6.3 Patterns. Definition: Design Patterns

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1 Subject/Topic/Focus: Analysis and Design Patterns Summary: What is a pattern? Why patterns? 6.3 Patterns Creational, structural and behavioral patterns Examples: Abstract Factory, Composite, Chain of Responsibility Applying patterns in UML Pattern taxonomy: idioms, mechanisms, frameworks Literature: Erich Gamma, Richard Helm, Ralph Johnson, John Vlissides; Design Patterns, Addison-Wesley, 1994 Patterns were were considered for for the the architecture of of buildings by by Christopher Alexander, Definition: Design Patterns Design Patterns are descriptions of of communicating objects and classes that can be be customized to to solve a general design problem in in a particular context. [Gamma 94] Most simply, a pattern is is a named problem/solution pair that can be be applied in in new contexts, with advice on on how to to apply it it in in novel situations. [Larman 97]

2 From Example to Pattern: Similarities Part * Graphic * Cost() Add(:Part) Remove(:Part) Draw() Add(:Graphic) Remove(:Graphic) parts lines BasePart CompositePart Line Picture Cost() Cost() Add(:Part) Remove(:Part) Draw() Draw() Add(:Graphic) Remove(:Graphic) These These examples look look quite quite similar. similar. They They lead lead to to a pattern pattern named named Composite OOA&D and Patterns There is great variability in the potential quality of object structuring, interaction design and responsibility assignment. Design patterns capture simple and elegant solutions to specific recurring problems in object-oriented software design that were developed and evolved over time by object-oriented experts to transfer their many years of experience. Systems built with design patterns are robust, maintainable, understandable, reusable, and extensible

3 Patterns: The Idea Experienced oo-developers build up a repertoire of both, general principles and idiomatic solutions that guide them in the creation of software. These principles and idiomatic solutions, if codified in a structured format describing the problem and solution, and given a name may be called patterns. Patterns do do not not contain new new ideas! The point of patterns is not to discover and express new software engineering principles; quite the opposite is the case. Patterns attempt to codify existing knowledge, idioms and principles Pattern: Elements In general a pattern has four essential elements: The pattern name, is a handle to describe a design problem, its solutions and consequences in a word or two. The problem, describes when to apply the pattern. It explains the problem and its context. The solution, describes the elements that make up the design, their relationships, responsibilities and collaborations. The solution does not describe a particular concrete design or implementation, because a pattern is used like a template in many different situations. The consequences are the results and trade-offs of applying the pattern. Though consequences are often unvoiced when design decisions are described, they are critical for evaluating design alternatives

4 Pattern Taxonomy Reuse Level Idioms: programming style low programming-language-dependent patterns generally accepted conventions e.g., exception handling in C++ Focus Focus Mechanisms: object collaboration structure common problem solutions which guarantee constraints building blocks of good system architecture programming language independent Frameworks: micro architecture collection of classes (class library) and mechanisms domain specific high Grady Booch: Best of Booch: Designing Strategies for Object Technology. (ed. by Ed Eykholt); SIGS Well-known Design Patterns [Gamma 94] Scope Purpose Creational Patterns Structural Patterns Behavioral Patterns Class Level Factory Method Adapter (class) Interpreter, Template Method Object Level Abstract Factory, Builder, Prototype, Singleton Adapter (object), Bridge, Composite, Decorator, Facade, Flyweight, Proxy Chain of Responsibility, Command, Iterator, Mediator, Memento, Observer, State, Strategy, Visitor Abstract Factory: Provide an interface for creating families of related objects without specifying their concrete classes. Composite: Compose objects into tree structures to represent part-whole hierarchies. Chain of Responsibility: Avoid coupling the sender of a request to its receiver by giving more than one object a chance to handle the request. Chain the receiving objects and the request along the chain until an object handles it

5 Creational Pattern: Abstract Factory Provide an an interface for for creating families of of related objects without specifying their concrete classes. Example (see also next slide): Portable GUI (Graphical User Interface) toolkit for Mac, Windows95,... GUI elements called widgets (windows, scrollbars, buttons,...) are implemented differently for all platforms. Client applications should be easily portable to each platform, i.e., without changing all GUI classes and operations. Therefore the client does not use the platform-specific classes, e.g. MacWindow or W95Window, but interfaces, e.g. Window. Objects are created by a concrete class. Therefore the creation is encapsulated in platform-specific factories, e.g. MacWidgetFactory. The client uses the factory through the interface WidgetFactory. When porting the application, only the factory has to be exchanged Factory Example: Structure WidgetFactory Client CreateWindow() CreateScrollBar() interfaces/ abstract classes classes uses uses new(:widgetfactory) Window MacWidgetFactory CreateWindow() CreateScrollBar() W95WidgetFactory CreateWindow() CreateScrollBar() creates creates objects objects W95Window Scrollbar MacWindow W95Scrollbar MacScrollbar

6 Factory Example: Behavior wf := new() wf : MacWidgetFactory new(wf) aclient seen seen by by aclient aclient as as WidgetFactory CreateScrollBar()... new() seen seen by by aclient aclient as as Scrollbar Scrollbar amacscrollbar aclient aclient does does not not know know which which concrete classes classes are are used used -- it it uses uses only only the the interfaces Abstract Factory: Structure AbstractFactory CreateProductA() CreateProductB() Client new(:abstractfactory) AbstractProductA ConcreteFactory1 CreateProductA() CreateProductB() ConcreteFactory2 CreateProductA() CreateProductB() ProductA2 ProductA1 AbstractProductB ProductB2 ProductB

7 Abstract Factory: Behavior f := new() f : ConcreteFactory1 new(f) aclient CreateProductA() seen seen by by aclient aclient as as AbstractFactory seen seen by by aclient aclient as as AbstractProductA new() aproducta1... aclient aclient uses uses only only the the interfaces AbstractFactory, AbstractProductA, Using the Abstract Factory Pattern in UML Patterns are are represented as as parameterized use-cases (compare to to parameterized classes) in in class class diagrams. Texteditor Client WidgetFactory AbstractFactory MacWidgetFactory ConcreteFactory instantiate as as role role Abstract Factory AbstractProductA Scrollbar ProductA1 MacScrollbar the the pattern pattern structure and and behavior diagrams are are placed placed in in the the use use case case

8 Use Abstract Factory when... a system should be independent of how its products are created, composed and represented; a system should be configured with one of multiple families of products; a family of related product objects is designed to be used together, and you need to enforce this constraint; you want to provide a class library of products, and you want to reveal just their interfaces, not their implementations Structural Pattern: Composite Compose objects into tree structures to to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of of objects uniformly. Example (see next slide): Complex pictures are built from simple components, e.g., lines and circles. Pictures themselves may be components of larger pictures leading to nested hierarchies. Simple components (lines, circles) and complex components (pictures) are treated identically by the client, e.g. all can be drawn. apicture object object instances describing this this picture picture apicture aline aline acircle

9 Composite Example: Structure Graphic * Draw() Add(:Graphic) Remove(:Graphic) graphics Line Draw() Circle Draw() Add(), Remove(), GetChild() not implemented Picture Draw() Add(g :Graphic) Remove(:Graphic) forall g in graphics g.draw() add g to graphics Composite Example: Behavior add(aline1) aline1 acircle apicture1 aline2 apicture2 add(acircle) add(apicture1) add(aline2) draw() draw() draw() draw() draw()

10 Composite: Structure Client Component * acomposite Operation() Add(:Component) Remove(:Component) children aleaf acomposite aleaf aleaf Leaf Operation() Add(), Remove(), GetChild() not implemented Composite Operation() Add(:Component) Remove(:Component) forall c in children c.operation() Using the Composite Pattern in UML PaintApp Graphic Line Circle Picture Client Component Leaf Leaf Composite Composite Use Use Composite when when you you want want to to represent part-whole hierarchies of of objects; you you want want clients clients to to be be able able to to ignore ignore the the difference between compositions of of objects objects and and individual objects. Clients Clients will will treat treat all all objects objects in in the the composite structure uniformly

11 Behavioral Pattern: Chain of Responsibility Avoid coupling the sender of of a request to to its its receiver by by giving more than one object a chance to to handle the request. Chain the receiving objects and the request along the chain until an an object handles it. it. Example: The F1 key gives context-sensitive help depending on the position of the mouse cursor, i.e., the GUI element the mouse cursor points to. If the GUI element under the mouse cursor, e.g., a print button, cannot give help on himself, he delegates giving help to its surrounding GUI element, e.g., the print dialog, and so forth. It is not known when F1 is pressed which GUI element will give help, i.e., to which object the help request should be sent. Print me? Print Cancel F handler Chain of Responsibility Example HelpHandler HandleHelp() ^handler.handlehelp() Structure Application Widget Dialog Button HandleHelp() ShowHelp() if can handle now then ShowHelp() else HelpHandler::HandleHelp() aprintbutton aprintdialog anapplication HandleHelp() HandleHelp() HandleHelp() Behavior

12 Chain of Responsibility: Structure Client 0..1 Handler handlerequest() Successor ConcreteHandler1 handlerequest() ConcreteHandler2 handlerequest() Chain of Responsibility: Behavior Successor Successor :Client ahandler ahandler ahandler handlerequest() handlerequest() handles request does does not not get get a chance chance to to handle handle the the request request (this (this time) time)

13 Using the Chain of Resp. Pattern in UML Keyboard aprintbutton aprintdialog anapplication Client Handler Handler Handler Chain of Responsibility Use Use Chain Chain of of Responsibility when when more more than than one one object object may may handle handle a request, and and the the handler isn t isn t known known a priori. priori. The The handler handler should should be be ascertained automatically. you you want want to to issue issue a request to to one one of of several several objects objects without specifying the the receiver explicitly. the the set set of of objects objects that that can can handle handle a request should should be be specified dynamically Benefits of design patterns are Patterns: Benefits common design vocabulary: system developers use conceptual structures and their abstraction rather than the syntax of a programming language. a documentation and learning aid: design patterns are often used and therefore structures of already existing software systems are easier understood in terms of the patterns they were built from. useful in turning an analysis model into an implementation model: a smooth transition from analysis to design (and reengineering) is supported by common design patterns, explaining the additional objects in a design model and its implementation

14 Summary: Software Systems Engineering Activity: Application Analysis and Software Systems Design Goal: Sound Software Systems with a potential for deliberate improvements Means: Object Orientation in Analysis and Design

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