Factory Constructors - Dart Programming

A factory constructor does not always build a fresh instance. It can pick an instance based on input, return a cached value, or fall back to a default. A library-private Color._(...) generative constructor builds the values the factory hands out.

Program

Play the program to ask for two colors and watch the factory dispatch to a known case and a fallback.

factory_constructors.dart

class Color {
  final String name;
  final int code;
  Color._(this.name, this.code);

  factory Color.named(String name) {
    switch (name) {
      case 'red':
        return Color._('red', 1);
      case 'green':
        return Color._('green', 2);
      default:
        return Color._('unknown', 0);
    }
  }
}

void main() {
  var a = Color.named('red');
  var b = Color.named('purple');
  print('${a.name}=${a.code} ${b.name}=${b.code}');
}

factory `factory Color.named(...)` returns an instance it chooses, instead of always creating a fresh one.

private constructor `Color._(...)` is a library-private generative constructor the factory uses to build values.

dispatch and fallback The factory inspects its argument and picks a case, with a `default` branch for unknown inputs.