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 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 Patterns What do you need to change for software? 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?

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 The Book: 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 Personal finance application 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 filter pipe filter filter filter filter pipe filter Filters: processes Pipes: input streams

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

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 Façade «exposed» Client 1 2 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.

Parallel Communicating Processes Customer: customer n withdraw customer n+1 Session: session k session m deposit create Account: customer n+1 saving checking retrieve 3 types of processes, 2 instances of each Duration of process processes actions sequence diagram Adapted from Software Engineering: An Object-Oriented Perspective by Eric J. Braude (Wiley 2001), with permission.

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 3 Gamma et al

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 Analysis process 1 n …... Control Adapted from Software Engineering: An Object-Oriented Perspective by Eric J. Braude (Wiley 2001), with permission.

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

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 Layout Encounter Environment Encounter Game Application layer 3D engine layer «uses» 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.

Recap Model-View-Controller Data flow systems Independent components 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