Patterns Design

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PatternsDesign

• 13 February

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User Interfaces

• Introducing the Book

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Patterns
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What is a Pattern?

• A solution to a problem in a context
• A structured way of representing design
  information in prose and diagrams
• A way of communicating design information from an
  expert to a novice
• Requirement shows when and how to apply

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Origin of Patterns

• Came from architecture
• Christopher Alexander, late 70s
• The Timeless Way of Building
• Describes
• Common architectural motifs
• How they come together to form a cohesive,
  livable environment
• Patterns from town planning to decorative detail

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Architectural Example Door Placement

• If room has two doors and people move through it,
  keep both doors at one end of the room

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Alexanders Patterns

• Entries have five parts
• Name A short familiar, descriptive name or
  phrase, usually more indicative of the solution
  than of the problem or context.
• Example One or more pictures, diagrams, and/or
  descriptions that illustrate prototypical
  application.
• Context Delineation of situations under which
  the pattern applies.
• Problem A description of the relevant forces and
  constraints, and how they interact.
• Solution Static relationships and dynamic rules
  describing how to construct artifacts in accord
  with the pattern, often listing several variants.
• What do you need to change for software?

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Properties of Patterns

• Independent, specific, and formulated precisely
  enough to make clear when they apply
  (encapsulation)
• Describes how to build a realization
  (generativity)
• Identifies a solution space containing an
  invariant that minimizes conflict among
  constraints (equilibrium)
• Represent abstractions of empirical experience
  and everyday knowledge (abstraction)
• May be extended down to arbitrarily fine levels
  of detail. Like fractals, patterns have no top or
  bottom (openness)
• Hierarchically related. Coarse grained patterns
  are layered on top of, relate, and constrain fine
  grained ones (composibility)
• What do you need to change for software?

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Design Patterns

• All the same benefits are true in software
• Cunningham and Beck recognized in late 80s
• Community formed in early 90s
• The Book
• Gamma, Helm, Johnson and Vlissides, Design
  Patterns Elements of Reusable Object-Oriented
  Software
• Define 23 patterns
• Three categories
• Structural ways to represent ensembles of
  information
• Creational creating complex objects
• Behavioral capturing the behavior of object

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Patterns at All Levels
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Software Architecture

• What is an architecture?
• External view
• What does that mean for software?
• The highest level design
• Well refer to it as system design though not
  consistent in the industry

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System Design Goals

• Extensibility adding new features
• Tradeoff of generality and time
• How might it be extended?
• Changeability requirements changes
• Simplicity ease of understanding and
  implementing
• Efficiency speed and size

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Key System Design Characteristics

• Cohesion
• degree to which communication takes place within
  the module
• Coupling
• degree to which communication takes place between
  modules
• Min-max problem minimize coupling while
  maximizing cohesion

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Examples

• Role-playing game
• Decompose into 4 modules environment, game
  control, participants and artifacts
• High cohesion and coupling
• When two characters meet, all 4 modules are
  involved
• Personal finance application
• Decompose into user activities accounts, bill
  playing, loans, investments
• Low cohesion and high coupling
• Accounts are pretty independent
• Loan payment would involve the first 3 modules

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Categorizing System Designs(Shaw and Garlan)

• Model-View Controller
• Data flows
• Viewed as data flowing among processes
• Independent components
• Components operating in parallel and
  communicating occasionally
• Virtual machines
• Treats an application as a program written in a
  special-purpose language
• Repository
• Application built around data
• Layered architectures
• Packages of function with a strong hierarchical
  uses relationship

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Why Categorize?

• Recognize patterns
• Reuse designs
• Learn from other similar applications
• Reuse classes

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Data Flow Design

• Data flowing among processes
• Two categories
• Pipes and filters
• Filters processes
• Pipes input streams
• Batch sequential
• Pipe and filter where input streams are batches
  of data

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Pipe and Filter
filter
pipe
filter
filter
filter
filter
pipe
filter
Filters processes Pipes input streams
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Example of Batch Sequential
Collect mortgage funds
Mortgage pool
Account balances
Collect unsecured funds
Unsecured pool
Pipe batch input
Processes
Pipe and filter where input streams are batches
of data
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Independent Components

• Components operating in parallel and
  communicating occasionally
• Three types
• Client-server
• Browser-web server most familiar example
• Separate systems with narrow interface
• Sometimes expanded to three tiers (why?)
• Façade pattern (single unified interface)
• Parallel communicating processes
• Several processes executing at the same time
• Typically modeled with sequence diagrams
• Observer pattern (one-to-many dependencies)
• Event systems
• Set of components waiting for input
• Example word processor waiting for user input
• State transition diagrams
• State pattern (alter behavior depending on state)

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Client-Server and Facade
Browser-web server most familiar
example Separate systems with narrow interface
Key concept limit exposed interface
Adapted from Software Engineering An
Object-Oriented Perspective by Eric J. Braude
(Wiley 2001), with permission.
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Parallel Communicating Processes
sequence diagram
Adapted from Software Engineering An
Object-Oriented Perspective by Eric J. Braude
(Wiley 2001), with permission.
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Observer Design Pattern
Single source of data with a number of clients
that need to be updated
Client of this system
1
Request others be notified
1..n
Source notify()
Observer update()
2
Notify all observers
ConcreteObserver observerState update()
ConcreteSubject state
Determines if change needed
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Gamma et al
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Event Systems and State Transition Diagrams
Set of components waiting for input
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Virtual machines

• Treats an application as a program written in a
  special language
• Payoff is that the interpreter code is the basis
  for multiple applications
• Two types
• Interpreters
• Rule-based systems

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Repository

• A system built around data
• Two types
• Databases
• Hypertext systems

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A Typical Repository System
Adapted from Software Engineering An
Object-Oriented Perspective by Eric J. Braude
(Wiley 2001), with permission.
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Iterator pattern
void setToFirst() points to first element void
increment() causes the iterator to
point to its next element C getcurrentElement()
return the element pointed to by the
iterator boolean isDone() true if all elements
processed
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Hypertext Basis of the Web

• Motivated by Vannevar Bush in 1945
• As We May Think (Atlantic Monthly)
• Theoretical machine, "memex," to enhance human
  memory by allowing the user to store and retrieve
  documents linked by associations
• Invented by Ted Nelson in the 1960s
• Popularized with HTML (Tim Berners-Lee)

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Ted Nelson

• "If computers are the wave of the future,
  displays are the surfboards."
• Xanadu 1974
• "give you a screen in your home from which you
  can see into the world's hypertext libraries...
  offer high-performance computer graphics and text
  services at a price anyone can afford... allow
  you to send and receive written messages... and
  make you a part of a new electronic literature
  and art, where you can get all your questions
  answered...
• Computer Lib/Dream Machines

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Layered Architecture
Coherent collection of software artifacts,
typically a package of classes
Adapted from Software Engineering An
Object-Oriented Perspective by Eric J. Braude
(Wiley 2001), with permission.
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Recap

• Model-View-Controller
• Data flow systems
• Pipes and filters
• Batch sequential
• Independent components
• Client-server
• Parallel communicating processes
• Event systems
• Virtual machines
• Interpreters
• Rule-based systems
• Repositories
• Databases
• Hypertext systems
• Layered architectures