Lecture 21: Design Patterns III

Transcription

1 Lecture 21: Design Patterns III Software System Design and Implementation ITCS/ITIS 6112/ Fall 2008 Dr. Jamie Payton Department of Computer Science University of North Carolina at Charlotte Nov. 18, 2008

2 Announcements D3 due today at 11:59 pm Project assessments due by 11/20/08 at 11:59 pm New links to corrected surveys! D4: Presentations Dates Dec. 2 (2 teams) Dec. 4 (2 teams) Dec. 9 (3 teams)

3 D4: Presentations Professional presentation Length Grading Slides/visual aids Polished Practice! 20 minutes 5 additional minutes for questions Presentation Content Presentation Style Presentation Time Management Project Progress

4 D4: Presentations Content Problem Statement What you re working on and why it is important to your stakeholders Big picture! Design Present an overview of your software architecture and mid-level design Discuss any consideration of patterns (architectural and/or design) Highlight important design decisions

5 D4: Presentations Content Progress Can show live prototype, recorded video of prototype, or screenshots If prototype is not viable, discuss your progress so far Expected outcome and project plan What you expect to be in your final prototype How you plan to design/implement to achieve goal Peer-assessment How did your teammates contribute to preparing the presentation? How did you teammates contribute to progress so far? How is your team dividing the remaining work?

6 Lecture Overview Patterns Course evaluations

7 Types of Patterns Behavioral Observer Mediator Structural Façade Adapter Proxy Creational Abstract Factory 7

8 Image Processing Application Requirements Problem Solution Must store many images Allows for image editing Supports animation Copying images requires significant time, memory Need to avoid copying images as much as possible Copy-on-write Have a placeholder keep a reference to original image, create a copy only when needed

9 The Proxy Pattern Description Provide a placeholder for another object Applicability Remote proxies Hide location of object Virtual proxies Create expensive objects on demand Protection proxies Control access to an object Smart proxies may: Count number of references to object (so it can be freed) Load persistent object into memory on first reference Prevent changes by other objects In general: Need a more versatile or sophisticated reference to an object than a pointer 9

10 The Proxy Pattern Participants Proxy Maintains a reference that lets Proxy access RealSubject Provides an interface identical to Subject s so that it can substitute for the real object Controls access to RealSubject May create and delete RealSubject Subject Defines the common interface for RealSubject and Proxy so that Proxy can substitute for RealSubject RealSubject Defines the real object that the Proxy represents 10

11 Proxy Pattern Structure «interface» Subject request () Client Proxy RealSubject

12 Proxy Pattern Behavior sd ProxyBehavior :Client :Proxy :RealSubject request() request() request()

13 Example: Image Proxy

14 Proxy Pattern Consequences Remote proxy can hide the fact that an object resides in a different address space Virtual proxy can perform optimizations such as creating an object on demand When used for copy-on-write, requires reference counting on real object

15 Proxy Analogy ATM card substitutes for cash Manager sends subordinate to meeting 15

16 Proxy vs. Adapter Adapter Provides a different interface to object it adapts Proxy provide same interface to object

17 Types of Patterns Behavioral Describe how classes interact and distribute responsibility Observer Mediator Structural Creational Describe how classes or objects should be composed Façade Adapter Proxy Concern the process of object creation Factory method Abstract Factory 17

18 Traffic Simulator Want to design a traffic simulator to study traffic patterns We have Road and Vehicles Vehicles are of different types SUV Compact MiniVan MidSize Vehicles should be generated to match expected distribution Which class should do this? Road? Class? Neither, really 18

19 The Factory Method Pattern Applicability Clients do not need to know details of object implementation, just need it to implement particular interface Consequences Client does not have to create objects Created by generator Allows choice of actual object type at runtime 19

20 Factory Method Examples Analogy Automobile production SUVs: Ford Explorer, Acura MDX, Subaru Outback Rental car company Ask for a compact car, but you don t care if it s a Civic Design Example Vehicle traffic simulator Vehicle and Road classes Must randomly create new Vehicle to adhere to expected distribution of vehicle types Compact, SUV, Minivan, Truck Which class chooses Vehicle type? Road? VehicleFactory! 20

21 Factory Method Pattern Description Defines an interface for creating an object, but lets subclasses decide which class to instantiate Applicability A class can t anticipate the class of objects it must create A class wants its subclasses to specify the objects it creates

22 Factory Method Pattern Participants Product Defines the interface of objects the factory method creates Concrete Product Implements the Product interface Creator (aka Factory) Declares the factory method Returns an object of type Product May also define a default implementation that returns a default ConcreteProduct May call the factory method to create a Product object

23 Factory Method Pattern Structure

24 Factory Method Behavior

25 Factory Method Pattern Consequences Eliminates need to bind application-specific classes into code Code only deals with Product interface Code can work with any user-defined ConcreteProduct Clients might need to subclass Creator just to create a particular ConcreteProduct

26 The Iterator and Factory Method Patterns

27 Types of Patterns Behavioral Observer Mediator Structural Façade Adapter Proxy Creational Factory Abstract Factory 27

28 The Abstract Factory Pattern Description Provides an interface for creating families of related or dependent objects without specifying their concrete classes 28

29 The Abstract Factory Pattern (2) Applicability Need to abstract from details of implementation of products Need to have multiple families of products Need to enforce families of products that must be used together Need to hide product implementations and just present interfaces Consequences Isolates concrete classes Makes exchanging product families easy Promotes consistency among products Supporting new kinds (in each family) of products is difficult 29

30 The Abstract Factory Pattern AbstractFactory Client CreateProductA() CreateProductB() AbstractProductA ProductA2 ProductA1 ConcreteFactory1 AbstractProductB ConcreteFactory2 CreateProductA() CreateProductB() ProductB2 ProductB1 30

31 Abstract Factory Example WidgetFactory CreateScrollbar() CreateWindow() Window Client MacWindow WWindow WWidgetFactory ScrollBar MacWidgetFactory MacScrollBar WScrollBar One for each standard. 31

32 Factory Method versus Abstract Factory Abstract Factory uses Factory Method 32

33 Summary Design patterns are generic, reusable design templates Design patterns provide a vocabulary for discussing design Three types of patterns Behavioral Structural Creational 33

34 Resources Introduction to Software Engineering Design Christopher Fox GoF book PoSA book Articles posted on course website 34