Separates the construction of a complex object from its representation, allowing the same construction process to create different representations.

Useful when an object has many optional parameters.

class Pizza {
  final String size;
  final bool cheese;
  final bool pepperoni;

  Pizza._({required this.size, this.cheese = false, this.pepperoni = false});
}

class PizzaBuilder {
  String _size = 'medium';
  bool _cheese = false;
  bool _pepperoni = false;

  PizzaBuilder size(String size) { _size = size; return this; }
  PizzaBuilder addCheese() { _cheese = true; return this; }
  PizzaBuilder addPepperoni() { _pepperoni = true; return this; }

  Pizza build() => Pizza._(size: _size, cheese: _cheese, pepperoni: _pepperoni);
}

// Usage
final pizza = PizzaBuilder().size('large').addCheese().addPepperoni().build();