Design Patterns

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Design Patterns

• Ric Holt
• U Waterloo, March 2009

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What is a Design Pattern

• A Design Pattern systematically names, explains,
  and implements an important recurring design.
• These define well-engineered design solutions
  that practitioners can apply when crafting their
  applications.

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Why Design Patterns?

• Good designers do not solve every problem from
  first principles. They reuse solutions.
• Practitioners do not do a good job of recording
  experience in software design for others to use.
  Patterns help solve this problem.

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Two Example Patterns

• MVC (Model View Controller)
• Adapter

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1. MVC Pattern(Model View Controller)

• There is a model (subject) which can be thought
  of as the application
• There are one or more views of the object (by
  observers)
• There is a controller that mediates between the
  model and the view(s).

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Example MVC
a 50 b 30 c 20
Views
a b c
Controller
Model
Mancoridis picture
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Example MVC
a 50 b 30 c 20
Views
a b c
Controller
See
Interact
Model
Mancoridis picture
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Multiple View Problem

• Need to keep all the views consistent
• If user (or one of users) changes a view, all
  other views should be updated

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Information HidingInsulating Model from
Presentation

• The model is the back end or business logic,
  or application
• To minimize complexity, the model should know as
  little as possible about
• How users interact with system
• How many users are there
• Any other models

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Implementing MVC

• There are many choices
• Where is list of views (observers) kept?
• How is notification of change transmitted?
• How manage web interface?
• Should a view ask for (or should it be told of)
  details about changes?

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What Kind of Pattern is MVC?

• Design pattern proper?
• Use it for structuring modest configurations
• Architectural pattern?
• Use it for overall structure of a software
  system, defining interactions among services,
  browsers etc.

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2. Adapter Pattern

• You have an existing client (application) that
  uses an old interface to an existing support
  package.
• You are given a new interface to a new support
  package
• You need to produce an adapter so that
• The client can use the new interface instead of
  the old one (without changing the client)

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Illustration of Adapter Pattern
Old interface
Old package
Client
Old interface
New interface
New package
Client
Adapter
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Implementing Adapter Pattern Using Object
Composition
Old Interface
New Interface
Client
Request()
SpecificRequest()
inherit
adaptee
Adapter
Request()