Dart Builder pattern

builder-pattern.dart

class Pizza {
  final String sauce;
  final List<String> toppings;
  final bool hasExtraCheese;
  
  Pizza._builder(PizzaBuilder builder) : 
    sauce = builder.sauce,
    toppings = builder.toppings,
    hasExtraCheese = builder.hasExtraCheese;
}

class PizzaBuilder {
  static const String neededTopping = 'cheese';
  
  final String sauce;

  PizzaBuilder(this.sauce);

  List<String> toppings;
  bool hasExtraCheese;

  void setToppings(List<String> toppings) {
    if (!toppings.contains(neededTopping)) {
      throw 'Really, without $neededTopping? :(';
    }
    
    this.toppings = toppings;
  }
  
  Pizza build() {
    return Pizza._builder(this);
  }
}

void main() {
  print('___PIZZA BBQ___');
  
  Pizza pizza = (
    PizzaBuilder('bbq')
      ..setToppings(['tomato', 'cheese', 'chicken'])
      ..hasExtraCheese = true
    ).build();

  print(pizza.sauce);           // bbq
  print(pizza.toppings);        // [onion, cheese, chicken]
  print(pizza.hasExtraCheese);  // true
  
  print('___PIZZA Carbonara___');
  
  Pizza pizza2 = (
    PizzaBuilder('cream')
      ..hasExtraCheese = true
    ).build();
  
  print(pizza2.sauce);          // cream
  print(pizza2.toppings);       // null
  print(pizza2.hasExtraCheese); // true
  
  print('___PIZZA Margherita___');
  
  Pizza pizza3 = (
    PizzaBuilder('olive-oil')
      ..setToppings(['PINEAPPLE', 'tomato'])
      ..hasExtraCheese = false
    ).build();
                                // Uncaught Error: Really, without cheese? :( 
  print(pizza3.sauce);          // No output due to Uncaught Error
  print(pizza3.toppings);       // No output due to Uncaught Error
  print(pizza3.hasExtraCheese); // No output due to Uncaught Error
}

Console output

___PIZZA BBQ___
bbq
[onion, cheese, chicken]
true

___PIZZA Carbonara___
cream
null
true

___PIZZA Margherita___
Uncaught Error: Really, without cheese? :(