Design Patterns

1
Design Patterns

• An Introduction
• By Ian M. Mac Donald

2
History

• Difficulties in designing reusable object
  oriented software
• Must be specific to problem, yet general enough
  to handle future problems
• Reoccurring patterns appear in classes and
  communicating objects in many O.O. systems.
• You may get design déjá-vu

3
What Was Needed

• We need to locate, explain and evaluate
  important reoccurring designs in object oriented
  system.
• We need a generation of successful designs and
  architectures that can be reused.
• We need a way to improve documentation and
  maintenance of existing systems.

4
Definition of a Design Pattern

• In general, a design pattern describes a
  recurring problem and the core of its solution
  with the goal of creating reusable software.
• A design pattern will identify the participating
  classes and instances, their roles and
  collaborations, and the distribution of
  responsibilities.

5
The Four Elements of a D.P.

• Pattern Name handle for a design problem,
  solutions and consequences. It is a good idea to
  keep this name down to one or two words.

6
The Four Elements of a D.P.

• The Problem When to apply the pattern.
  Explains problem and context, prerequisite
  conditions.

7
The Four Elements of a D.P.

• The Solution Elements that make up design,
  relationships, responsibilities, and
  collaborations. Does not give concrete, rather
  abstract description of problem and how a general
  arrangement of classes/objects can solve it.

8
The Four Elements of a D.P.

• The Consequences Results/trade-offs of applying
  the pattern.
• Ex Space and time, language implementation,
  impact on existing system.

9
Attributes of a Design Pattern

• Pattern Name and Classification
• Intent
• Also Known As
• Motivation
• Applicability
• Structure

• Participants
• Collaborations
• Consequences
• Implementation
• Sample Code
• Known Users
• Related Patterns

10
Defined Design Patterns

• Interpreter
• Iterator
• Mediator
• Memento
• Observer
• Prototype
• Proxy
• Singleton
• State
• Strategy
• Template Method
• Visitor

• Abstract Factory
• Adapter
• Bridge
• Chain of Responsibility
• Command
• Composite
• Decorator
• Façade
• Factory Method
• Flyweight

11
Design Pattern Classifications

• Creational Patterns
• Concerned with the process of object creation
• Structural Patterns
• Concerned with the composition of classes or
  objects
• Behavioral Patterns
• Concerned with the characterizing the ways in
  which classes or objects interact and distribute
  responsibility.

12
Organization of Design Patterns

• We organize our catalog of defined design
  patterns based on two criteria
• Purpose That is, whether the pattern has a
  creational, structural, or behavioral purpose.
• Scope That is, whether the pattern applies
  primarily to objects or classes.

13
Design Pattern Classification
14
Deciding Which D.P. to Use

• Consider how design patterns solve design
  problems
• Scan the intent attributes of each design pattern
• Study how patterns interrelate
• Study patterns of like purpose
• Examine a cause of redesign
• Consider what should be variable in your design

15
How to Use a Design Pattern

1. Read a pattern once through for an overview
2. Study the structure
3. Look at the sample code
4. Choose names for pattern participants that are
   meaningful in the application context
5. Define the classes

16
How to Use a Design Pattern

1. Define application-specific names for operations
   in the pattern
2. Implement the operations to carry out the
   responsibilities and collaborations in the pattern

17
Design Pattern Example

• Lets look at the simplest design pattern in the
  catalog, the singleton.

18
Singleton

• Intent
• Ensure a class has only one instance, and
  provide a global point of access to it.
• Motivation
• It is important that some classes have only one
  instance.
• Ex There can be many printers in a system, but
  there should only be one print spooler.
• Make the class itself responsible for keeping
  track of its sole instance
• Applicability
• Use when there must be exactly one instance of a
  class, yet must be accessible to clients from a
  well known access point.

19
Singleton

• Structure

20
Singleton

• Benefits
• Controlled access to sole instance.
• Reduced name space better than using global
  variables.
• Permits a variable number of instances if you
  change your mind and want to allow more than one
  instance of the Singleton class.
• Implementation
• Ensuring a unique instance.
• The design pattern will give you a lot of some
  sample code to illustrate how to implement.

21
Singleton

• Class Definition
• class Singleton (
• public
• static Singleton Instance()
• protected
• Singleton()
• private
• static Singleton _instance
• )

• Corresponding Implementation
• Singleton Singleton_instance 0
• Singleton SingletonInstance()
• if (_instance 0)
• _instance new Singleton
• return _instance

22
Singleton

• Sample Code
• In addition to the sample code given in the
  implementation section, sample code for a
  practical implementation is given here.
• class MazeFactory (
• public
• static MazeFactory Instance()
• // existing interface goes here
• protected
• MazeFactory()
• private
• static MazeFactory _instance
• )

• MazeFactory MazeFactory_instance 0
• MazeFactory MazeFactoryInstance()
• if (_instance 0)
• _instance new MazeFactory
• return _instance
• The sample code section will also demonstrate
  example subclasses.

23
Singleton

• Known Uses
• Smalltalk-80Par90
• InterViews user interface toolkitLCI02 used
  this design pattern to access the unique instance
  of its Session and WidgetKit classes.
• Related Patterns
• Many patterns can be implemented by using the
  Singleton pattern. Example Abstract Factory,
  Builder, and Prototype.