Design Patterns

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Design Patterns

• CS341
• Western Washington University

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What is a Design Pattern?

• A solution to a problem in a context
• 4 essential elements
• Pattern name
• a handle used to describe a design problem, its
  solutions, and consequences
• Problem
• describes when to apply the pattern
• Solution
• describes the elements that make up the design,
  their relationships, responsibilities, and
  collaborations
• Consequences
• the results and trade-offs of applying the pattern

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The Gang of Four(GOF) Book

• Design Patterns, Elements of Reusable
  Object-Oriented Software
• Erich Gamma, Richard Helm, Ralph Johnson, and
  John Vlissides
• Design patterns can be subjectivein the GOF
  book, design patterns are descriptions of
  communicating objects and classes that are
  customized to solve a general design problem in a
  particular context

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MVC Classes

• Model/View/Controller
• Model is the application object
• View is its screen representation
• Controller defines the way the user interface
  reacts to user input
• Decouples views and models by establishing a
  subscribe/notify protocol
• This is how user interfaces are designed in
  Smalltalk-80

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Describing a Pattern

• Pattern name and classification
• Intent
• Also known as
• Motivation
• Applicability
• Structure
• Participants
• Collaborations
• Consequences
• Implementation
• Sample Code
• Known Uses
• Related Patterns

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The Catalog of Design Patterns

• Patterns are classified by 2 criterionpurpose
  and scope
• Purpose reflects what a pattern does
• Creational
• concern the process of object creation
• Structural
• deal with the composition of classes or objects
• Behavioral
• characterize the ways classes or objects
  interact and distribute responsibility

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• Scope specifies whether the pattern applies
  primarily to classes or objects
• Class patterns deal with relationships between
  classes and their subclasses
• established via inheritance, at compile-time
• Object patterns deal with object relationships
• are more dynamic
• most patterns are object patterns

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Solving Problems with Design Patterns

• Finding appropriate objects
• Determine object granularity
• Specifying object interfaces
• Specifying object implementations
• Putting reuse mechanisms to work
• Relating run-time and compile-time structures
• Designing for change

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Choosing a Design Pattern

• Consider how design patterns solve design
  patterns
• Scan Intent sections
• Study how patterns interrelate
• Study patterns of like purpose
• Examine a cause of redesign
• Consider what should be variable

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Using a Design Pattern

• 1. Read pattern once through for an overview
• 2. Go back and study the Structure,
  Participants, and Collaborations sections
• 3. Look at the Sample Code section to see a
  concrete example of the pattern in code
• 4. Choose names for pattern participants that
  are meaningful in the application context
• 5. Define the classes
• 6. Define application-specific names for
  operations in the pattern
• 7. Implement the operations to carry out the
  responsibilities and collaborations in the pattern