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  1. PatternsDesign 13 February

  2. User Interfaces • Introducing the Book

  3. Patterns

  4. 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

  5. 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

  6. Architectural Example: Door Placement If room has two doors and people move through it, keep both doors at one end of the room

  7. Alexander’s 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?

  8. 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?

  9. 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

  10. Patterns at All Levels

  11. “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

  12. 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

  13. 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

  14. 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

  15. 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

  16. Why Categorize? • Recognize patterns • Reuse designs • Learn from other similar applications • Reuse classes

  17. 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

  18. Pipe and Filter filter pipe filter filter filter filter pipe filter • Filters: processes • Pipes: input streams

  19. 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

  20. 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)

  21. Façade «exposed» 1 Client 2 «not exposed» «not exposed» P «not exposed» «not exposed» «not exposed» «not exposed» 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.

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

  23. 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

  24. Event Systems and State Transition Diagrams • Set of components waiting for input

  25. 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

  26. Repository • A system built around data • Two types • Databases • Hypertext systems

  27. GUI Analysis process 1 Analysis process n Control …... …... DBMS Database A Typical Repository System Adapted from Software Engineering: An Object-Oriented Perspective by Eric J. Braude (Wiley 2001), with permission.

  28. 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

  29. 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)

  30. 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

  31. 3D engine layer Role-playing game layer «uses» RolePlayingGame Layout Characters «uses» Application layer Encounter Characters Encounter Environment Encounter Game 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.

  32. 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