Information Systems Concepts Design Patterns

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Information Systems Concepts Design Patterns

• Dell Zhang
• Birkbeck, University of London
• Spring 2009

Based on Chapter 15 of Bennett, McRobb and
Farmer Object Oriented Systems Analysis and
Design Using UML, (3rd Edition), McGraw Hill,
2005.
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Outline

• What Are Patterns
• Part of Section 8.2 (pp. 223 229)
• Framework
• Section 15.2.1
• Pattern Templates
• Section 15.3.1 (p. 418)
• Design Patterns
• Section 15.4 (pp. 419 435)
• Benefits and Dangers of Using Patterns
• Section 15.6 (pp. 436 437)

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What Are Patterns
A pattern describes a problem which occurs over
and over again in our environment, and then
describes the core of a solution to that problem,
in such a way that you can use this solution a
million times over, without ever doing it the
same way twice. Alexander et al. (1977)
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The Tower Bridge Bascule Suspension
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What Are Patterns

• A Pattern
• is a solution to a recurring problem
• is proven to be good (effective, )
• is not obvious / trivial
• is elegant

Patterns capture and document proven good
practices, while anti-patterns capture and
document demonstrably bad practices.
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What Are Patterns

• Patterns in software development
• Organization patterns
• the structures, roles and interactions in the
  software development organisation itself
• Analysis patterns
• the group of concepts useful in modelling
  requirements
• Architectural patterns
• the structure of major components of a software
  system
• Design patterns
• the structure and interaction of smaller software
  components

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Pattern vs. Framework

• A pattern is used to resolve a problem by
  applying a suitably contextualized solution.
• A framework is a partially developed system that
  is essentially the core to a set of similar
  applications and is instantiated by adding the
  necessary program code.
• Frameworks are similar to software libraries in
  that they are reuseable abstractions of code
  wrapped in a well-defined API.
• Unlike libraries, however, the overall programs
  flow of control is not dictated by the caller,
  but by the framework.

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Pattern vs. Framework

• Web application frameworks
• Java Struts, Cocoon, WebWork, etc.
• PHP symfony, CodeIgniter, Zend, etc.
• Ruby Ruby on Rails, etc.
• Python Django, etc.

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Pattern vs. Framework

• Django
• Django is an open source web application
  framework, written in Python, which loosely
  follows the model-view-controller design pattern.
• Django's primary goal is to ease the creation of
  complex, database-driven websites.
• Django emphasizes reusability and 'pluggability'
  of components, rapid development, and the
  principle of DRY (Don't Repeat Yourself).
• Django also provides an optional administrative
  'CRUD' interface that is generated dynamically
  through introspection and configured via admin
  models.

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Pattern vs. Framework

• Django
• The core Django framework consists of
• an object-relational mapper which mediates
  between data models (defined as Python classes)
  and a relational database
• a regular-expression-based URL dispatcher
• a view system for processing requests
• a templating system

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Pattern vs. Framework

• Their major differences
• Patterns are more abstract and general than
  frameworks.
• A pattern is a description of the way that a type
  of problem can be solved, but the pattern is not
  itself a implemented solution.
• Unlike a framework, a pattern cannot be directly
  implemented in a particular software environment.
• A successful implementation is only an example of
  a design pattern.
• Patterns are more primitive than frameworks.
• A framework can employ several patterns but a
  pattern cannot incorporate a framework.

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Pattern Template

• 5 main elements of a design pattern
• Name a meaningful name that reflects the
  knowledge embodied by the pattern
• Problem the problem that the pattern addresses,
  i.e., the intent of the pattern
• Context the circumstances or preconditions under
  which it can occur
• Forces the constraints or issues that must be
  addressed by the solution
• Solution the description of the static and
  dynamic relationships among the components of the
  pattern

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GOF Design Patterns

• A catalogue of 23 design patterns presented by
  Gamma et al. (1995) known as the Gang of Four
  (GOF)

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GOF Design Patterns

• Typically address issues concerning changeability
  that involves several different aspects
• Maintainability
• Extensibility
• Restructuring
• Portability

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GOF Design Patterns

• Creational Patterns
• Abstract Factory, Builder, Factory Method,
  Prototype, Singleton
• Structural Patterns
• Adapter, Bridge, Composite, Decorator, Façade,
  Flyweight, Proxy
• Behavioural Patterns
• Chain of Responsibility, Command, Interpreter,
  Iterator, Mediator, Memento, Observer, State,
  Strategy, Template Method, Visitor

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Creational Patterns

• Concerned with the construction of object
  instances
• Separate the operation of an application from how
  its objects are created
• Gives the designer considerable flexibility in
  configuring all aspects of object creation

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Creational Patterns Singleton

• How does one ensure that only one instance of the
  company class is created?

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Creational Patterns Singleton

• Solution restrict access to the constructor!

The use of class-scope operations allows global
access
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Sequence Diagram for the Singleton Pattern
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Different subclasses of Company can be
instantiated as needed.
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Creational Patterns Singleton
General form of the Singleton pattern
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Structural Patterns

• Concerned with the way in which classes and
  objects are organized
• Offer effective ways of using object-oriented
  constructs such as inheritance, aggregation and
  composition to satisfy particular requirements

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Structural Patterns Composite

• How can we present the same interface for a media
  clip whether it is composite or not?

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Structural Patterns Composite

• How can we incorporate composite structures?

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Structural Patterns Composite
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Structural Patterns Composite
General form of the Composite pattern
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Behavioural Patterns

• Address the problems that arise when assigning
  responsibilities to classes and when designing
  algorithms
• Suggest particular static relationships between
  objects and classes and also describe how the
  objects communicate

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Behavioural Patterns State

• Consider the class Campaign
• It has four states
• Commissioned, Active, Completed and Paid
• A Campaign object has different behaviour
  depending upon which state it occupies.
• Operations have case statements giving this
  alternative behaviour
• The class factored into separate components one
  for each of its states

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The State pattern could be applied to the
Campaign class
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Behavioural Patterns State
The Campaign class with the State pattern applied
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Behavioural Patterns State
Some State pattern objects for Agate there are
6 Campaign objects sharing the four State objects.
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Behavioural Patterns State
Sequence Diagram for the State Pattern
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Behavioural Patterns State
General form of the State pattern
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Benefits and Dangers

• Benefits of Using Design Patterns
• The introduction of a reuse culture at the design
  level
• The rich source of development experience they
  offer
• Dangers of Using Design Patterns
• The inappropriate application of a pattern
• Some limitation on creativity

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Take Home Messages

• What Are Patterns
• Pattern vs Framework
• Pattern Templates
• GOF Design Patterns
• Creational Patterns (e.g., Singleton)
• Structural Patterns (e.g., Composite)
• Behavioral Patterns (e.g., State)
• Benefits and Dangers of Using Patterns