Intent: Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.

Applicability: Use Decorator

• to add responsibilities to individual objects dynamically and transparently, that is, without affecting other objects
• for responsibilities that can be withdrawn
• when extension by subclassing is impractical. Sometimes a large number of independent extensions are possible and would produce an explosion of subclasses to support every combination. Or a class definition may be hidden or otherwise unavailable for subclassing

Credits

• Design Patterns: Elements of Reusable Object-Oriented Software