软件工程英文教学课件：Ch8 Design Concepts

1、1Chapter 8Design ConceptsSoftware Engineering: A Practitioners Approach, 7/e by Roger S. Pressman2DesignGood software design should exhibit:Firmness(稳定性): A program should not have any bugs that inhibit its function. Commodity(适用性): A program should be suitable for the purposes for which it was inte

2、nded. Delight(令人愉快): The experience of using the program should be pleasurable one.3Analysis Model = Design Model4Three Characters of Design Qualitythe design must implement all of the explicit(明确的) requirements contained in the analysis model, and it must accommodate all of the implicit(隐含的) requir

3、ements desired by the customer.the design must be a readable, understandable guide for those who generate code and for those who test and subsequently support the software.the design should provide a complete picture(全貌) of the software, addressing the data, functional, and behavioral domains from a

4、n implementation perspective.5 Design Quality GuidelinesA design should exhibit an architecture that (1) has been created using recognizable architectural styles or patterns, (2) is composed of components that exhibit good design characteristics and (3) can be implemented in an evolutionary fashion.

5、A design should be modular; that is, the software should be logically partitioned into elements or subsystems.A design should contain representations of data, architecture, interfaces, and components.A design should lead to data structures that are appropriate for the classes to be implemented.A des

6、ign should lead to components that exhibit independent functional characteristics.A design should lead to interfaces that reduce the complexity of connections between components and with the external environment.A design should be derived using a repeatable method that is driven by information obtai

7、ned during software requirements analysis.A design should be represented using a notation that effectively communicates its meaning.6Design PrinciplesThe design should be traceable to the analysis model. The design should “minimize the distance” between the software and the problem as it exists in t

8、he real world. The design should exhibit integration. The design should be structured to accommodate change. The design should be assessed for quality as it is being created, not after the fact. Design is not coding, coding is not design. From Davis DAV957Design Fundamental ConceptsAbstractiondata,

9、procedureArchitecture(架构)the overall structure of the softwarePatternsconveys(承载) the essence(精髓) of a proven design solutionModularitycompartmentalization(划分) of data and functionInformation hidingcontrolled interfacesFunctional independencesingle-minded function and low couplingAspectsa mechanism

10、for understanding how global requirements affect designRefactoring(重构)a reorganization technique that simplifies the designDesign Classesprovide design detail that will enable analysis classes to be implemented8Data Abstractiondoorimplemented as a data structuremanufacturermodel numbertypeswing dire

11、ctionweightopening mechanism9Procedural Abstractionopenimplemented with a knowledge of the object that is associated with enterdetails of enter algorithm10Architecture & its Properties“The overall structure of the software and the ways in which that structure provides conceptual integrity for a syst

12、em.”Structural properties. This aspect of the architectural design representation defines the components of a system (e.g., modules, objects) and the manner in which those components are packaged and interact with one another. For example, objects encapsulate both data and the processing that manipu

13、lates the data. (即:考虑功能) Extra-functional properties. The architectural design description should address how the design architecture achieves requirements for performance, capacity, reliability, security, adaptability, and other system characteristics. (即:考虑性能)Families(族) of related systems. The ar

14、chitectural design should draw upon repeatable patterns that are commonly encountered in the design of families of similar systems. The design should have the ability to reuse architectural building blocks. (即:考虑重用)11ArchitectureIn the simplest form, architecture is the structure or organization of

15、program components(modules), the manner in which these components interact, and the structure of data that are used by the components. In a broader sense, architecture can be generalized to represent major system elements and their interactions.12Modularitymodularity is the single attribute of softw

16、are that allows a program to be intellectually manageable. Monolithic(整块庞大的) software (i.e., a large program composed of a single module) cannot be easily grasped(理解) by a software engineer. The number of control paths, span of reference(引用跨度) , number of variables, and overall complexity would make

17、 understanding close to impossible. In almost all instances, you should break the design into many modules, hoping to make understanding easier and as a consequence, reduce the cost required to build the software.13Modularity: number of modules What is the right number of modules for a specific soft

18、ware design?optimal number of modules cost of softwarenumber of modulesmoduleintegrationcostmodule development cost 14Why Information Hiding?reduces the likelihood of “side effects”limits the global impact of local design decisionsemphasizes communication through interfacesdiscourages the use of glo

19、bal dataencapsulationan attribute of high quality designresults in higher quality software15Information Hidingmoduleinterfacesecret algorithm data structure resource allocation policyclientsa specific design decision16Sizing Modules: Two Views17Functional IndependenceFunctional independence is achie

20、ved by developing modules with “single-minded” function and an “aversion(避免) to excessive interaction with other modules.Cohesion(内聚性) is an indication of the relative functional strength of a module.A cohesive module performs a single task, requiring little interaction with other components in othe

21、r parts of a program. Stated simply, a cohesive module should (ideally) do just one thing. Coupling(耦合性) is an indication of the relative interdependence among modules.Coupling depends on the interface complexity between modules, the point at which entry or reference is made to a module, and what da

22、ta pass across the interface.18AspectAn aspect is a representation of a cross-cutting concern. (横切关注点: 利用“横切”技术，剖开封装的对象内部，将那些影响多个类的公共行为封装成一个可重用对象，称为Aspect)。 Consider two requirements A and B. B crosscuts A if a software decomposition has been chosen in which A cannot be satisfied without taking B. A

23、OP(Aspect Oriented Programming)是OOP的补充和完善。如：业务处理的主要流程是核心关注点，与之关系不大的部分是横切关注点, 如权限认证、日志、事务处理等，可以将核心关注点和横切关注点分离开来。19AspectAn ExampleConsider two requirements for the SafeHomeA WebApp. Requirement A is described via the use-case Access camera surveillance via the Internet. A design refinement would focu

24、s on those modules that would enable a registered user to access video from cameras placed throughout a space. Requirement B is a generic security requirement that states that a registered user must be validated prior to using SafeHomeA. This requirement is applicable for all functions that are avai

25、lable to registered SafeHome users. As design refinement occurs, A* is a design representation for requirement A and B* is a design representation for requirement B. Therefore, A* and B* are representations of concerns, and B* cross-cuts A*. An aspect is a representation of a cross-cutting concern.

26、Therefore, the design representation, B*, of the requirement, a registered user must be validated prior to using SafeHomeA, is an aspect of the SafeHome WebApp. 20显示更新的需求：无论图元何时移动、移动到哪里，都要通知屏幕管理器（Display）其位置发生了改变。Aspect - Example21 采用OOP，典型做法是在每个移动图元的操作代码中，插入一段通知Display其位置发生改变的代码，即：调用Display.update(

27、 )。Class Line private Point _p1,_p2; Point getP1( ) return _p1; Point getP2( ) return _p2; void setP1(Point p1)this._p1=p1; Display.update( ); void setP2(Point p2)this._p2=p2; Display.update( ); Class Point private int _x1,_x2; int getX( ) return _x1; int getY( ) return _x2; void setX (int x1)this._

28、x1=x1; Display.update( ); void setY(int x2)this._x2=x2; Display.update( ); Aspect - Example22Aspect DisplayUpdating Pointcut move(): call(void Line.setP1(Point)| call(void Line.setP2(Point)| call(void Point.setX(int)| call(void Point.setY(int); after() returning:move()Display.update(); Class Line pr

29、ivate Point _p1,_p2; Point getP1( ) return _p1; Point getP2( ) return _p2; void setP1(Point p1)this._p1=p1; void setP2(Point p2)this._p2=p2; Class Point private int _x1,_x2; int getX( ) return _x1; int getY( ) return _x2; void setX (int x1)this._x1=x1; void setY(int x2)this._x2=x2; 采用AOP，典型做法是在将所有移动

30、图元的代码封装成一个Aspect: DisplayUpdating 。Aspect - Example23RefactoringRefactoring is the process of changing a software system in such a way that it does not alter the external behavior of the code design yet improves its internal structure. 重构是指优化软件的处理过程和内部结构而不改变软件的外部行为的方法。When software is refactored, th

31、e existing design is examined for redundancyunused design elementsinefficient or unnecessary algorithmspoorly constructed or inappropriate data structuresany other design failure that can be corrected to yield a better design.24The Design Model25Design Model ElementsData elementsData model - data structuresData model - database architectureArchitectural elementsApplication domainAnalysis classes, their relationships, collaborations and behaviors are transformed into design realizationsInterface elementsComponent elementsDeployment elements26Architectural ElementsThe architectura