Today’s educational objective CSE 403 � Understand the basics of design Lecture 14 patterns � Be able to distinguish them from design approaches such as information hiding Design Patterns and layering � Be able to find patterns that meet specific needs � Know what the Gang of Four is Experts vs. Novices Examples of expertise � Experience � Chess playing � Higher level thought � Experts view pieces in groups � Mathematics � Chunking � Idioms � Integration by trigonometric substitution � Programming � Techniques � Examples for (int i = 0; i < n; i+ + ) a[i] = b[i]; Design patterns in Architecture Design Pattern � Alexander: "Each pattern describes a � Pattern name: Strip mall. problem which occurs over and over � Problem: Make small commercial establishments and franchises again in our environment, and then accessible to car driving customers. describes the core of the solution to the problem. In such a way that you can � Solution: Parking area with store fronts facing parking. Uniform construction. use this solution a million times over, � Consequences: Traffic flow, congestion, without ever doing it the same way parking availability, building rents. twice." 1
Gang of Four Case study � Lexi Editor (Calder) � Gamma, Helm, Johnson, Vlissides � Document structure � Catalog of design � Formatting patterns for � Embellishing UI software � Multiple look and feel standards � Multiple window systems � User operations � Spelling checking and hyphenation Document structure Composite pattern � Characters, pictures, lines, words, � Intent paragraphs, columns, tables, . . . � Tree structures � Represent uniformly � Structure � Recursive solution � Glyph – display object Composite Pattern Composite Pattern � Participants � Consequences � Component (Graphic) � Simple client � Leaf � Easy to extend � Composite � Runtime check required to restrict components � Client 2
Composite Pattern Document structure � Implementation issues � An object allocated for every character! column � Explicit parent references � Solution (trick, hack) � Sharing components row row row � Table of character � Child ordering objects a p p a r e n t � Responsibility for deletion (in non-GC � Reference into the language) a b c d e f g h i j k l m table n o p q r s t u v w x y z Flyweight pattern Formatting � Break text into lines FlyweightFactory Flyweight � Approach – insert row glyphs to break text GetFlyweight(key) Operation(extrinsicState) flyweights into lines � Want to allow different algorithms for formatting ConcreteFlyweight UnsharedConcreteFlyweight Operation(extrinsicState) Operation(extrinsicState) � Compositor class – formatting algorithm intrinsicState allState � Composition glyph has a compositor Client � Compositor is responsible for formatting children Strategy pattern UI Embellishment � Context, strategy pair � Add border or scrollbar to component � Specific algorithms subclass strategy � MonoGlyph extends Glyph � Border extends MonoGlyph � ConcreteStrategy � ScrollBar extends MonoGlyph � Decorator Pattern 3
Multiple look and feel Supporting Multiple Window standards Systems � Window Class Hierarchy � Motif menus, Mac menus � GuiFactory guiFactory = new MotifFactory(); � WindowImp Class Hierarchy � ScrollBar sb = guiFactory.CreateScrollBar(); � Extend WindowImp for each different system � Button bu = guiFactory.CreateButton(); � Avoid polluting Window Class with system dependencies � Abstract Factory Pattern � Bridge Pattern � Link between Window and WindowImp User commands and spell check/hyphenation Classification of patterns � User commands Creational � Original GoF patterns Abstract factory, builder, � factory method, prototype, � Command Pattern singleton Structural � � Includes Undo functionality � Adapter, bridge, composite, decorator, façade, flyweight, � Spell check and hyphenation proxy Behavioral � � Iterate over words of document � Chain of responsibility, command, interpreter, iterator, � Iterator Pattern and Visitor pattern mediator, memento, observer, state, strategy, template method, visitor 4
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