Lecture 20: Design Patterns II

Transcription

1 Lecture 20: Design Patterns II Software System Design and Implementation ITCS/ITIS 6112/ Fall 2008 Dr. Jamie Payton Department of Computer Science University of North Carolina at Charlotte Nov. 13, 2008

2 Design Patterns A pattern is a way of reusing abstract knowledge about design a description of the problem the essence of its solution Patterns allow programmers to share knowledge about design 2

3 Alexander s Patterns Different Chairs (251)...People are different sizes; they sit in different ways. And yet there is a tendency in modern times to make all chairs alike....therefore Never furnish any place with chairs that are identically the same. Choose a variety of different chairs, some big, some small, some softer than others, some rockers, some very old, some new, with arms, without arms, some wicker, some wood, some cloth. 3

4 Pattern Elements Name A meaningful pattern identifier Problem description (Applicability) Solution description Not a concrete design instead, a template for a design solution Consequences The results and trade-offs of applying the pattern 4

5 Purpose of Patterns Behavioral Describe how classes or objects interact and distribute responsibility Observer (last time) Mediator Structural Creational Describe how classes or objects should be composed Façade Adapter Proxy Concern the process of object creation Factory method Abstract Factory 5

6 Scope of Patterns Scope Another dimension for classifying patterns Class patterns Relationships between classes and subclasses Established through inheritance Fixed at compile-time Object patterns Relationships between objects Changeable at run-time (most patterns) 6

7 Purpose and Scope Behavioral Class Use inheritance to describe algorithms and flow of control Object Describe how a group of objects cooperate to perform a task Structural Class Use inheritance to compose classes Object Describe how to compose objects Creational Class Defer some part of object creation to a subclass Object Defer some part of object creation to another object 7

8 Scope Purpose Creational Structural Behavioral Class Factory Method Adapter (class) Interpreter Template Method Object Abstract Factory Builder Prototype Singleton Adapter (object) Bridge Composite Decorator Façade Proxy Flyweight Chain of Responsibility Command Iterator Mediator Memento Observer State Strategy Visitor 8

9 Boggle Let s design an networked Boggle game Players find words in the jumble of letters on the board within given time frame Player gets one point for each identified real word that is unique unique = not found by another player Player with most points wins What are the basic classes? (ignore the user interface for now) How do they work together? How should we keep score? 9

10 Scoring in the Players Problems? Now every player object has to know about: Board Player Dictionary EVERY OTHER PLAYER Tightly coupled!!! Scoring algorithm is in the player class May not be the best place for it Not cohesive!!! 10

11 Mediator Pattern (1) Applicability Solution Complex interaction exists You do not want to bury the interaction in the objects Distributed behavior should be customizable Introduce a mediator class that is responsible for controlling and coordinating group of objects Objects refer only to mediator, not each other 11

12 Mediator (2) Participants Mediator (interface) Defines interface for communicating with Colleagues Concrete Mediator Coordinates Colleague objects Knows and maintains Colleagues Colleague Each Colleague knows its Mediator Each Colleague communicates with Mediator instead of Colleagues 12

13 Mediator (3) Consequences Decouples colleagues Abstracts how objects cooperate Centralizes control Simplifies object interaction protocols but can cause a performance hit Mediator can become complex and unwieldy 13

14 The Mediator Pattern: Static Structure 14

15 Mediator-driven Scoring 15

16 Comparing Boggle Scoring Design Approaches Without mediator Each Player must have references to Board, Dictionary, and all Player objects to do scoring Scoring algorithm is in Player class May not really belong there Complicates Player class With Mediator Player needs references to Board and Tiles only to make plays Scoring is in Scorer class More logically organized, easier to make changes 16

17 Applying the Mediator Pattern Analogies Meeting Scheduler Emergency Call Dispatcher Air Traffic Control Center Chat Applications Boggle 17

18 Types of Patterns Behavioral Observer Mediator Structural Façade Adapter Proxy Creational Abstract Factory 18

19 Compiler Example A compilation subsystem consists of: Scanner Parser ProgramNode BytecodeStream ProgramNodeBuilder Most applications that use the compilation subsystem don t care about the details of these classes Complicated if the clients have to use the classes directly 19

20 The Façade Pattern (1) Description Provide a unified interface to a set of interfaces in a subsystem Facade defines a higher-level interface that makes the subsystem easier to use 20

21 The Façade Pattern (2) Applicability Need to provide a simple interface to a complex system Need to decouple a subsystem from its clients Want to treat subsystem as a software layer Façade is used as interface to a software layer 21

22 The Façade Pattern (3) Participants Façade Knows which subsystems classes are responsible for handling a particular request Delegates client requests to appropriate subsystem objects Subsystem classes Implement subsystem functionality Handle work assigned by Facade Have no knowledge of Facade 22

23 The Façade Pattern (4) Consequences Shields clients from subsystem components Promotes weak coupling between the subsystem and its clients Shield clients from changes to subsystems Only Façade needs to be aware of changes 23

24 Facade Pattern: Solution Client Classes Facade Subsystem classes 24

25 Façade Example Analogy Travel agent Design Example A friendlier Java Calendar class 25

26 Mediator and Façade Mediator coordinates interaction among multiple objects Defines some function that is not in Colleagues Colleagues know about Mediator Façade provide a simplified interface to subsystems Subsystems do not know about Façade Can think of a Mediator as a multi-way Façade Objects involved in interaction subsystem Objects interact with subsystem through Mediator 26

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

28 Word Processor Example Want to build a word-processor Need to be able to click a save button to save work Need to be able to print a file by clicking print button We have a Button and a ButtonListener class Upon Button click, will notify registered ButtonListeners of event We have a DocManager class already Has a printdocument() method Can t change DocManager; have to use as-is How can we combine these together? 28

29 The Adapter Pattern Description Convert interface of a class into another interface clients expect Adapter lets classes work together that could not otherwise because of incompatible interfaces Applicability Need to use an existing class whose interface does not match Need to make use of incompatible classes 29

30 The Adapter Pattern (2) Participants Target (ButtonListener) Defines the domain-specific interface that the Client uses Client (Button) Collaborates with objects conforming to the Target interface Adaptee (DocManager) Defines an existing interface that needs adapting Adapter (a new thing we re creating) Adapts the interface of Adaptee to the Target Interface 30

31 The Adapter Pattern: Approaches Class Adapter Adapter class is a subclass of adaptee Inherits operations Overrides operations if needed Add new operations to provide new interface Object Adapter Adapter object holds a reference to adaptee Delegates work to adaptee object 31

32 The Adapter (Class) Pattern 32

33 The Adapter (Object) Pattern 33

34 Class Adapter for Word Processor Example Button ButtonListener DocManager MyDocManager buttonpressed(e){ print(); } buttonpressed(event e) 34

35 The Adapter Pattern: Consequences Consequences Class adapter Adapter commits to the concrete Adapter class Won t work when we want to adapt a class and its subclasses Lets Adapter override some of Adaptee s behavior Introduces only one object, no pointer indirection Cannot block access to public attributes and methods of Adaptee Object adapter Lets a single Adapter work with Adaptee and its subclasses Makes it harder to override Adaptee behavior Would have to create Adaptee subclass, make Adapter call subclass operations 35

36 Applying the Adapter Pattern Analogy Electrical travel adapters Design example Thread safe PriorityQueue Design choice between Object and Class Adapter Must use Object Adapter if: Adaptee has changeable public attributes Cannot subclass adaptee Cannot override methods of adaptee 36

37 Adapter versus Facade Façade Adapter Used to provide a simplified interface to a subsystem Used to provide a different interface to a single class Used when operations with slightly different functionality are needed 37

38 Types of Patterns Behavioral Observer Mediator Structural Façade Adapter Proxy Creational Abstract Factory 38

39 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 39

40 The Proxy Pattern 40

41 Proxy Examples Analogy ATM card substitutes for cash Manager sends subordinate to meeting Design example Image manipulation 41

42 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 42

43 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 43

44 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 44

45 Factory 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! 45

46 Types of Patterns Behavioral Observer Mediator Structural Façade Adapter Proxy Creational Factory Abstract Factory 46

47 The Abstract Factory Pattern Description Provides an interface for creating families of related or dependent objects without specifying their concrete classes 47

48 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 48

49 The Abstract Factory Pattern AbstractFactory Client CreateProductA() CreateProductB() AbstractProductA ProductA2 ProductA1 ConcreteFactory1 AbstractProductB ConcreteFactory2 CreateProductA() CreateProductB() ProductB2 ProductB1 49

50 Abstract Factory Example WidgetFactory CreateScrollbar() CreateWindow() Window Client MacWindow WWindow WWidgetFactory ScrollBar MacWidgetFactory MacScrollBar WScrollBar One for each standard. 50

51 Factory Method versus Abstract Factory Abstract Factory uses Factory Method 51

52 Summary Design patterns are generic, reusable design templates Design patterns provide a vocabulary for discussing design Three types of patterns Behavioral Structural Creational 52

53 Resources Introduction to Software Engineering Design Christopher Fox GoF book PoSA book Articles posted on course website 53