Attaches additional behavior to an object dynamically without modifying its class.

An alternative to subclassing for extending functionality.

abstract class Coffee {
  String get description;
  double get cost;
}

class PlainCoffee implements Coffee {
  @override
  String get description => 'Plain coffee';
  @override
  double get cost => 2.0;
}

class MilkDecorator implements Coffee {
  final Coffee _coffee;
  MilkDecorator(this._coffee);

  @override
  String get description => '${_coffee.description} + milk';
  @override
  double get cost => _coffee.cost + 0.5;
}

class SugarDecorator implements Coffee {
  final Coffee _coffee;
  SugarDecorator(this._coffee);

  @override
  String get description => '${_coffee.description} + sugar';
  @override
  double get cost => _coffee.cost + 0.25;
}

// Usage
Coffee order = PlainCoffee();
order = MilkDecorator(order);
order = SugarDecorator(order);
print('${order.description}: \$${order.cost}'); // Plain coffee + milk + sugar: $2.75