Session 2A: Design Patterns

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Sitecore for Experts Sitecore skills for
real men (and Max -))

• Session 2A Design Patterns
• Wednesday 14 March 2007

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Todays Program

• Introduction to the subject
• Decorator
• Factory Façades
• Observer
• Questions
• Homework!
• End

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Next Months Program

• Refresh of the subject
• Prototyping Commands
• Async Client
• Algoritmic / Concurrency Semphore
• Questions
• End

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Introduction
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Everything was better in the 70s

• Less computers
• Feminism wasnt fully propagated -)
• Elvis and Bob were still alive
• Buildings could be constructed without any
  environmental restrictions

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Christopher Alexander

• American architect
• Building on huge projects(even cities!)

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Building patterns

• Christopher looked at his work and discovered
• Building and infrastructure design have some many
  repeatable steps.
• Those steps have got the same results
• Simplified results to unified building blocks
• Proved their existence afterwards
• Patterns were born!

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In the 80s

• Two computer scientist worked on this idea

Ward Cunningham FoundereXtreme Programming
Wikis
Kent Back FoundereXtreme Programming Agile
Programming
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90s Gang of Four

• Ralph Johnson, Richard Helm, Erich Gamma, and
  John Vlissides

• Gamma presented the others his Thesis work on
  OOP-SLA 90
• 4 years of work contributed toDesign Pattern
  Elements of Reusable Object Oriented Software

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These days

• Design Patterns are part of regular IT- education
• All original patterns still exists(!!)
• Some are added and some slightly improved
• Used as base for nearly all applications
• Invented the idea of Application blocks(design
  pattern on Architecture level)
• Sitecore uses m...

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Pattern Categories

• Fundamental PatternsFeeding the general need of
  a developer such as Interface design
• Creational PatternsPatterns which gives you
  handhelds to construct your objects
• Structural PatternsThe design of the relations
  between objects
• Behavioural PatternsWays to cocommunicate
  between objects
• Concurrency PatternsBest pratices on
  implementing concurrency(not just threads!)

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Last but not least...

• Design patterns arent...
• Hard to understand
• Codesnippets you directly apply on your code
• Something youll learn in a day
• Answer to everything...

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Decorator
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Introduction the case

• Sometimes youve got a object which identifies
  itself by a property / action.
• Example
• Santa Claus a Person with
• a red nose...

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Introduction the case

• Santa has the same properties like every Person
• Class Person
• public string FirstName get set
• public string LastName get set
• public DateTime BirthDate get set

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Introduction the solution

• Santa is also Person
• Class SantaClaus Person
• public Color Nose get return Color.Red
• Yep Ive Decorated that Person, hes now Santa.

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Introduction implementing the pattern

• Class SantaClaus Person
• SantaClaus(Person)
• public Person Person get set
• public Color Nose get return Color.Red

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Introduction the UML again
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Practical usage

• GUI componentsBuilding up functionalityWindow,
  WindowWithBorder, WindowWithBorderAndScrollBar
• StreamsRuntime base functionalityStream,
  TextStream, IOStream, NetStream
• Security layersHiding field which shouldnt be
  accessed by other developer. Make your base
  private and just display the fields you want!

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Sitecore usage

• Simplify access to data
• CustomItem
• Security UserItem / RoleItem
• Content MediaItem
• Presentation DeviceItem / LayoutItem
• Structure TemplateItem / MasterItem
• CustomField
• HtmlField
• LinkField
• SecurityField
• XmlField

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Advantages of the pattern

• Functionality on runtime
• Easily to implement, no rocket sience
• Provides better interface for libraries
• Code readability improves

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Disadvantages and possible problems

• More classes, might be (a bit) slower
• Original class doesnt define the
  possiblitiesE.g. What kind of CustomField is X?
• Requires a well organized object structureE.g.
  Should be possible but where is that Decorator?
  Owwwww.... There it is......

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Additional reading

• General
• http//www.dofactory.com/Patterns/PatternDecorator
  .aspx
• http//en.wikipedia.org/wiki/Decorator_pattern
• Sitecore specific Alexey Rusakovs blog
• http//www.alexeyrusakov.com/sitecoreblog/2007/02/
  04/CustomItemSitecoreAPIPatternExplained.aspx
  
• http//www.alexeyrusakov.com/sitecoreblog/2007/02/
  08/CustomFieldSitecoreAPIPatternExplained.asp
  x

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Factory Façades
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Introduction Factory

• Factory Building objects.

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Introduction Factory

• Class Factory
• public ICar CreateCar(CarType)
• public CarChair CreateCarChair(Material)
• public Wheel CreateWheel()

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Introduction Façade

• Façade Doing something somewhere else you dont
  want to know about.

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Introduction Façade

• Class Façade
• public CRMConnection crm get set
• public BizTalkConnToSAP bizSap get set
• public Façade()
• public bool ExistsInSapAndCrm(Person p)
• return crm.Exists(p) bizSap.Exists(p)

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Practical usage Factory

• Building objects which are hard to construct
• Construct object which need some more actions
  direct after construction
• Examples
• Configreaders
• Document creation
• Parameters as a part of a connection

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Practical usage Façade

• You dont want to know anything about the other
  subsystems
• You want to make sure this is really the end of
  the other subsystem
• So communications starts and stops here

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Combining Factory Façade

• Software is built on blocks(subsystem)
• Factories allows you to create objects for
  communication with all different subsystems
  (interfaces)
• Those interfaces are façades
• Or sometimes objects are constructed using
  façades when they need data from different sources

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Sitecore usage

• Sitecore.ConfigurationClass Factory

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Advantages of the patterns

• Factory Construction on runtime
• Factory Construction without writing complex
  code
• Façade Hiding implementation and communication
  details of other subsystems

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Disadvantages and possible problems

• Factory No abilities to control the amount of
  instances
• Façades As youre hiding details, you might do
  not efficient reuse your code

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Last note

• Sitecore.Configuration.Factory might also contain
  another pattern Abstract Factory.
• Not described as it would take to much time.
• See also http//en.wikipedia.org/wiki/Abstract_fa
  ctory_pattern

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Additional reading

• Factory method
• http//en.wikipedia.org/wiki/Factory_method_patter
  n
• Façade
• http//en.wikipedia.org/wiki/FaC3A7ade_pattern

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Observer
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Introduction

• You want to be updated when something changes
• For example
• Someone places a bid on Ebay
• A user clicks on a button

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Introduction the steps

• First initialize a dispatcher
• Add observers / listeners
• Send notify to
• All listeners
• Listeners with a specific signatureE.g. Those
  who accept specific arguments

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Practical usage

• All systems which run more then one independant
  proces
• To keep different threads synchronized
• Connect between object who havent got a direct
  relation

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Sitecore usage

• Item handling (creating, editing, deleting)
• Publishing (before, during, after)
• Shell (Window events)

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Advantages of the pattern

• No needs of creating not existing relations
• When good implemented
• Incredible fast
• Thread-safe
• Native support in .NET languages

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Disadvantages and possible problems

• Sometimes hard to imagine
• Hard to debug

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Additional reading

• General
• http//en.wikipedia.org/wiki/Observer_pattern
• http//www.dofactory.com/Patterns/PatternObserver.
  aspx
• Sitecore specific
• http//sdn5.sitecore.net/Articles/API/Using20Even
  ts.aspx

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Questions?

• This is your last change

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Homework

• Find out what Sitecore.Data.Comparers.
  ComparerFactory does. Write it down in at most
  200 words so anyone who isn't technical
  understands it (my and your mum).
• Provide me a sample implementation of another
  comparer and explain in approx. 15 steps what the
  system does to retrieve end return the comparer.
• Find out what the Abstract Factory Pattern is.
  Promote 2 method in the whole Sitecore API which
  might have implemented this pattern. Explain in
  at most 250 per method why and how. You may make
  1 drawing which applies for both methods.
• Sent it to a.degroot_at_lectric.nl before Monday 2
  April.

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The end

• Thank you for your attention!
• And please, think about it and spread the word
• Mistakes, corrections and additions can be mailed
  to Alex de Groot a.degroot_at_lectric.nlThe
  presentation is part of LECTRIC / Alex de Groot
  but can be used without any premission.