Learn Design Patterns fun by code.

1. OMG! Patterns.
Decorator
By Max Titov
maxtitov.me
Ninja Software Operations

2. OMG! Patterns?
▶ About mostly used patterns
▶ Examples of usage in real life
▶ Learning by code examples
▶ Learning new language

3. Example
Beauty salon.
Salon business
Salon provides following services:
▶ Haircut
▶ Coloring
▶ Styling

4. The Objective
▶ Salon going to add list of new services
▶ Salon want to have different combinations
of services in their pricelist
▶ Salon want’s to expand for new market
and provide services for males

6. System Overview
class Salon
{
string HaircutDescription();
string StylingDescription();
string ColoringDescription();
}

7. Don’t you feel like?
WHY?

8. Identifying the problems
▶ There is only one thing wrong with this
approach: everything.
▶ God class. Single responsibility principle?
Never heard about…
▶ Code duplication
▶ Hard to test
▶ #@#$&%%!!!

9. Lets do a bit of refactoring
▶ Identify common interface (if any)
▶ Apply single responsibility principle
▶ Use of old good inheritance

10. System Overview
interface ISalonService
{
string getDescription();
}

11. System Overview
class Haircut: ISalonService
{
string getDescription()
{
return ‘Haircut’;
}
}

12. System Overview

13. Muuuuuch better!

14. Adding new functionality

15. Requirements update
Client: Hey, we want to add two
other services and remove one.
That wouldn’t take much time
right?

16. Requirements update
F@#k
Client: Right?

17. Identifying the problems
▶ Requires new class for each combination
of services
▶ Hard to extend and maintain

18. Decorator
▶ The Decorator Pattern attaches additional
responsibilities to an object dynamically.
Decorators provide a flexible alternative to
subclassing for extending functionality

19. Decorator UML

20. Abstract Decorator
class abstract SalonServiceDecorator:
ISalonService
{
SalonServiceDecorator (ISalonService service);
string getDescription();
}

21. Concrete Decorator
class HaircutDecorator
{
HaircutDecorator (ISalonService service);
string getDescription();
}

22. Use of Decorators
salonService = new SalonService();
salonService = new
HaircutDecorator(salonService)
salonService = new
StylingDecorator(salonService)
salonService.getDescription()
Output:
> Salon service: Haircut, Styling

23. System Overview

24. Success!

25. Code Time!

26. Where Used
▶ GUI Frameworks
BaseElement
ScrollbarBehavior(BaseElement)
ShadowBehavior(BaseElement)
▶ Streams
FileStream
GZipStream(ByteStream)

27. Pros and Cons
Pros:
+ The cure for ugly code
+ Composition over inheritance (Adds flexibility)
+ Add behavior at runtime without class modification
+ Behavior can be reused
Cons:
– Too many small classes
– Could be not obvious for other developers
(example: Java streams)

28. Books
▶ Head First Design Patterns
By Eric Freeman
▶Design Patterns : Elements of
Reusable Object-Oriented Software
By (GoF) Gamma, Helm, Johnson, Vlissides
▶ Search Google for: Design patterns

29. What next?
More patterns!


30. Questions?
OMG! Patterns. Decorator
By Max Titov
Get examples: www.maxtitov.me
https://github.com/eolexe
Get in touch: eolexe@gmail.com
Twitter: eolexe