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Patterns Design. 13 February. User Interfaces. Introducing the Book. Patterns. 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 - PowerPoint PPT Presentation
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PatternsDesign
13 February
User Interfaces Introducing the Book
Patterns
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
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
Architectural Example: Door Placement
If room has two doors and people move through it, keep both doors at one end of the room
Alexander’s PatternsEntries 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?
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?
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
Patterns at All Levels
“Software Architecture” What is an architecture? External view What does that mean for software? The highest level design We’ll refer to it as “system design”
– though not consistent in the industry
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
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
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
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
Why Categorize? Recognize patterns Reuse designs Learn from other similar
applications Reuse classes
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
Pipe and Filter
filterfilter
filter
filter filter
filter
pipe
pipe
Filters: processesPipes: input streams
Example of Batch Sequential
Collectmortgage funds
Accountbalances
Mortgagepool
Unsecuredpool
Collectunsecured funds
Pipe: batch input
Processes
Pipe and filter where input streams are batches of data
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)
Client-Server and Facade
«not exposed»
P
«not exposed»
Façade«exposed»
Client1
2
«not exposed» «not exposed»
«not exposed»
«not exposed»
Adapted from Software Engineering: An Object-Oriented Perspective by Eric J. Braude (Wiley 2001), with permission.
Key concept: limit exposed interface
Browser-web server most familiar example:Separate systems with narrow interface
Parallel Communicating Processes
Adapted from Software Engineering: An Object-Oriented Perspective by Eric J. Braude (Wiley 2001), with permission.
Customer:customer n
withdraw
Customer:customer n+1
Session:session k
Session:session m
depositcreate
Account:customer n+1 saving
Account:customer nchecking
create retrieve
retrieve
3 types of processes, 2 instances of eachDuration of process
processes
actions
sequence diagram
Observer Design Pattern
Gamma et al
Sourcenotify()
Observerupdate()
ConcreteSubjectstate
ConcreteObserverobserverState
update()
Client of thissystem
1
2
3
1..nRequest others be notified
Notify all observers
Determines if change needed
Single source of data with a Single source of data with a number of clients that need to number of clients that need to be updatedbe updated
Event Systems and State Transition Diagrams
Set of components waiting for input
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
Repository A system built around data Two types
Databases Hypertext systems
A Typical Repository System
Database
DBMS
GUI
Analysisprocess
1
Analysisprocess
n…...…...
Control
Adapted from Software Engineering: An Object-Oriented Perspective by Eric J. Braude (Wiley 2001), with permission.
Iterator patternvoid 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
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)
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
Layered Architecture
Role-playing game layer
Characters LayoutRolePlayingGame
EncounterCharacters
EncounterEnvironment
Encounter Game
Application layer
3D engine layer
«uses»
«uses»
Adapted from Software Engineering: An Object-Oriented Perspective by Eric J. Braude (Wiley 2001), with permission.
Coherent collection of software artifacts, typically a package of classes
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