潘爱民 http://www.icst.pku.edu.cn/CompCourse 软件设计模式(二) 潘爱民 http://www.icst.pku.edu.cn/CompCourse

内容 复习 续：介绍一些重要的模式 Structural Patterns Behavioral Patterns

复习：pattern定义 定义： 关于pattern的研究状况 pattern与框架 pattern的分类 特定环境中问题的成功解决方案中的静态、动态结构，以及结构元素相互之间的协作关系 Design patterns represent solutions to problems that arise when developing software within a particular context 关于pattern的研究状况 研究历史 现状 pattern与框架 pattern的分类 粒度

复习：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

复习：creational patters Factory Method 本质：用一个virtual method完成创建过程 Abstract Factory 一个product族的factory method构成了一个factory接口 Prototype 通过product原型来构造product，Clone+prototype manager Builder 通过一个构造算法和builder接口把构造过程与客户隔离开 Singleton 单实例类型，如何构造这单个实例？如何访问这单个实例？ Finder 把对象的获取过程与客户隔离开

creational patterns小结 了解每一种模式的实质 具体实现的时候可能会有变化情况，或者扩展，或者退化 factory method是基础，abstract factory是它的扩展 factory method、abstract factory、prototype都涉及到类层次结构中对象的创建过程，有所取舍 prototype需要prototype manager factory method需要依附一个creator类 abstract factory需要一个平行的类层次 根据应用的其他需求，以及语言提供的便利来决定使用哪种模式

creational patterns小结(续) builder往往适合于特定的结构需要，它所针对的product比较复杂 singleton有比较强烈的物理意义，可以用在许多细微的地方，不一定与类层次关联 finder模式需要有一定范围内的对象管理功能 这些patterns都很常见，有时需要结合两种或者多种模式完成系统中对象的构造过程

Structural Patterns Adapter Bridge Composite * Decorator Facade Flyweight * Proxy

模式 7： Adapter (一) Aliases：Wrapper Intent Motivation Convert the interface of a class into another interface clients expect. Adapter lets classes work together that couldn't otherwise because of incompatible interfaces. Motivation Sometimes a toolkit class that's designed for reuse isn't reusable only because its interface doesn't match the domain-specific interface an application requires.

Adapter模式(二) Applicability：Use the Adapter pattern when you want to use an existing class, and its interface does not match the one you need. you want to create a reusable class that cooperates with unrelated or unforeseen classes, that is, classes that don't necessarily have compatible interfaces. (object adapter only) you need to use several existing subclasses, but it's impractical to adapt their interface by subclassing every one. An object adapter can adapt the interface of its parent class.

Adapter模式(三) Struct class adapter object

Adapter模式(三) Participants Collaborations Client、Target、Adaptee、Adapter class adapter —— delegation object adapter —— container

Adapter模式(四) Evaluation 本质上是两种重用模型 class adapter： object adapter 无法adapt adaptee的子类，但是可以重载adaptee的行为 object adapter 可以adapt adaptee的所有子类 How much adapting does Adapter do? Pluggable adapters Using two-way adapters to provide transparency 针对class adapter，用多重继承来实现

Adapter模式(五) Implementation 使用C++继承机制实现class adapter 使用内嵌对象技术实现object adapter Pluggable adapters，三种实现方案 使用抽象方法定义 使用代理对象 参数化技术 这三种方法的实质：如何在一个类中定义抽象操作，供客户插入？—— hook 技术

Adapter模式(六) Related Patterns Examples Bridge Decorator Proxy DrawCli：COleDrawObj C++ STL COM中的site

模式 8：Bridge(一) Aliases：Handle/Body Intent Motivation Decouple an abstraction from its implementation so that the two can vary independently Motivation 要做到“抽象(接口)与实现分离”，最常用的办法是定义一个抽象类，然后在子类中提供实现。也就是说，用继承机制达到“抽象(接口)与实现分离” 但是这种方法不够灵活，继承机制把实现与抽象部分永久地绑定起来，要想独立地修改、扩展、重用抽象(接口)与实现都非常困难。

Bridge模式(二) Applicability：Use the Bridge pattern when 编译时刻无法确定抽象(接口)与实现之间的关系 抽象部分与实现部分都可以通过子类化而扩展 对一个实现的修改不影响客户(无须重新编译) 在C++中，对客户完全隐瞒实现细节 因为扩展的原因，需要把一个类分成两部分，(以便灵活组合) 在多个对象之间共享数据，但客户不需要知道

Bridge模式(三) Struct Participants Collaborations Client, Abstraction, RefinedAbstraction, Implementor, ConcreteImplementor Collaborations

Bridge模式(四) Evaluation 抽象部分与实现部分的分离，可以在运行时刻连接起来 二进制兼容性 提高可扩充性：抽象与实现两部分可以单独扩充 对客户隐藏实现细节

Bridge模式(五) Implementation Only one Implementor Creating the right Implementor object 如何创建？根据客户环境，或者通过factory Sharing implementors 资源管理：引用计数技术 Using multiple inheritance

Bridge模式(六) Related Patterns Examples Abstract Factory可以用来创建和配置Bridge模式 与Adapter模式的区别 Examples handle：文件handle、窗口handle

插：Handle/Body Counted Handle/Body class StringRep { friend class String; StringRep(const char *s); ~StringRep(); int count; char *rep; }; class String { public: String(); String(const String &s); String &operator=(const String &s); ~String(); String(const char *s); . . . . private: StringRep *rep; Counted Handle/Body

模式 9：Composite(一) Intent Motivation Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly. Motivation 一些部件对象经过组合构成的复合部件对象仍然具有单个部件对象的接口，这样的复合部件对象被称为“容器(container)” 复合部件与单个部件具有同样的接口，所有接口包含两部分：单个部件的功能、管理子部件的功能 递归组合

Composite模式(二) Applicability：Use the Composite pattern when you want to represent part-whole hierarchies of objects. you want clients to be able to ignore the difference between compositions of objects and individual objects. Clients will treat all objects in the composite structure uniformly.

Composite模式(三) Struct Participants Collaborations Client, Component, Leaf, Composite Collaborations

典型的composite对象结构

Composite模式(四) Evaluation defines class hierarchies consisting of primitive objects and composite objects。定义了包含leaf对象和composite对象的类层次接口。——递归结构 makes the client simple。客户一致地处理复合对象和单个对象 makes it easier to add new kinds of components。易于增加新类型的组件 can make your design overly general。使得系统过于一般化，无法限制类型的组合，可以在运行时刻通过类型检查加以弥补

Composite模式(五) Implementation Explicit parent references Sharing components Maximizing the Component interface Declaring the child management operations Should Component implement a list of Components? Child ordering Caching to improve performance Who should delete components? What's the best data structure for storing components?

Composite模式(六) Related Patterns Examples Decorator、Flyweight、Iterator、Visitor Examples 广泛应用于OO领域 MFC中的CWnd 组件层次：ActiveX Container

模式 10：Facade(一) Intent 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. Motivation 使系统的各子系统之间的关联最小，引入一个facade对象，为子系统提供一个简单的、泛化的设施

Facade模式(二) Applicability：Use the Facade pattern when 为一个复杂的子系统提供一个简单接口时。 子系统往往会非常复杂，但是其接口应该尽可能地简单，特别是对于一般用户而言 客户与抽象类的实现部分之间必然存在一定的依赖性，facade可以降低这种依赖性 在多个子系统的结构中，使用facade模式定义子系统的入口点，有助于降低各子系统之间的依赖性

Facade模式(三) Struct Participants Collaborations facade, subsystem classes Collaborations

Facade模式(四) Evaluation Implementation 简化子系统的接口，方便客户使用子系统 化“紧耦合”为“松耦合” —— 实现组件软件的关键技术 facade模式并不限制客户直接访问子系统的内部类和对象 Implementation 以抽象类的形式定义facade，进一步decouple，从而完全隔离子系统的细节 Public versus private subsystem classes

Facade模式(五) Related Patterns Examples Netscape Browser MDI Framework facade对象的创建：singleton、abstract factory Examples Fake MFC library Netscape Browser CDrawView CDrawDoc Map Plugin Module Map Module MFC library CDrawView CDrawDoc MDI Framework Map Module

模式 11：FlyWeight(一) Intent Motivation Use sharing to support large numbers of fine-grained objects efficiently. Motivation 当对象的粒度太小的时候，大量对象将会产生巨大的资源消耗，因此考虑用共享对象(flyweight)来实现逻辑上的大量对象。Flyweight对象可用于不同的context中，本身固有的状态不随context发生变化，而其他的状态随context而变化

FlyWeight模式(二) Applicability：Use the FlyWeight pattern when all of the following are true: An application uses a large number of objects. Storage costs are high because of the sheer quantity of objects. Most object state can be made extrinsic. Many groups of objects may be replaced by relatively few shared objects once extrinsic state is removed. The application doesn't depend on object identity. Since flyweight objects may be shared, identity tests will return true for conceptually distinct objects.

FlyWeight模式(三) Struct

FlyWeight模式(四) Struct(续) Participants Collaborations client, flyweight, concreteFlyweight, FlyweightFactory, UnsharedConcreteFlyweight Collaborations

FlyWeight模式(五) Evaluation Implementation 把对象的状态分开：intrinsic and extrinsic 节约存储空间：内部状态的共享节约了大量空间，外部状态可通过计算获得从而进一步节约空间 flyweight与composite结合。Flyweight为leaf节点，并且父节点只能作为外部状态 Implementation Removing extrinsic state，尽可能做到实时计算(通过一个小的数据结构) Managing shared objects，客户不能直接实例化flyweight，必须通过管理器，例如FlyweightFactory。 flyweight的生命周期管理，引用计数和回收

FlyWeight模式(六) Related Patterns Examples 与Composite模式组合 可以用flyweight实现State和Strategy模式中的对象 Examples Excel中cell的管理 IOleItemContainer接口允许客户发现每一个cell对象 用flyweight实现cell对象 —— tearoff技术 对状态的有效管理是对象技术的一个进步 “Design Patterns”中提到的文档编辑器的例子

Structural Patterns ：Decorator

Structural Patterns ：Proxy

Structural patterns小结 Adapter 、bridge、facade adapter用于两个不兼容接口之间的转接 bridge用于将一个抽象与多个可能的实现连接起来 facade用于为复杂的子系统定义一个新的简单易用的接口 composite、decorator和proxy composite用于构造对象组合结构 decorator用于为对象增加新的职责 proxy为目标对象提供一个替代者 flyweight 针对细粒度对象的一种全局控制手段

Behavioral Patterns Chain of Responsibility * Command Interpreter * Iterator Mediator * Memento * Observer State * Strategy Template Method * Visitor

模式 12：Command(一) Aliases Intent Motivation Action, Transaction functor (function object) Intent Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations. Motivation 把请求信息和请求执行过程封装起来 framework往往需要把命令请求与处理请求的对象分开，command模式可以把调用操作的对象与操作的目标对象分开 允许通过多种途径调用同一个请求。——请求的重用

Command模式(二) Applicability：Use the Command pattern when : parameterize objects by an action to perform，代替回调 specify, queue, and execute requests at different times support undo support logging changes so that they can be reapplied in case of a system crash structure a system around high-level operations built on primitives operations —— transactions

Command模式(三) Struct Participants Client, Command、ConcreteCommand、Invoker、Receiver

Command模式(四) Collaborations

Command模式(五) Implementation Evaluation Command decouples the object that invokes the operation from the one that knows how to perform it. Commands are first-class objects. They can be manipulated and extended like any other object. You can assemble commands into a composite command. An example is MacroCommand. It's easy to add new Commands, because you don't have to change existing classes. Implementation How intelligent should a command be? Supporting undo and redo Avoiding error accumulation in the undo process Using C++ templates

Command模式(六) Related Patterns Examples Composite模式可用来实现command组合 为实现undo/redo，可以用其他行为模式来管理状态，如memento模式。Command被放到history list之前，可以用prototype模式复制自身 Examples

模式 13：Iterator(一) Aliases ：Cursor Intent Motivation Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation. Motivation An aggregate object such as a list should give you a way to access its elements without exposing its internal structure. Separating the traversal mechanism from the List object lets us define iterators for different traversal policies without enumerating them in the List interface.

Iterator模式(二) Applicability：Use the Iterator pattern when : to access an aggregate object's contents without exposing its internal representation. to support multiple traversals of aggregate objects. to provide a uniform interface for traversing different aggregate structures (that is, to support polymorphic iteration).

Iterator模式(三) Struct Participants Collaborations Iterator、ConcreteIterator、Aggregate、ConcreteAggregate Collaborations

Iterator模式(四) Evaluation Implementation It supports variations in the traversal of an aggregate Iterators simplify the Aggregate interface More than one traversal can be pending on an aggregate Implementation 实现可以非常灵活 Who controls the iteration? external iterator versus internal iterator Who defines the traversal algorithm? Aggregate本身定义算法 —— Cursor mode iterator定义算法 —— iterator如何访问数据 How robust is the iterator?

Iterator模式(五) Implementation(续) Additional Iterator operations. 基本操作：First, Next, IsDone, and CurrentItem Using polymorphic iterators —— iterator资源释放 Iterators may have privileged access Iterators for composites —— 适合于internal iterator或者cursor方式的iterator Null iterators

Iterator模式(六) Related Patterns Examples Composite：iterator常被用于composite模式的复合结构 Polymorphic iterators rely on factory methods to instantiate the appropriate Iterator subclass. Examples COM enumerator：connectable object、… ADO/OLE DB C++ STL 在STL中，iterator是连接algorithm和container的桥梁

模式 14：Observer(一) Aliases ：Dependents, Publish-Subscribe Intent Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically. Motivation 把系统分成一些相互关联的类或者对象，如何维护这些类的实例一致性？ The key objects in this pattern are subject and observer One-to-many relationship A subject may have any number of dependent observers. All observers are notified whenever the subject undergoes a change in state.

Observer模式(二) Applicability：Use the Observer pattern when : When an abstraction has two aspects, one dependent on the other. Encapsulating these aspects in separate objects lets you vary and reuse them independently. When a change to one object requires changing others, and you don't know how many objects need to be changed. When an object should be able to notify other objects without making assumptions about who these objects are. In other words, you don't want these objects tightly coupled.

Observer模式(三) Struct Participants Subject、ConcreteSubject、Observer、ConcreteObserver

Observer模式(四) Collaborations

Observer模式(五) Evaluation Implementation Abstract coupling between Subject and Observer Support for broadcast communication Unexpected updates Implementation Mapping subjects to their observers. Observing more than one subject Who triggers the update? Client or subject？ Making sure Subject state is self-consistent before notification subject向observer传递变化信息 中间插入ChangeManager

Observer模式(六) Related Patterns Examples Mediator：用Mediator模式封装复杂的更新语义 COM property page COM+ Event Model， MVC

模式 15：Strategy(一) Aliases ：Policy Intent Motivation Define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients that use it. Motivation 有些算法对于某些类是必不可少的，但是不适合于硬编进类中。客户可能需要算法的多种不同实现，允许增加新的算法实现或者改变现有的算法实现 我们可以把这样的算法封装到单独的类中，称为strategy

Strategy模式(二) Applicability：Use the Strategy pattern when : many related classes differ only in their behavior. you need different variants of an algorithm. an algorithm uses data that clients shouldn't know about. a class defines many behaviors, and these appear as multiple conditional statements in its operations.

Strategy模式(三) Struct Participants Collaborations Strategy、ConcreteStrategy、Context Collaborations Strategy and Context interact to implement the chosen algorithm A context forwards requests from its clients to its strategy

Strategy模式(四) Evaluation Families of related algorithms An alternative to subclassing Strategies eliminate conditional statements Clients must be aware of different Strategies Communication overhead between Strategy and Context Increased number of objects Implementation Defining the Strategy and Context interfaces Strategies as template parameters Making Strategy objects optional

Strategy模式(五) Related Patterns Examples flyweight：考虑用flyweight模式来实现strategy对象 Examples ATL中COM对象的线程模型支持

模式 16：Visitor(一) Intent Motivation Represent an operation to be performed on the elements of an object structure. Visitor lets you define a new operation without changing the classes of the elements on which it operates. Motivation 为了把一个操作作用于一个对象结构中，一种做法是把这个操作分散到每一个节点上。导致系统难以理解、维护和修改 把这样的操作包装到一个独立的对象(visitor)中。然后在遍历过程中把此对象传递给被访问的元素。

不用visitor的compiler例子 Operation Class TypeCheck GenerateCode PrettyPrint VariableRefNode AssignmentNode ...

使用visitor的compiler例子 class Class operation VariableRefNode AssignmentNode TypeCheckVisitor VisitVariableRef VisitAssignment GenerateCodeVisitor PrettyPrintVisitor

Visitor模式(二) Applicability：Use the Visitor pattern when 一个对象结构包含许多对象类，我们想执行一些依赖于具体类的操作 要对一个对象结构中的对象进行很多不同的并且不相关的操作，又不想改变这些对象类 定义对象结构的类很少改变，但是经常要在此结构上定义新的操作。改变对象结构类，需要重定义所有visitor的接口

Visitor模式(三) Struct

Visitor模式(四) Participants Collaborations Client、Visitor、ConcreteVisitor、ObjectStructure、Element、ConcreteElement Collaborations client先创建一个ConcreteVisitor，然后遍历ObjectStructure

Visitor模式(五) Evaluation Implementation Visitor makes adding new operations easy A visitor gathers related operations and separates unrelated ones Adding new ConcreteElement classes is hard. 即使不是类层次，visitor也可以实施 状态累积 visitor要访问每个元素的状态，所以要打破封装 Implementation double-dispatch，Accept实现了double dispatch This is the key to the Visitor pattern: The operation that gets executed depends on both the type of Visitor and the type of Element it visits. Who is responsible for traversing the object structure?

Visitor模式(六) Related Patterns Composite：visitor常常被用于composite模式组成的结构中 Examples 编译器实现

其他Behavioral Patterns Chain of Responsibility 请求的处理过程，沿着链传递，decouple发送和接收方 Interpreter 在类层次结构中，在特定环境的“interpret”过程 Mediator 用一个mediator来decouple各同等单元 Memento 在对象之外保存对象的内部状态 State 把一个对象的状态独立出来，动态可变换状态对象的类型 Template Method 在基类中定义算法的骨架，把某些细节延迟到子类中

Behavioral Patterns小结 Strategy、Iterator、Mediator、State、command 用一个对象来封装某些特性，比如变化、交互、状态、行为、命令 Mediator、Observer Observer建立起subject和observer之间的松耦合连接 mediator把约束限制集中起来，-〉中心控制 command、Chain of Responsibility、interpreter command模式侧重于命令的总体管理 Chain of Responsibility侧重于命令被正确处理 interpreter用于复合结构中操作的执行过程