The Decorator pattern allows behavior to be added to individual objects dynamically at runtime without affecting other objects. It works by having decorator classes that contain and inherit from the class being decorated. This allows multiple behaviors to be independently added and removed from objects through composition rather than inheritance. Decorators can be used to flexibly add responsibilities to objects on an individual basis.

1. Structural Pattern
DECORATOR
Mohammad Reda Katby
Solution Architect
MOHE

2. Definition
• Provide a way of attaching new state and behavior to an object dynamically.
• The object does not know it is being “decorated”, which makes this a useful
pattern for evolving systems.
• A key implementation point in the Decorator pattern is that decorators both
inherit the original class and contain an instantiation of it.

3. Description
The importance of this pattern is that:
• The original object is unaware of any decorations.
• There is no one big feature-laden class with all the options in it.
• The decorations are independent of each other.
• The decorations can be composed together in a mix-and-match fashion.

4. Design
• Component
• An original class of objects that can have operations added or modified (there may be more than one such
class)
• Operation
• An operation in IComponent objects that can be replaced (there may be several operations)
• IComponent
• The interface that identifies the classes of objects that can be decorated (Component is one of these)
• Decorator
• A class that conforms to the IComponent interface and adds state and/or behavior (there may be more than
one such class)

5. Design

6. Quiz
Pattern Elements
• IComponent
• Component
• Operation
• Decorator
• Client
Example Elements
• Any photo
• A plain photo
• To display a photo
• A tagged photo
• Creator of photos and tagged photos

7. Options
• Multiple components
• Different components that conform to the interface
• Multiple decorators
• Multiple operations

8. Use
• Fits well in the graphics world
• Used in System.IO.Stream
• Used in Screen Creation inWPF

9. Use
Use the Decorator pattern when…
• You have:
• An existing component class that may be unavailable for subclassing.
• You want to:
• Attach additional state or behavior to an object dynamically.
• Make changes to some objects in a class without affecting others.
• Avoid subclassing because too many classes could result.