潘爱民 http://www.icst.pku.edu.cn/CompCourse 软件设计模式(一) 潘爱民 http://www.icst.pku.edu.cn/CompCourse

Why Patterns? 每一个库或者软件都有自己独特的地方, 有自己优劣的地方, 不能因为劣而看轻了这些技术,而要客观地看待这些成果,它们有其自己的背景,有其自己的用途,那么,沉淀下来的是什么呢? 这就是patterns，这是值得我们学习和研究的

内容 从一个例子看模式 关于模式的研究情况 介绍一些重要的模式(部分) 第三次作业

一个设计例子 VC/Samples/MFC/OLE/DrawCli

DrawCli的基础 MFC提供的基础 Doc/View结构 CWinApp/CMainFrame 对OLE的封装 数据结构管理功能 splitwnd功能 CWinApp/CMainFrame 提供了一套命令处理流程 对OLE的封装 Active Container OLE Clipboard Property Page 数据结构管理功能 CObject/CObList

我们的设计焦点 用C++对象来表示每一个图元 定义图元的公共接口 如何处理用户的操作 鼠标的动作 图元对象的创建和管理

图元基类 class CDrawObj : public CObject { // Attributes CDrawDoc* m_pDocument; // owner virtual int GetHandleCount(); virtual CPoint GetHandle(int nHandle); virtual HCURSOR GetHandleCursor(int nHandle); virtual void SetLineColor(COLORREF color); virtual void SetFillColor(COLORREF color); // Operations virtual void Draw(CDC* pDC); virtual void DrawTracker(CDC* pDC, TrackerState state); virtual void MoveTo(const CRect& positon, CDrawView* pView = NULL); virtual int HitTest(CPoint point, CDrawView* pView, BOOL bSelected); virtual BOOL Intersects(const CRect& rect); virtual void MoveHandleTo(int nHandle, CPoint point, CDrawView* pView = NULL); virtual void OnOpen(CDrawView* pView); virtual void OnEditProperties(); virtual CDrawObj* Clone(CDrawDoc* pDoc = NULL); virtual void Remove(); virtual void Serialize(CArchive& ar); // …… };

图元层次结构 CDrawObj CDrawRect CDrawPoly CDrawOleObj …… CDrawObj CDrawRect

图元的创建 永久支持： CDrawObj::Clone DrawTool：创建图元对象 CDocument::Serialize->CObList::Serialize ->CObject::Serialize CDrawObj::Clone DrawTool：创建图元对象

交互操作 键盘和菜单命令 鼠标操作 MFC内部机制 在CDrawView的鼠标处理函数中处理 工具箱：工具链，有一个当前活动工具 转交给当前活动工具来处理

工具箱和CDrawTool class CDrawTool { // Constructors public: CDrawTool(DrawShape nDrawShape); // Overridables virtual void OnLButtonDown(CDrawView* pView, UINT nFlags, const CPoint& point); virtual void OnLButtonDblClk(CDrawView* pView, UINT nFlags, const CPoint& point); virtual void OnLButtonUp(CDrawView* pView, UINT nFlags, const CPoint& point); virtual void OnMouseMove(CDrawView* pView, UINT nFlags, const CPoint& point); virtual void OnEditProperties(CDrawView* pView); virtual void OnCancel(); // Attributes DrawShape m_drawShape; static CDrawTool* FindTool(DrawShape drawShape); static CPtrList c_tools; static DrawShape c_drawShape; // …… };

DrawTool层次结构 CDrawTool CSelectTool CRectTool CPolyTool …… CDrawTool

例子中的模式 CDrawObj和CDrawTool合起来构成了Factory Method模式 CDrawTool::Clone用到了原型创建模式 每一个CDrawTool都是一个singleton Adapter模式：把OLE对象封装成CDrawObj * 可以增加Composite模式 facade模式：通过CDrawDoc/View与MFC通信 chain of responsibility：如鼠标处理工作 * 用Command模式增加undo/redo功能

patterns 定义： 几个近义词 A physical arrangement of elements Repeating;with some degree of correspondence in successive trials or observations 典范、范例，事物的标准样式 In the book “Design Patterns”, the design patterns are descriptions of communicating objects and classes that are customized to solve a general design problem in a particular context Design patterns represent solutions to problems that arise when developing software within a particular context 几个近义词 idiom、technique、paradigm

About patterns About patterns Documentation of expert software engineers' "behavior" Documentation of specific reoccurring problems (and solutions) Abstraction of common design occurrences Properties of design patterns A pattern addresses a recurring design problem that arises in specific design situations, and presents a solution to it. Patterns document existing, well-proven design experience. Patterns provide a common vocabulary and understanding for design principles. Patterns are a means of documenting software architectures. Patterns support the construction of software with define properties. Patterns help you to manage software complexity.

pattern与framework Patterns支持软件结构和设计的重用 frameworks支持细节设计和代码的重用 抓住了特定领域中问题的成功解决方案中的静态、动态结构和相互之间的协作关系 patterns与开发语言无关，但是建立在一定的环境基础上 例如：经典的MVC、Factory Method frameworks支持细节设计和代码的重用 framework是一组组件的综合，这些组件相互协作，为一族相关应用提供了一个可重用的框架结构 例如：MMC、MS Script Engine 两者结合起来， design patterns and frameworks有助于提高软件的质量 比如：重用性，扩展性，性能，可维护性

Design pattern与framework(续) a framework supplies the infrastructure and mechanisms that execute a policy for interaction between abstract components with open implementations. frameworks are often said to abide by the Hollywood Principle ("Don't call us, we'll call you.") 比较： Design patterns are more abstract than frameworks Design patterns are smaller architectural elements than frameworks Design patterns are less specialized than frameworks framework与class library(toolkit)

Pattern的研究情况 关于pattern研究的历史 A Pattern Language，Christopher Alexander，1977 “Advanced C++：Programming Styles and Idioms”，James Coplien，1992 “Design Patterns: Elements of Reusable Object-Oriented Software”，GOF，1995 “Pattern-Oriented Software Architecture: A System of Patterns” (简称为“POSA”) ，GoV，1996 …...

Pattern is a hot topic 在amazon上查找包含patterns的书(2002.12.12)

Pattern的研究现状 pattern与Java pattern与CORBA pattern与系统结构 pattern与generic programming结合 其他(例如UML等)

POSA中的模式分类 Architectural Patterns Design Patterns Idioms 表达了软件系统的基本结构组织形式或者结构方案 它包含一组预定义的子系统，规定了这些子系统的责任，同时还提供了用于组织和管理这些子系统的规则和向导 Design Patterns 为软件系统的子系统、组件或者组件之间的关系提供一个精炼之后的解决方案 它描述了在特定环境下，用于解决通用软件设计问题的组件以及这些组件相互通信时的可重现结构 Idioms 是一个与编程语言相关的低级模式 它描述了如何实现组件的某些功能，或者利用编程语言的特性来实现组件内部要素之间的通信功能

POSA: Architectural Patterns(1) Architectural Patterns are very high-level structural patterns. Also called “Conceptual Patterns ” From Mud to Structure: Organize components. Layers: Organize components into layers where layer i's services are only used by layer i+1. Pipes and Filters: Divide the task into several sequential processing steps -- the output of task i is the input of task i+1. Blackboard: Several independent programs work cooperatively on a common data structure. Distributed Systems :Handle distributed computation. Broker: Introduce a broker component to to achieve better decoupling of clients and servers -- brokers accept requests from clients and forward the requests to servers, then return the results back to the clients.

POSA: Architectural Patterns(2) Interactive Systems: Keep a program's functional core independent of the user interface Model - View - Controller: Divides the application into processing, output, and input. View and controller parts are usually observers of the model via the observer pattern Presentation - Abstract - Control: Divides the application up to heirarchies or MVC-like components. Each component is dependent upon and provides functionality for the a higher-level component. There is only one top-level component Adaptable Systems : Design for change Microkernel Encapsulate the fundamental services of the application Reflection Divide the application into a meta-level and a base level to make the application "self-aware". The meta level encapsulates knowledge of the system; the base level encapsulates knowledge about the problem domain

POSA: Design Patterns(1) Structure Decomposition: Decompose subsystems and complex components into cooperating parts. Whole - Part: Define a component that encapsulates smaller objects. Prevent clients from directly accessing the contained objects, but provide a interface for the aggregate. Organization of Work : Components collaborate to solve complex problems. Master - Slave The master divides a task among identical (but independent) slaves, the combines the slave's partial results to arrive a solution. Access Control : Guard and control access to services and components. Proxy: Clients communicate with a representative (proxy) rather than the target object itself. The proxy can perform pre- and post-processing to provide validation checking, access control, remote object access, extra computation, etc. See also Gamma et al's proxy pattern

POSA: Design Patterns(2) Management: Handle homogenous collections of objects, services and components in their entirety. Command Processor: Extends Gamma et al's command pattern by adding an explicit command processor View Handler: Separate the the management of views from the code required to present or control specific views. Similar to Gamma et al's Abstract Factory and Mediator. Communication :Organize communication between components. Forward - Receiver: Contain all system-specific communication functionality in separate components so distributed peers can communicate without loosing portability Client - Dispatcher - Server: A dispatcher acts as an intermediate layer between clients and servers. The dispatcher provides the communication channel and a name service to hide physical locations. Publisher - Subscriber: Same as Gamma et al's Observer pattern.

Idioms Also called “Programming Patterns” Idioms are low-level patterns specific to a programming language. Counted Pointer: Simplifies memory management of shared objects providing reference counting. 其他 Virtual constructor smart pointer handle/body

关于“Design Pattern” 对已有模式的整理、分类 一套描述模式的词汇，可用于交流和文档化 为软件设计总结了宝贵的经验，这些设计经验可以被重用，但不是简单的代码重用 分类： Creational Patterns Structural Patterns Behavioral Patterns 在软件设计模式领域，起到先驱的作用

重提：指导模式设计的三个概念 重用(reuse)：是目标 接口与实现分离 Decouple 两种重要的重用手段 接口保持不变，分离带来灵活性 Inheritance & composition 接口与实现分离 接口保持不变，分离带来灵活性 多态性(polymorphism) Decouple 降低复杂性

如何描述一个模式 关键要素 Design pattern name，Aliases or Also Known As Problem，Intent or Goal Forces，Constraints，Motivation Context， Applicability Solution Structure Participants Collaboration Implementation Evaluation，Resulting Context，Consequences Related Patterns Examples，Known uses

creational patterns Abstract Factory(kit) Builder Factory Method(virtual constructor) Prototype Singleton * Finder

模式 1：Factory Method (一) Aliases：virtual constructor Intent Motivation Define an interface for creating an object, but let subclasses decide which class to instantiate. Factory Method lets a class defer instantiation to subclasses. Motivation Frameworks use abstract classes to define and maintain relationships between objects. A framework is often responsible for creating these objects as well.

Factory Method模式(二) Applicability：Use the Factory Method pattern when a class can't anticipate the class of objects it must create. a class wants its subclasses to specify the objects it creates. classes delegate responsibility to one of several helper subclasses, and you want to localize the knowledge of which helper subclass is the delegate.

插：virtual constructor intent： 在一个类层次中，客户在runtime要创建一个对象，对象的子类型未确定，根据客户的需要和环境情况，确定对象的类型 problem： 只知道对象的一般类型，不知道确切类型(需要从环境中获取类型信息) Forces ： 隐藏对象的类型层次，只发布基接口 如何确定最合适的派生类 客户必须有办法使用派生类的服务

插：virtual constructor(续) solution： 使用Envelope/Letter或者Handle/Body pattern 由envelope或者handle根据环境信息选择适当的派生类型 例子： 1 根据stream动态创建对象 2 COM对象

Factory Method模式(三) struct Participants Collaborations Product、ConcreteProduct、Creator、ConcreteCreator Collaborations

Factory Method模式(四) Evaluation 多态性：客户代码可以做到与特定应用无关，适用于任何实体类 缺点：需要Creator和相应的子类作为factory method的载体，如果应用模型确实需要creator和子类存在，则很好；否则的话，需要增加一个类层次 优点： (1) Provides hooks for subclasses。基类为factory method提供缺省实现，子类可以重写新的实现，也可以继承父类的实现。 体现了：加一层间接性，增加了灵活性 (2) Connects parallel class hierarchies

Factory Method模式(五) Connects parallel class hierarchies

Factory Method模式(六) Implementation (1) 父类是否提供缺省的实现 (3) Language-specific variants and issues SmallTalk，使用类型 C++，使用lazy initialization技术 (4) Using templates to avoid subclassing

Factory Method模式(七) Related Patterns Examples Abstract factory Prototype Examples

模式 2 ：Abstract Factory(一) Aliases：Kit Intent Provide an interface for creating families of related or dependent objects without specifying their concrete classes. Motivation 为了解决一族相关或者相依对象的创建工作，专门定义一个用于创建这些对象的接口(基类)。客户只需与这个基接口打交道，不必考虑实体类的类型。

Abstract Factory(二) Applicability，Use the Abstract Factory pattern 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.

Abstract Factory(三) Struct Participants： Collaborations Client、AbstractFactory、ConcreteFactory、AbstractProduct、ConcreteProduct Collaborations

Abstract Factory(四) Evaluation 与factory method的关系 多个factory method合在一起 factory method一定是virtual的

Abstract Factory(五) Evaluation(续) 优点： 缺点： factory把product的类型封装起来，分离了具体的类 易于变换product族 保证不同族之间的product相互不会碰撞，即保证products的一致性 缺点： factory对象的方法数目对应product数目，增加新的product种类比较困难，要影响到factory的基类，进而影响到所有的子类

Abstract Factory(六) Implementation Factories as singletons, 每个product族往往只需要一个factory对象就可以了 Creating the products, 对于product族比较多的情况，可以使用prototype模式来实现这些factories，而不必对于每一个具有细微差别的product族都使用一个concrete factory class Defining extensible factories，针对Evaluation中提到的缺点，通过参数化技术提高factory的适应能力和扩展性 问题在于，返回给客户什么样的类型？

Abstract Factory(七) Related Patterns Examples Factory Method、Prototype 、Singleton Examples WidgetFactory

插：COM中的class factory 兼有两种模式：factory method和abstract factory IClassFactory是abstract factory的接口 CreateInstance是factory method 对于每一个coclass，class object就是Concrete Factory， 每一个产品的抽象接口为IUnknown，COM对象是真正的concrete product IPSFactoryBuffer也是abstract factory的接口 通过factory method创建对象 比客户直接创建对象，要灵活 ConcreteFactory也是一个concrete product，所以可以重用底层的许多机制，如套间机制、跨进程机制等

模式三：Builder (一) Intent Motivation Separate the construction of a complex object from its representation so that the same construction process can create different representations Motivation 在复杂对象的构造过程中，允许同样的构造过程能够加入新的被构造元素 “结构化构造过程” Applicability, Use the Builder pattern when the algorithm for creating a complex object should be independent of the parts that make up the object and how they're assembled. the construction process must allow different representations for the object that's constructed.

Builder (二) Structure Participants Director、 Builder、ConcreteBuilder、 Product

Builder (三) Collaborations

Builder (四) Evaluation Implementation It lets you vary a product's internal representation It isolates code for construction and representation It gives you finer control over the construction process Implementation Builder interface(Assembly and construction) Why no abstract class for products? Empty methods as default in Builder. Related patterns Abstract Factory 区别：(1) builder重在构造过程，最后一步返回结果; (2) builder构造许多复杂对象

Builder (五) Examples readers、parsers、converters the persistence of OLE documents

模式四：Prototype(一) Intent Specify the kinds of objects to create using a prototypical instance, and create new objects by copying this prototype. Motivation 以一个已有的对象作为原型，通过它来创建新的对象。在增加新的对象的时候，新对象的细节创建工作由自己来负责，从而使新对象的创建过程与框架隔离开来 Applicability 当产品的创建过程要独立于系统时 当产品的类型是在runtime时被指定的情况下 避免创建一个与product层次平行的factory层次时

Prototype(二) Structure Participants Collaborations Prototype、ConcretePrototype、Client Collaborations

Prototype(三) Evaluation Implementation Adding and removing products at run-time Specifying new objects by varying values，降低系统中类的数目 Configuring an application with classes dynamically 要求：每一个product类都必须实现Clone操作 对于C++语言特别有意义：C++的class不是first-class objects Implementation Using a prototype manager Implementing the Clone operation shallow copy versus deep copy Save & Load Initializing clones 两阶段构造

Prototype(四) Related patterns Examples Prototype与Abstract Factory往往是相互竞争的 factory method Examples DrawCli，music editor

模式五：Singleton(一) Intent Motivation Ensure a class only has one instance, and provide a global point of access to it. Motivation It's important for some classes to have exactly one instance. Instance-controlled class Applicability, Use the Singleton pattern when there must be exactly one instance of a class, and it must be accessible to clients from a well-known access point. when the sole instance should be extensible by subclassing, and clients should be able to use an extended instance without modifying their code.

Singleton(二) Structure Participants Collaborations Singleton Clients access a Singleton instance solely through Singleton's Instance operation.

Singleton(三) Evaluation Implementation Ensuring a unique instance Controlled access to sole instance Reduced name space Permits refinement of operations and representation，允许子类化 Permits a variable number of instances More flexible than class operations (static member functions in C++ ) 这种思想比较适用于Object-Based中的许多情形 Implementation Ensuring a unique instance 考虑使用lazy initialize 使用global/static object的缺点 Subclassing the Singleton class

Singleton(四) Related patterns Examples Singleton与其他创建型模式并不矛盾，可以用singleton来实现其他模式中的对象。包括Abstract Factory、Builder、Prototype等。 多个实例对于构造过程往往并无意义，所以在许多情况下singleton模式比较符合应用背景 Examples MFC中的CWinApp派生类实例theApp ……

增加模式六：Finder(一) Intent Alias Motivation 利用环境信息，根据客户的请求，找到已有的、符合要求的对象，返回给客户 Alias Object-retriever Motivation 在有些情况下，客户希望连接到一个对象上，它提供一些状态标识信息，由Finder返回已经被创建的对象，或者重新创建新的对象(如果当前不存在满足条件的对象) Applicability, Use the Finder pattern when 当需要在软件不同部分之间建立Client/Object连接时 把获取对象的过程隐藏起来 view finder：The application demands user customizability of the actions taken when a particular file format is encountered in the browser.

Finder(二) Structure Participants Collaborations Finder table Lookup client、finder、product-table、constructor、product Collaborations Finder GetObject table Lookup client product product create product constructor

Finder(三) Evaluation Implementation 避免同样的对象被实例化两次，从而提高资源利用率，避免发生资源竞争 把连接对象的过程与客户隔离开 带来的问题：多个客户共享同样的资源，如何有效管理对象的所有权？ Implementation 实现product table以及相应的管理设施，保证查找过程的有效性 每一个product的生命周期管理 如何标识product的类型，客户如何多态地提供状态标识信息 Finder对象本身可以是一个singleton

Finder(四) Related patterns Examples 与Singleton的区别：singleton是一个类的单个实例；而Finder是避免相同的对象(通常是类型和状态信息都相同)被创建两次。 与Prototype的区别 在创建product子步骤中，需要与其他创建型模式结合使用 Examples moniker in COM 在Netscape浏览器中，根据MIME类型，找到插件，然后创建view

第三次作业 写出你认为理解最深刻的一个pattern， 可以是你自己学习、工作中体会出来的， 也可以是书上已经总结好的。