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