SOEN%206011%20Software%20Engineering%20Processes

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SOEN 6011Software Engineering Processes

• Section SS Fall 2007
• Dr Greg Butler
• http//www.cs.concordia.ca/gregb/home/soen6011-f2
  007.html

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Week 9

• Design patterns

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Gang Of Four

• Gamma, Helm, Johnson, Vlissides
• Some patterns in Larman, Chap. 23,
• All patterns in XDE
• As documentation.
• As dynamic templates.

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Overview of Patterns
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Design Repeat Successes

• Has a (successful) similar product been built?
• Yes, then reuse domain specific
• Architectural
• Style (e.g. client/server, database, process
  control)
• Patterns.
• Design Patterns ( idioms).
• Use Domain model as source of inspiration.

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Design New Application Area?

• Has a (successful) similar product been built?
• No, then choose among general
• Architectural
• Style (e.g. client/server, database, process
  control)
• Patterns.
• Design Patterns ( idioms).
• Use Domain model as source of inspiration.

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GoF Pattern Classification

• Behavioral Patterns
• Creational Patterns
• Structural Patterns

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GoF Pattern Summary Relationships
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GoF Behavioral Patterns

• Chain of Responsibility
• Command
• Interpreter
• Iterator
• Mediator
• Memento

• Observer
• State
• Strategy
• Template Method
• Visitor

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GoF Creational Patterns

• Abstract Factory
• Builder
• Factory Method
• Prototype
• Singleton

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GoF Structural Patterns

• Adapter
• Bridge
• Composite
• Decorator
• Facade
• Flyweight
• Proxy

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Command Pattern(for Front Controller)

• Problem How to allow the same command to be
  invoked by
• Menu selection
• Alt-ctrl shortcut
• Commandline text entry, etc
• How to allow (unlimited) undo/redo
• How to keep a log/audit/history of commands
  invoked
• How to allow macro commands to be defined

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Command Pattern (for Front Controller)

• You have commands that need to be
• executed,
• undone, or
• queued
• Command design pattern separates
• Receiver from Invoker from Commands
• All commands derive from Command and implement
  do(), undo(), and redo()
• Also allows recording history, replay

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Adapter

• Context / problemHow to resolve incompatible
  interfaces, or provide a stable interface to
  similar components with different interfaces?
• SolutionConvert the original interface of a
  component into another interface, through an
  intermediate adapter object.

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Adapter

• Suppose we have a tax calculation class (or
  external library) but the interface is not well
  suited for our application.

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Adapter

• Adapter providesan interface suitedto the
  application

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Adapter (For More than One Class)

• What if more than one class (library) needs to be
  adapted?

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Adapter
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Singleton Pattern

• Notice
• Constructor is no longer public.
• To access the instance use getUniqueInstance().
• All other attribute and method declarations of C
  stay the same.

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Facade
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Facade
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Factory

• Context / problemWho should be responsible for
  creating objects when there are special
  considerations, such as complex creation logic, a
  desire to separate the creation responsibilities
  for better cohesion, and so forth?
• SolutionCreate a Pure Fabrication object called
  a Factory.

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(Abstract) Factory Example (GoF)
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Factory (in EAs)
FrontControllerServlet
FrontCommand
processRequest ( )
init ( )
- getCommand ( ) FrontCommand
processRequest ( )
- getCommandClass ( )
ViewStudInfoCommand
RemoveStudentCommand
processRequest ( )
processRequest ( )
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Strategy

• Context / problemHow to design for varying, but
  related, algorithms or policies? How to design
  for the ability to change (even dynamically)
  these algorithms or policies?
• SolutionDefine each algorithm/policy/strategy
  in a separate class with a common interface

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Strategy
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How Do We Create a Strategy?
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Composite (Larman05, 26.8)

• Context/problem
• How do you treat a group or composite structure
  of objects the same way (polymorphically) as a
  non-composite (atomic) object?
• Solution
• Define classes for composite and atomic objects
  so that they implement the same interface.

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Composite Ex. Class Diagram
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Composite Ex. Objects
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Must add be implemented by Line?

• In C add declared virtual subclass need not
  implement it.
• In Java if add is abstract, then subclasses must
  implement it.
• String add(Graphic g)
• throw new UnsupportedOperationException()
• Can you think of a better solution?

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Composite Clients point-of-view
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Composite Pricing Strategies

• Interface
• Realization

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Observer Pattern
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Observer

• How shall we have the display be updated?
• Why not have the Sale inform the display when
  it changes value.

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What is Wrong With This?
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Observer Pattern

• Context / ProblemDifferent kinds of subscriber
  objects are interested in the state changes or
  events of a publisher object, and want to react
  in their own way when the publisher generates the
  event.

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Observer Pattern

• SolutionDefine a subscriber or listener
  interface. Subscribers implement this interface.
  The publisher can dynamically register
  subscribers who are interested in an event, and
  notify them when an event occurs.
• Clarification Publisher can dynamically process
  registration requests from subscribers.

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Observer Pattern (GoF book)
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Observers Illustration