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