Design Patterns

Design Patterns Overview

What is a design pattern?

• A general expression of a solution to common problems within a context.

• "... a named nugget of insight that conveys the essence of a proven solution to a recurring problem within a certain context amidst competing concerns" (from Brad Appleton's pattern overview)

• Each named pattern forms a vocabulary - i.e., a pattern language.

• Pattern languages can lead to:

  • Idea reuse

  • A media for communication and documentation

    • "This is a composite pattern"

    • "This is a singleton pattern"

  • Building blocks for expressing larger constructs - i.e., frameworks.

Becoming a Master Chess Player

• Learn rules and physical requirements
  • board configuration, pieces, legal moves, etc.

• Learn principles
  • Relative value of pieces, value of board positions, etc.

• To become a master you must study the games of other masters.
  • These games contain patterns that are reused and must be understood and reused.
  • Famous opening games
  • Famous end games

• There are 1000's of these patterns

Example inspired by Douglas C. Schmidt, schmidt@cs.wustl.edu

Becoming a Master Software Designer

• Learn rules and physical requirements
  • language syntax, algorithms, data structures

• Learn principles
  • Modularization, recursion, object-oriented programming

• To become a master you must study the programs of other masters.
  • These programs contain patterns that are reused and must be understood and reused.

• There are 1000's of these patterns

Example inspired by Douglas C. Schmidt, schmidt@cs.wustl.edu

Meta-Patterns

• Encapsulation (chunking?)

  • Type based (e.g., class)

  • Object based (i.e., object method invocation)

• Delegation

  • Intra-object (i.e., self method call, virtual methods)

  • Inter-object (i.e., object method invocation)

Meta-Patterns Examples

Abstract Factory

Intent

Applicability

• A system should be independent of how its products are created, composed and represented

• A system should be configured with one of multiple families of products

• A family of related product objects is designed to be used together, and you need to enforce this constraint.

• You want to provide a class library of products and you want to reveal just their interfaces.

Consequences

• It isolates concrete classes... encapsulation.

• It makes exchanging product families easy.

• Promotes consitency among products.

• Adding new product families is difficult, but ... see 2 above.

Implementation issues

• Factories as singletons. When not??

• Uniformity of access

• Creating the products

  • Prototype pattern

  • Concrete/abstract product mismatch. Proxy?? Adapter??

• How do you figure out what factory to use, e.g., Factory factory

  • Compile time

  • Determined (fixed) at runtime based on environment, e.g., ET++

  • Dynamic loading

  • Data driven dynamic dynamic loading

Related pattern

ET++ example

	if (gWindowSystem == 0 && Env::Defined("GLX"))
		gWindowSystem= Load("glxserver", "GLXWinSystem");
	
	if (gWindowSystem == 0 && Env::Defined("DISPLAY"))
		gWindowSystem= Load("xwindow", "XWinSystem");

	if (gWindowSystem == 0 && Env::Defined("WINDOW_PARENT"))
		gWindowSystem= Load("sun", "SunWindowSystem");
	
	if (gWindowSystem == 0) {
		fprintf(stderr, "can't initialize window system\n");
		gSystem->Exit(-1);
	}

Pattern Format

• Pattern name and classification
  

• Intent
  A short synopsis of what this pattern does.

• AKA
  What else is this pattern known by?

• Motivation
  A concrete example of the problem this pattern addresses

• Applicability
  When should you use this pattern?

• Structure
  Class structure

• Participants
  A description of the classes and their responsibilities.

• Collaborations
  What actualls happens at run time? e.g., object collaboration diagram

• Consequences
  What is the concrete benefits and constraints using this pattern provides.

• Implementation
  When implementing a pattern waht are some of the actual issues that have to be addressed.

• Sample Code
  

• Known uses
  

• Related patterns
  

Pizza Inversion