Lecture two, An introduction to Design Pattern History Pattern Language, Categorization according to GoF MVC Creational Design Patterm Factory Method Abstract Factory Singleton Builder

1. Design Pattern
By Julie Iskander
MSc. Communication and Electronics

2. Outlines
Lecture 2
• What is Design Patterns?
• History of Design Patterns
• How to learn Design Patterns?
• Design Patterns Classifications acc. to GoF
• Creational DP:
• Singleton
• Factory Method
• Abstract Method
• Builder
• Prototype

3. What is Design Pattern
• A pattern is a solution to a common problem,
• NOT a code solution ready for copy-and-paste but
a best practice, a useful abstraction, and a template
for solving categories of problems.

4. What is Design Pattern
• Design Patterns are frequently used ways
of organizing objects to make solutions
easier to extend and modify
• Design Patterns name and describe
effective solutions for common problems
• Design Pattern help a designer get a
design right faster.
• Never reinvent the wheel.

5. What is Design Pattern
• “Each pattern
describes a problem
which occurs over and
over again, and then
describes the core of
the solution in such a
way that you can use
this solution many
times without ever
doing it the same way
twice”, Christopher

6. History of Design Pattern
• Dr. Christopher Alexander (in civil
engineering), in1977, A Pattern
Language.
• In 1978–79, Prof. Trygve Reenskaug
(in computer science), wrote “The
design pattern: Model-View–
Controller (MVC)”.
• In 1995, a catalog of 23 design
patterns by Drs. Gamma, Helm,
Johnson, and Vlissides “Gang of
Four” (GoF), “Design Patterns:
Elements of Reusable Object-
Oriented Software (Addison-Wesley,
1995)”.

7. How to learn Design
patterns ?
The best way to
use patterns is to
load your brain with
them and then
recognize places in
your designs and
existing
applications where
you can apply them

8. “Designing OO software is hard and designing
reusable OO software even harder”,
Erich Gamma
Experience  Reusing solutions that worked before
Knowing patterns that worked in the past  make
designer more productive and creating flexible and
reusable designs

9. Instead of code reuse,
with design patterns you
get experience reuse

10. Advantages of learning Design
Pattern
• It helps us to communicate in an efficient manner,
creates a common vocabulary
• It enables us to discuss low-level implementation in
a high-level, abstract manner.
• It enables us to apply a proven solution (experience
reuse)
• It helps extendibility and avoid solution redesign
• It help present design alternatives if found.
• It can help improve maintenance and documentation

11. Design Pattern elements
• Pattern Name
• Problem : When to apply? Problem and
Context
• Solution
• Consequence : results, trade offs of
applying it

12. Model-View-Controller
MVC

14. MVC
• What , Where
• a requirement to vary perspectives or views (output)
without changing the underlying information source
(model).
• Why
• it is more manageable and less expensive to
encapsulate change in a set of controller objects than
it is to disfigure the model object through constant
reengineering.

15. MVC
• How
• Encapsulating the client requirements in a
controller object that transposes the
information from the model object into the
variety of formats required by the client.
• a model object (model)
• a view interface (IView), or abstract class, if you
prefer;
• Deriving any number of views (GraphView and
BarView classes) from the interface;
• creating a controller object (controller), to represent
each view; and then wiring up the functionality.

16. Design Patterns
according to the
GOF

17. DP Classifications
• According to scope
Class Object
Deals with relationships Deals with objects
between classes and their relationships that can be
subclasses changed at runtime,
dynamically
Done by inheritance

18. DP Classifications
• According to purpose
Creational Structural Behavioral
Gives program the Deals with the Characterizes the
flexibility in composition of way classes and
deciding which classes and objects objects interact and
object to create. to create larger distribute
When and how to structures responsibilities
create objects Or add
functionalities

19. DP Classifications
• According to purpose
Creational Structural Behavioral
Singleton Adapter (Class Chain of Responsibility
Factory Method Pattern) Command
(Class Pattern) Bridge Iterator(Class Pattern)
Abstract Factory Composite Mediator
Builder Decorator Memento
Prototype Façade Observer
Flyweight State
Proxy Strategy
Template Method
(Class Pattern)
Visitor
Interpreter

20. Creational Patterns

21. Singleton
(object – creational dp)

22. Singleton
(object – Creational DP)
• What, Why
• A class has one and only one instance
created
• How
• private constructor

23. Singleton
(object – Creational DP)

24. Singleton
(object – Creational DP)
• Notes:
• Can permit a variable # of instances

25. Factory method
(Class – Creational DP)
There is more to making objects than just using
the new operator. Instantiation is an activity that
shouldn’t always be done in public and can often
lead to coupling problems.

26. Factory method
(Class – Creational DP)
• What
• A design that defines an interface for creating
objects and delegates the choice of what objects to
create to subclasses.
• A creator class is required to be decoupled from
creating specific objects.
• Why
• Flexibility in creating a variety of objects
• How
• Subclasses, with overridden factory method create
specific types, which in turn create content objects.

27. Factory method
(Class – Creational DP)

28. Factory method
(Class – Creational DP)
• Example
• An application with multiple documents
• Document type to create depends on
specific application
• Drawing Application  Drawing Documents
• Writing Application  Writing Documents

29. Factory method
(Class – Creational DP)

30. Factory method
(Class – Creational DP)
• Another implementation:
• Parametrized Factory Method
• Parameter specifies the type of product to
instantiate
• No inheritance in creator class
• All Products must implement the product
interface

31. Abstract Factory DP
(object– Creational DP)
• What
• Client deals with a high-level abstraction for creating
families of products while leaving the factory
abstraction to deal with coupling the implementation.
• A requirement not to couple the client to an
implementation that creates sets or families of objects.
The pattern enables the client to commit to an interface
and avoid commitment to an implementation.
• Why
• Enhancing the client’s ability to switch between a set or
family of objects.
• How
• Client code has an association with the interface of the
factory class. The factory objects create sets of
objects .

32. Abstract Factory DP
(object– Creational DP)

33. Abstract Factory DP
(object– Creational DP)

34. Abstract Factory DP
(object– Creational DP)

35. Abstract Factory DP
(object– Creational DP)
• Another Example
• Web application that renders a set of controls
appropriate for a given web browser type or
version.

36. Abstract Factory DP
(object– Creational DP)
• Factories can be singletons
• Factory Method for each Product, each
ConcreteFactory class overrides it
choosing the type of products to create
•  a new concrete factory subclass for each
product family
• If many product families used Prototype
DP

37. Builder DP
(object– Creational DP)

38. Builder DP
(object– Creational DP)
• What
• Separate Construction of complex objects from the
representation of its parts
• Algorithm for creating a complex object should be
independent of the parts that make up the object
and how they're assembled.

39. Builder DP
(object– Creational DP)

41. Builder DP
(object– Creational DP)

42. Builder DP
(object– Creational DP)
• Example
• A Schedule Builder where there are two types
of Schedules(Inhouse or OnSite).
• A RTF Converter where there may be text or
Images (convert to PDF or ASCII text or……)

43. Report #1:
Prototype DP with
code example
N.B. Hand Written 