An Introduction to Design Patterns

Transcription

1 An Introduction to Design Patterns 1

2 Definitions A pattern is a recurring soution to a standard probem, in a context. Christopher Aexander, a professor of architecture Why woud what a prof of architecture says be reevant to software? A pattern describes a probem which occurs over and over again in our environment, and then describes the core of the soution to that probem, in such a way that you can use this soution a miion times over, without ever doing it the same way twice. Jim Copein, a software engineer: I ike to reate this definition to dress patterns What are dress patterns?... I coud te you how to make a dress by specifying the route of a scissors through a piece of coth in terms of anges and engths of cut. Or, I coud give you a pattern. Reading the specification, you woud have no idea what was being buit or if you had buit the right thing when you were finished. The pattern foreshadows the product: it is the rue for making the thing, but it is aso, in many respects, the thing itsef.

3 Patterns in engineering How do other engineers find and use patterns? Mature engineering discipines have handbooks describing successfu soutions to known probems Automobie designers don't design cars from scratch using the aws of physics Instead, they reuse standard designs with successfu track records, earning from experience Shoud software engineers make use of patterns? Why? Be sure that you make everything according to the pattern I have shown you here on the mountain. Exodus 25:40. Deveoping software from scratch is aso expensive Patterns support reuse of software architecture and design

4 The gang of four (GoF) Erich Gamma, Richard Hem, Raph Johnson & John Vissides (Addison-Wesey, 1995) Design Patterns book cataogs 23 different patterns as soutions to different casses of probems, in C++ & Smatak The probems and soutions are broady appicabe, used by many peope over many years What design pattern did we discover with the Undo probem? Why is it usefu to earn about this pattern? Patterns suggest opportunities for reuse in anaysis, design and programming GOF presents each pattern in a structured format What do you think of this format? Pros and cons?

5 Eements of Design Patterns Design patterns have 4 essentia eements: Pattern name: increases vocabuary of designers Probem: intent, context, when to appy Soution: UML-ike structure, abstract code Consequences: resuts and tradeoffs

6 Design Patterns are NOT Data structures that can be encoded in casses and reused as is (i.e., inked ists, hash tabes) Compex domain-specific designs (for an entire appication or subsystem) If they are not famiiar data structures or compex domain-specific subsystems, what are they? They are: Descriptions of communicating objects and casses that are customized to sove a genera design probem in a particuar context.

7 Three Types of Patterns Creationa patterns: Dea with initiaizing and configuring casses and objects Structura patterns: Dea with decouping interface and impementation of casses and objects Composition of casses or objects Behaviora patterns: Dea with dynamic interactions among societies of casses and objects How they distribute responsibiity

8 Creationa Patterns Abstract Factory: Factory for buiding reated objects Buider: Factory for buiding compex objects incrementay Factory Method: Method in a derived cass creates associates Prototype: Factory for coning new instances from a prototype Singeton: Factory for a singuar (soe) instance

9 Structura patterns Describe ways to assembe objects to reaize new functionaity Added fexibiity inherent in object composition due to abiity to change composition at run-time not possibe with static cass composition Exampe: Proxy Proxy: acts as convenient surrogate or pacehoder for another object. Remote Proxy: oca representative for object in a different address space Virtua Proxy: represent arge object that shoud be oaded on demand Protected Proxy: protect access to the origina object

10 Structura Patterns Adapter: Transator adapts a server interface for a cient Bridge: Abstraction for binding one of many impementations Composite: Structure for buiding recursive aggregations Decorator: Decorator extends an object transparenty Facade: Simpifies the interface for a subsystem Fyweight: Many fine-grained objects shared efficienty. Proxy: One object approximates another

11 Behaviora Patterns Chain of Responsibiity: Request deegated to the responsibe service provider Command: Request or Action is first-cass object, hence re-storabe Iterator: Aggregate and access eements sequentiay Interpreter: Language interpreter for a sma grammar Mediator: Coordinates interactions between its associates Memento: Snapshot captures and restores object states privatey Which ones do you think you have seen somewhere?

12 Behaviora Patterns (cont.) Observer: Dependents update automaticay when subject changes State: Object whose behavior depends on its state Strategy: Abstraction for seecting one of many agorithms Tempate Method: Agorithm with some steps suppied by a derived cass Visitor: Operations appied to eements of a heterogeneous object structure

13 Patterns in software ibraries AWT and Swing use Observer pattern Iterator pattern in C++ tempate ibrary & JDK Façade pattern used in many studentoriented ibraries to simpify more compicated ibraries! Bridge and other patterns recurs in middeware for distributed computing frameworks

14 More software patterns Design patterns idioms (ow eve, C++): Jim Copein, Scott Meyers I.e., when shoud you define a virtua destructor? design (micro-architectures) [Gamma-GoF] architectura (systems design): ayers, refection, broker Refection makes casses sef-aware, their structure and behavior accessibe for adaptation and change: Meta-eve provides sef-representation, base eve defines the appication ogic Java Enterprise Design Patterns (distributed transactions and databases) E.g., ACID Transaction: Atomicity (restoring an object after a faied transaction), Consistency, Isoation, and Durabiity Anaysis patterns (recurring & reusabe anaysis modes, from various domains, i.e., accounting, financia trading, heath care) Process patterns (software process & organization)

15 Benefits of Design Patterns Design patterns enabe arge-scae reuse of software architectures and aso hep document systems Patterns expicity capture expert knowedge and design tradeoffs and make it more widey avaiabe Patterns hep improve deveoper communication Pattern names form a common vocabuary

16 Singeton Synopsis or Intent: Singeton pattern is one of the simpest design patterns This type of design pattern comes under creationa pattern provides one of the best ways to create an object. Context: invoves a singe cass which is responsibe to create an object whie making sure that ony singe object gets created. This cass provides a way to access its ony object which can be accessed directy without need to instantiate the object of the cass. Impementation cass provides a static method to get its static instance to outside word

17 Singeton (exampe)

18 Prototype Synopsis or Intent: creating dupicate object whie keeping performance in mind. This type of design pattern comes under creationa pattern provides one of the best ways to create an object. Context: invoves impementing a prototype interface which tes to create a cone of the current object. This pattern is used when creation of object directy is costy. For exampe, an object is to be created after a costy database operation. We can cache the object, returns its cone on next request and update the database as and when needed thus reducing database cas. Impementation to create an abstract cass and concrete casses extending the abstract cass. A third cass is defined as a next step which stores objects in a Hashtabe and returns their cone when requested.

19 Prototype (exampe)

20 Adapter Pattern Synopsis or Intent: works as a bridge between two incompatibe interfaces comes under structura pattern combines the capabiity of two independent interfaces Context: This pattern invoves a singe cass which is responsibe to join functionaities of independent or incompatibe interfaces A rea ife exampe coud be a case of card reader which acts as an adapter between memory card and a aptop You pugin the memory card into card reader and card reader into the aptop so that memory card can be read via aptop.

21 Adapter (exampe)

22 Command Pattern Synopsis or Intent: Encapsuate a request as an object, thereby etting you parameterize cients with different requests, queue or og requests, and support undoabe operations Context: You want to mode the time evoution of a program: What needs to be done, e.g. queued requests, aarms, conditions for action What is being done, e.g. which parts of a composite or distributed action have been competed What has been done, e.g. a og of undoabe operations What are some appications that need to support undo? Editor, cacuator, database with transactions Perform an execute at one time, undo at a different time Soution: represent units of work as Command objects Interface of a Command object can be a simpe execute() method Extra methods can support undo and redo Commands can be persistent and gobay accessibe, just ike norma objects

23 Command pattern Consequences: You can undo/redo any Command Each Command stores what it needs to restore state You can store Commands in a stack or queue Command processor pattern maintains a history It is easy to add new Commands, because you do not have to change existing casses Command is an abstract cass, from which you derive new casses execute(), undo() and redo() are poymorphic functions

24 Command (exampe)