Design Patterns and the Real World

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Design Patterns and the Real World

• PHP-Conference 2003
• Mai 9th 2003, Amsterdam
• Gerd Schaufelberger

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Agenda

• About the speaker
• Software Engineering
• Pattern Introduction
• Exhausting Example
• Using Patterns How-to
• Object-Oriented Design and Patterns
• Pattern Catalogues and examples
• Further Reading

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Gerd Schaufelberger

• Web Application Developer at Metrix Internet
  Design GmbH in Karlsruhe/Germany
• Publishing OS on http//www.php-tools.net
• Contributor to the German PHP Magazine

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Software Engineering

• Goals
• Increase productivity
• Increase quality
• Decrease costs
• Methods
• Methodical design
• Reusability
• Better maintainability

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Engineering Problems

• Developers and maintainers need to know and
  understand the system.
• Classes are very small parts of a system
• Reusing classes often does not increase
  productivity
• (Class-)Libraries only support the reusability of
  code
• Methods that ease understanding software and
  increases reusability are needed

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What are Patterns?

• Many people say "A pattern is a proven solution
  to a problem in a context.
• Pattern must
• solve a problem
• be related to a context
• be general to fit in many situations
• impart knowledge (information how the pattern
  solves the problem)
• have a unique name

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Pattern Definition

• As an element in the world, each pattern is a
  relationship between a certain context, a certain
  system of forces which occurs repeatedly in that
  context, and a certain spatial configuration
  which allows these forces to resolve themselves.
• As an element of language, a pattern is an
  instruction which shows how this spatial
  configuration can be used, over and over again,
  to resolve the given system of forces, whenever
  the context makes it relevant.
• Alexander, 1979, A Timeless Way of Building

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Pattern Anatomy

• Essential contents of patterns
• Pattern name
• Description of the problem
• Description of the solution
• Consequences
• The quality of a pattern depends on the
• quality of the document.

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Pattern Prototype (1)

• Name Prototype (Creational Pattern)
• Intend Specify the kinds of objects to create
  using a prototypical instance, and create new
  objects by copying this prototype.
• Diagram

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Pattern Prototype (2)

• Solution
• Create an ancestor product class which defines an
  abstract or default clone method.
• Create specific product classes by inheriting
  from the ancestor product class and implementing
  the clone method to create a copy of itself.
• During system initialization, or other
  appropriate time, create prototype instances of
  each specific product.
• Whenever a real instance of a particular product
  is needed, call the appropriate products clone
  method and use the object returned.

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Pattern Prototype (3)

• Applicability
• Classes to instantiate are specified at run-time,
  or,
• To avoid building a hierarchy of factories, or
• It is more appropriate to clone objects with
  state information than to instantiate and
  initialize the class(es) manually.

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Pattern Prototype (4)

• Consequences
• Products may be dynamically added and removed at
  run-time.
• May reduce the number of object classes needed by
  varying object values.
• Simplifies construction of complex object
  aggregations
• Reduces or eliminates the need to implement a
  hierarchy of factories that parallels a hierarchy
  of product types

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Extract Information

• Extract essential information from the Pattern
  description
• Name The Name (Prototype) identifies the
  Pattern. Add these names to your vocabulary!
• Intent Tells what the Pattern is good for. Keep
  in mind!
• Diagram It show how to implement the Pattern.
  Usually drawn as UML-Diagrams. It gives you an
  idea how the Pattern work. Forget it you know
  where you can find it.
• Read this and you know whether it
• is worth reading more.

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More Information

• You find the Prototype Pattern may help you? Read
  more
• Solution Some more detailed view what the
  Pattern is good for.
• Applicability Tells you in which environment the
  Pattern makes sense.
• Consequences This very important part talks
  about disadvantages and pitfalls.
• often there is additional information available.

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Make Your Decision

• Ask yourself Is this pattern useful in my
  case?
• The description of the pattern can help you to
  make a decision You know
• what this pattern intents to solve
• how it can be implemented
• the positive and the negative aspects
• You know this in advance,
• before implementation and testing.

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No Magic!

• The Prototype Pattern contains nothing magical
• Easy to understand
• Easy to implement
• For sure you already used Patterns without
  knowing it
• Design Patterns are nothing mystic
• disappointed?

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Object Oriented Design

• Object-Oriented design and programming is widely
  spread and well known by programmers.
• Thinking in objects and classes are good methods
  in software development.
• PHP supports basic OO-features, but

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Objects and PHP (PHP4)

• Current version of PHP (PHP4)
• Only some OO-features (classes, inheritance,
  etc.)
• No destructors
• Only some information hiding
• No private attributes and methods
• No protection from inheritance
• PHP4 is not type-safe
• Object-oriented programming with PHP4 needs
  discipline

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Objects and PHP (PHP5)

• PHP5 grows-up to an adult OO-programming language
• Better encapsulation (private/protected
  attributes and member methods)
• Type-Hinting allows better type-safe programming
• New concept for constructors and destructors
• and more
• Object-oriented programming with PHP5 needs
• less discipline

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Patterns and Objects

• The implementation of Patterns were usually
  described as class-diagrams.
• Some Patterns (like Prototype) are about
  creating objects which only makes sense in an
  object-oriented environment.
• OO-programming is not mandatory for Patterns.
  (Patterns describe a solution for specific
  problems.)

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and with PHP

• Even if PHP (PHP4) is no real OO-programming
  language, Patterns are useful.
• Whether a Pattern can be implemented in PHP
  depends on the Pattern. Some Patterns dont make
  even sense for PHP-Programmes.
• Design Patterns want solve Problems and not to
  force you OO-programming.

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Pattern Catalogues (1)

• Patterns (like Prototype) were collected in
  Catalogues.
• In the Internet e.g. http//www.patterndigest.com
  
• In books like Design Patterns
• Catalogues contain complete Pattern-Description
  Name, Intent, Diagram, Solution, Consequences,
• Catalogues usually contain references to related
  Patterns, related problems, related environments,
  

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Pattern Catalogues (2)

• Catalogues (usually) sort Design Patterns by
  categories
• Structural Patterns
• Creational Patterns
• Behavioural Patterns
• Relationship Patterns
• Downcasting Patterns
• Property Patterns

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Pattern Catalogues (3)

• Usage of catalogues
• Browse through the catalogues for Patterns
  matching to your problem.
• Study class-diagrams before implementing
• Some catalogues also contain example code
  (usually in Java or C ?)

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Pattern Catalogues (4)

• These catalogues contain proven solutions for
  various problems. Finding a good solution within
  a catalogue is more easy than thinking about
  yourself.
• Stop thinking, start reading.

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Some Patterns

• Some example Patterns
• Iterator
• Prototype
• Command
• shameless stolen from a pattern catalogue.

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The Iterator (1)

• Name Iterator (Behavioural Pattern)
• Intent Provide a way to access the elements of a
  aggregate object without exposing its underlying
  representation.
• Diagram

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The Iterator (2)

• The Client uses Iterator-Objects to walk through
  any aggregate.
• Replaces loops ( for, while, etc.)
• Easy to implement
• Abstraction for accessing data
• Simple interface for all kind of aggregates
• Use more than one iterator at the same time

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The Iterator (3)

• agg new aggregate() //
  instantiate the aggregate
• iter1 agg-gtgetIterator() //
  get a matching iterator
• ele iter1-gtfirst()
  // get the first element
• while( ele )
• ele-gtdoSomething() //
  perform operation
• ele iter1-gtnext()
  // get the next element
• iter2 agg-gtgetIterator() //
  get independent iterator
• ele iter2-gtlast()
  // get the last element

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Prototype

• Name Prototype (Creational Pattern)
• This pattern was already shown.
• The class itself is responsible for duplicating
  the client just calls the clone() method.

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Command (1)

• Name Command (Behavioural Pattern)
• Intent Encapsulate a request as a parameterized
  object allow different requests, queue or log
  requests and support undoable operations
• Diagram

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Command (2)

• Some benefits
• Decouples objects from operations
• Allows logging, do, undo
• Easy to implement Macro-Commands

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Summarize

• Patterns names may ease communication
• Patterns ease understanding software
• Patterns are proven solutions
• Patterns are well documented
• Benefits and disadvantages are known in advance
• Dogs love patterns
• Design Patterns can ease your life
• use them.

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Further Reading

• WWW
• Pattern Homepage http//www.hillside.net
• Pattern Digest http//www.patterndigest.com
• Patterns and PHP http//www.phppatterns.com
• Books
• Graig Larman Applying UML and Patterns, 2001
• Van Duyne, Hong, Landay Design of Sites, 2002
• Gamma, Helm, Vlissides, Johnson Design Patterns,
  1994

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The End

• Thank you!
• Get in contact
• http//www.php-tools.net
• gerd_at_php-tools.net
• Thanks to
• Sebastian Mordziol, Stephan Schmidt,
• Metrix Internet Design GmbH