patterns/decorator.dart

//предназначен для динамического подключения дополнительного поведения к объекту

abstract class Beverage {
  double get cost;
  String get ingredients;
}

class Ingredient {
  double cost;
  String name;

  Ingredient(this.name, this.cost);

  @override
  String toString() => this.name;
}

var coffee = Ingredient("кофе", .25);
var milk = Ingredient("молоко", .5);
var sugar = Ingredient("сахар", .1);

class Coffee implements Beverage {
  Set<Ingredient> _ingredients = Set.from([coffee, milk, sugar]);
  double get cost => _ingredients.fold(0, (total, i) => total + i.cost);
  String get ingredients {
    var stringIngredients =
        _ingredients.fold("", (String str, i) => str + "${i.name}, ");
    var trimmedString =
        stringIngredients.substring(0, stringIngredients.length - 2);
    var lastComma = trimmedString.lastIndexOf(",");
    var replacement = ",".allMatches(trimmedString).length > 1 ? ", и" : " и";

    return trimmedString.replaceRange(lastComma, lastComma + 1, replacement);
  }
}

class PremiumCoffeDecorator implements Beverage {
  Beverage _coffee = Coffee();
  double get cost => _coffee.cost * 5;
  String get ingredients => _coffee.ingredients;
}

void main() {
  var coffee = Coffee();
  var premCoffee = PremiumCoffeDecorator();

  print(
      "Обычное кофе состоит из ${coffee.ingredients}. Оно стоит \$${coffee.cost}");
  print(
      "Лучшее кофе состоит из ${premCoffee.ingredients}. Оно стоит \$${premCoffee.cost}");

  // Coffee contains coffee, milk, and sugar. It costs $0.85
  // Starbucks coffee contains coffee, milk, and sugar. It costs $4.25
}