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Patterns Senior Member of Technical Staff and Pattern Languages Knowledge Systems Corp Kyle Brown An Introduction to Patterns Senior Member of Technical Staff and Pattern Languages Knowledge Systems Corp. 4001 Weston Parkway CSC591O Cary, North Carolina 27513-2303 April 7-9, 1997 919-481-4000 Raleigh, NC [email protected] http://www.ksccary.com Copyright (C) 1996, Kyle Brown, Bobby Woolf, and 1 2 Knowledge Systems Corp. All rights reserved. Overview Bobby Woolf Senior Member of Technical Staff O Patterns Knowledge Systems Corp. O Software Patterns 4001 Weston Parkway O Design Patterns Cary, North Carolina 27513-2303 O Architectural Patterns 919-481-4000 O Pattern Catalogs [email protected] O Pattern Languages http://www.ksccary.com 3 4 Patterns -- Why? Patterns -- Why? !@#$ O Learning software development is hard » Lots of new concepts O Must be some way to » Hard to distinguish good communicate better ideas from bad ones » Allow us to concentrate O Languages and on the problem frameworks are very O Patterns can provide the complex answer » Too much to explain » Much of their structure is incidental to our problem 5 6 Patterns -- What? Patterns -- Parts O Patterns are made up of four main parts O What is a pattern? » Title -- the name of the pattern » A solution to a problem in a context » Problem -- a statement of what the pattern solves » A structured way of representing design » Context -- a discussion of the constraints and information in prose and diagrams forces on the problem »A way of communicating design information from an expert to a novice » Solution -- a description of how to solve the problem » Generative: show when and how to apply solutions –This may include a list of the participants in the pattern » One of the hottest topics in OO Design O May have other sections 7 8 Patterns -- Origin (1) Patterns -- Origin (2) O Pattern concept came from architecture » A Pattern Language O A Pattern Language [Alexander77] describes » A Timeless Way of » Common architectural Building [Alexander79] motifs O Alexander used patterns » How they can come to together to form a » Express the interaction of cohesive, liveable forces in a problem environment » Arrive at an elegant solution 9 Patterns -- Doors Example (1) Patterns -- Doors Example (2) O Corner Doors O Solution: “...in most » [196 in Alexander77] rooms, especially small O Problem: How do you ones, put the doors as near place the doors in a room? the corners of the room as “untouched by O “If the doors create a possible.” movement” pattern of movement O “If a room has two doors, G! which destroys the places N and people move through BA in a room, the room will it, keep both doors at one never allow people to be end of the room.” comfortable.” Software Patterns -- What? Software Patterns -- Why? O Ward Cunningham and Kent Beck from O Abstractors and Tektronix Elaborators [Beck94] O In 1987, they applied Alexander’s ideas to » Abstractors create reusable pieces designing user interfaces (frameworks) » “Nouns in Lists, Verbs in Menus” » Elaborators use pieces to » “Window per Task” build applications » Abstractors must explain O The idea was independently discovered pieces to elaborators » Jim Coplien of AT&T [Coplien92] » Patterns are an efficient »Others explanation of intent 13 14 Software Patterns -- Why? Software Pattern -- GUIs O Patterns support O Nouns in Lists, Verbs in Menus reusable software O Problem: »Many more elaborators than abstractors (100:1?) »Designing a GUI: What goes in lists vs. menus? » More efficient explanation, more blah blah blah efficient reuse blah » More powerful than a blah blah faster compiler or better ? blah tools 15 16 Nouns in lists... Nouns in lists… (2) O Context: O Solution: » Principle of least astonishment » Put nouns in lists, verbs in menus » People expect to perform actions on subjects » List of actions without subjects looks incomplete edit tree » Select a subject first, then an action house copy ? cut 17 18 Patterns have exploded! Architectural Patterns Architectural Analysis Patterns O Deal with the highest levels of software Patterns development O Often discovered in very large projects Alexander’s O A few published examples Patterns Organizational Design » Coad, Object Models: Strategies Patterns and Patterns Patterns Applications » Buschmann, Pattern Oriented Software Architectures: A System of Patterns Language-Specific Patterns 19 20 Architectural Pattern - Pipes and Filters Pipes and Filters (2) O From [Buschmann96] O Solution: Divide the task O Problem: Building Lexical Analysis into sequential Systems to Transform processing steps and Process Streams of (filters). Syntax Analysis/Parsing Data O Steps are connected by » Different steps of the process may be data flow components Semantic Analysis represented differently (pipes) »Many intermediate data O Example: Compiler forms Code Generation 21 22 Architectural Pattern -- Four Layer Architecture (1) Four Layer Architecture (2) O Problem: »How do you structure an application with O Solution: significant presentation » Divide classes into layers and model components? » The MVC architecture of O Forces: Smalltalk-80 separated »Existing structures the “model” and “view” » Code reuse components » Distribution of work » More layers are needed among team members to model all necessary abstractions »Portability to new environments and language vendors Smalltalk Application Layers Layer purposes O View layer - provides the graphical view of an application. Handles user interaction O View layer Application layer - Mediates between different views of an application. Adapts the protocol Application layer of the View layer to the protocol of the domain model. Domain layer O Domain layer - Represents the business or Infrastructure layer “real-world” model of the application O Infrastructure layer - supports the domain layer with connections to external interfaces Analysis Patterns Analysis Patterns O Written by Martin Fowler O Deals with common O Patterns about Analysis business domains O Are they patterns about the Analysis process, » Accounting »Finance or patterns found in systems analysis? » Inventory O Both have been published... O Includes general purpose analysis patterns 27 28 An Analysis Pattern Account - Solution Account Entry Transaction entries O Account balance : Quantity amount : Quantity whenCharged : TimeStamp 2 O Problem: In many fields whenBooked : TimeStamp you must keep records Constraint: Constraint: of not just the current balance = sum(entries.amount) value of a thing, but the sum (entries.amount) = 0 changes that have affected that value. Basic Double-Entry Accounting! 29 30 Design Patterns Design Patterns -- Gang of Four O Gang of Four is Erich Gamma, Richard Helm, O Design Patterns are patterns about OO Ralph Johnson and John Vlissides design O Collaborated on finding patterns in software O Discuss tradeoffs inherent in the process over a series of years O Evaluate alternative designs O Finally decided to write a book 31 32 A Design Pattern -- Strategy Design Patterns (315) O Their book is Design Patterns: Elements of Reusable Object- O Often you have the “Choose 1 of N Oriented Software algorithms” problem. [GHJV95] O Example: Converting different file formats O 23 basic design patterns found in many large OO » GIF, JPEG, EPS, etc. systems O You could write one “FileReader” with a LOT O A pattern catalog of case statements. » Patterns are not dependent on each other 33 34 Strategy Solution Strategy Structure O Define a family of algorithms strategy Client AbstractStrategy » Various approaches to solving a problem » Encapsulate them as objects » Make them polymorphic O Client requests problem StrategyA StrategyB be solved O Strategy directs how problem is solved 35 36 Example Strategy Strategy Benefits strategy FileConverter AbstractReader O convertFrom:to: readFileNamed: Avoids case statements O Easily extensible » just add new Strategy subclasses O Strategy classes may be GIFReader JPGReader reusable elsewhere readFileNamed: readFileNamed: 37 38 A Design Pattern -- State (305) State Solution O Sometimes you have an object that acts O Define a set of State differently over time. objects. O Example: A TCPConnection O Defer the » You can’t open an opened one or close a closed implementation of the one. different methods to the O You could use methods like become: or state object. changeClassToThatOf: O The client changes its state from time to time. » leads to messy, difficult code 39 40 TCP Example STD TCP Example Hierarchy transmit state TCPConnection TCPState ActiveOpen( ) ActiveOpen( ) Close( ) Close( ) activeOpen PassiveOpen( ) PassiveOpen( ) Closed Established state.ActiveOpen() TCPClosed passiveOpen send TCPEstablished TCPListen ActiveOpen( ) ActiveOpen( ) ActiveOpen( ) close Close( ) Close( ) Close( ) PassiveOpen( ) PassiveOpen( ) PassiveOpen( ) Listen 41 42 State Benefits Why use Design Patterns O Proven solutions to common OO problems O Makes state transitions explicit O Confer expertise to beginners O Localizes behavior; better allocation of O Common vocabulary for designers responsibility O Documentation for existing frameworks O Eliminates the need for case statements O Once internalized, good design techniques O Since the States are objects, the State become second nature Machine can be reconfigured on the fly O Often combined to create more sophisticated designs 43 44 How to use Design Patterns Languages-Specific Patterns O Usually the pattern describes a role for an O Often called idioms object -- not a class! O Can be found in any language (not just OO) » Don’t blindly implement all 23 as classes! O They are the way
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