decorator

Decorator Pattern

The decorator pattern allow us to compose objects while keeping the same top-level interface, thus allowing for enhancing the behavior of the target objects in a flexible and run-timleable way.

Structure

classDiagram
class Component{
    MethodA();
    MethodB();
}

class ConcreteComponent{
    MethodA();
    MethodB();
}

class Decorator{
    Component wrappedObj
    MethodA();
    MethodB();
}

class ConcreteDecoratorA{
    Component wrappedObj
    MethodA();
    MethodB();
}

class ConcreteDecoratorB{
    Component wrappedObj
    MethodA();
    MethodB();
}

Component<|--ConcreteComponent
Component<|--Decorator
Decorator<|--ConcreteDecoratorA
Decorator<|--ConcreteDecoratorB

Loading

A client here would the Component without knowing wether it's a simple ConcreteComponent or a decorated one, because both of them implements the same abstract interface. Additionally, the decorator components (ConcreteDecoratorA and ConcreteDecoratorB) would implement their logic before or after calling MethodA() or MethodB() and the client would treat the result the same way.

Working example

For our working example, we are going to implement the beverages menu for Starbuzz Coffee. Currently, there are four types of base coffees and four types of condiments. Multiple condiments can be added to a base coffee, in any order.

+-------------------+
| Coffees           |
| - House Blend .89 |
| - Dark Roast .99  |
| - Decaf 1.05      |
| - Expresso 1.99   |
|                   |
| Condiments        |
| - SteamedMilk .10 |
| - Mocha .20       |
| - Soy .15         |
| - Whip .10        |
+-------------------+

Make sure to check out the StarbuzzCoffee for implementation details.