the result of type V.
  • toString() that works like with the default maps.

Do Not Put Generated Files Under Version Control #

You should never put generated files under version control. Instead you should generate them after each checkout. To do this, add the above command to your build pipeline before dart compile or flutter build.

If you store generated files under version control, there is a risk that these files get outdated. Then your program will only work until the next re-generation, and then fixing it will be hard. To ensure you do not store generated files under version control, add the following line to your .gitignore file:


There is however a safety in get() method. It uses switch (key) to return the result. If you add a value to your enum, the generated file will break the build because there will be an uncovered case that does not return a result. This will remind you to rebuild your generated files.

Support Chat #

Do you have any questions? Feel free to ask in the Telegram Support Chat.

Or even just join to say 'Hi!'. I like to hear from the users.

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A Map with compile-time check that every enum constant has an entry in it.

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