【Head First 模式设计】第1章 策略模式
2024-05-16 04:14:14
Intro to Design Patterns
Welcome to Design Patterns
SimUDuck应用程序
一个模拟鸭子游戏：SimUDuck。游戏中会出现各种鸭子，一边游泳，一边呱呱叫。此系统设计了一个鸭子超类 (Superclass)，并让各种鸭子继承此超类。
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每个鸭子的子类型负责实现自己的 display() 行为，在屏幕上显示其外观。MallardDuck 外观是绿头，RedheadDuck 外观是红头。
软件开发的一个不变真理：改变
问题：改变程序，让鸭子能飞。
如果在 Duck 类中添加成员 fly()，会导致所有子类都具备 fly()，连那些不该具备 fly() 的类都不能免除。
例如，橡皮鸭不会飞，叫声是吱吱叫。
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可以覆盖橡皮鸭中的 fly() 方法，变成什么事都不做。
但是规范会不断变化，比如增加诱饵鸭，它不会飞也不会叫。这样开发人员总是需要检查，且可能重写程序中每个新的 Duck 子类的 fly() 和 quack()。
利用继承来提供 Duck 行为，会导致以下缺点：
- 代码在多个子类中重复
- 运行时的行为不容易改变
- 很难知道所有鸭子的全部行为
- 改变会牵一发动全身，造成其他鸭子不想要的改变
换一种方式，使用接口：将 fly() 从 Duck 超类中取出，并使用 fly() 方法创建 Flyable() 接口。这样，只有应该会飞的鸭子才能实现该接口并具有 fly() 方法，同理，创建 Quackable 接口。
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虽然接口可以解决部分问题——不会再有会飞的橡皮鸭，但是却造成代码无法复用。甚至，鸭子的飞行姿势可能会有多种变化，而且会出现叫声不同的情况。
设计原则一
现在已经知道，继承与接口并不能很好地解决问题。有一个设计原则，可以适用这种情况。
设计原则1：找出应用中可能需要变化之处，把它们独立出来，不要和那些不需要变化的混在一起。
下面是这个原则的另一种思考方式：把会变化的部分取出并封装起来，以便以后可以轻易地改动或扩充此部分，而不影响不需要改变的其他部分。
结果如何？代码变化引起的不经意后果变少，系统变得更有灵活性。
分开变化和不会变化的部分
将 quack() 和 fly() 两个行为从 Duck 类中分开：把它们从 Duck 类中取出来，建立一组新类来代表每个行为。
设计原则二
如何设计一组实现 fly 和 quack 行为的类呢？
让鸭子的行为变得灵活。而且要将行为赋值给 Duck 实例。
例如，实例化一个新的 MallardDuck 实例，并使用特定类型的飞行行为对其初始化。这时，可以动态地改变鸭子的行为。即，应该在 Duck 类中包含设定行为的方法，以便在运行时改变 MallardDuck 的飞行行为。
设计原则2：针对接口编程，而不是针对实现编程。
利用接口表示每个行为，比如 FlyBehavior 和 QuackBehavior，一个行为的每个实现将实现其中一个接口。
这样，Duck 类不需要知道它们自己行为的实现细节。
“针对接口编程”真正的意思是“针对超类型编程”
“针对接口编程”的关键是利用多态，这可通过对超类型编程来实现，以使实际的运行时对象不会被锁定在代码中。
可以将“针对超类型编程”改写为“变量的声明类型应该是超类型，通常是抽象类或接口，以便赋值给这些变量的对象可以是超类型的任何具体实现，这意味着声明它们的类不必知道实际的对象类型！”
例如，假设有一个抽象类 Animal，它有两个具体的实现类 Dog 和 Cat。
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.edge-pattern-dashed{stroke-dasharray:3}#mermaid-svg-tAqaUW9c2m5tTblh .edge-pattern-dotted{stroke-dasharray:2}#mermaid-svg-tAqaUW9c2m5tTblh .marker{fill:#333}#mermaid-svg-tAqaUW9c2m5tTblh .marker.cross{stroke:#333}:root { --mermaid-font-family: "trebuchet ms", verdana, arial;}#mermaid-svg-tAqaUW9c2m5tTblh {color: rgba(0, 0, 0, 0.75);font: ;}DogmakeSound() //bark()bark()AnimalmakeSound()CatmakeSound() //meow()meow()
针对实现编程：
Dog *d = new Dog();
d->bark();
针对接口/超类型编程：
Animal *animal = new Dog();
animal->makeSound();
知道对象是 Dog，但可以动态地使用 Animal 进行调用。
更好的是，与其将子类型的实例化 (例如 new Dog()) 硬编码到代码中，不如在运行时赋值具体的实现对象：
a = getAnimal();
a->makeSound();
不知道实际的 Animal 子类型，只关心它知道如何正确进行 makeSound() 的动作。
实现鸭子的行为
两个接口 FlyBehavior 和 QuackBehavior，以及它们对应的实现每个具体行为的类。
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使用这种设计，对象的其他类型可以复用 fly 和 quack 行为，因为这些行为不再隐藏在 Duck 类中。
而且，可以添加新的行为，而无需修改任何现有的行为类，也不会影响到任何使用 fly 行为的 Duck 类。
整合鸭子的行为
关键在于，Duck 现在将委托 (delegate) 其飞行和呱呱叫行为，而不是使用 Duck 类 (或子类) 中定义的 fly 和 quack 方法。
做法：
- 首先，在 Duck 类中添加两个变量 flyBehavior 和 quackBehavior，它们被声明为接口类型。每个鸭子对象都将多态设置这些变量，以在运行时引用其想要的特定行为类型 (如 FlyWithWings)。
从 Duck 类 (和所有子类) 中删除 fly() 和 quack() 方法，因为这些行为已被移到了 FlyBehavior 和 QuackBehavior 类中。
用两种类似的方法 performDly() 和 performQuack() 替换 Duck 类中的 fly() 和 quack()。
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.edge-pattern-dashed{stroke-dasharray:3}#mermaid-svg-E98P9jtUOY1g0S4V .edge-pattern-dotted{stroke-dasharray:2}#mermaid-svg-E98P9jtUOY1g0S4V .marker{fill:#333}#mermaid-svg-E98P9jtUOY1g0S4V .marker.cross{stroke:#333}:root { --mermaid-font-family: "trebuchet ms", verdana, arial;}#mermaid-svg-E98P9jtUOY1g0S4V {color: rgba(0, 0, 0, 0.75);font: ;}Duck#flyBehavior : FlyBehavior#quackBehavior : QuackBehavior+performQuack()+swim()+display()+performFly()
- 现在，实现 performQuack()。
为了执行呱呱叫动作，Duck 只需允许 quackBehavior 引用的对象为它呱呱叫。
在下面这段代码中，我们不关心对象的类型是什么，我们关心的是它知道如何 quack()！
class Duck {protected:std::unique_ptr<QuackBehavior> quackBehavior;// more
public:void performQuack() {// Duck对象本身并不处理呱呱叫行为，而是将这个行为委托给quackBehavior引用的对象quackBehavior->quack();}
};
- 如何设定 flyBehavior 和 quackBehavior 实例变量？以 MallardDuck 类为例：
class MallardDuck : public Duck {public:MallardDuck() {quackBehavior = std::unique_ptr<QuackBehavior>(new Quack());flyBehavior = std::unique_ptr<FlyBehavior>(new FlyWithWings());}void display() const {std::cout << "I'm a real Mallard duck" << std::endl;}
};
当 MallardDuck 被实例化时，其构造函数将 MallardDuck 的继承的 quackBehavior 实例变量初始化为 Quack 类型 (QuackBehavior 的具体实现类) 的新实例。
测试 Duck 的代码
// Duck.h
#include <iostream>
#include <memory>
#include "FlyBehavior"
#include "QuackBehavior"class Duck {protected:std::unique_ptr<FlyBehavior> flyBehavior;std::unique_ptr<QuackBehavior> quackBehavior;public:Duck() { }virtual void display() const = 0;void performFly() const {flyBehavior->fly();}void performQuack() const {quackBehavior->quack();}void swim() const {std::cout << "All ducks float, even decoys!" << std::endl;}
};class MallardDuck : public Duck {public:MallardDuck() {quackBehavior = std::unique_ptr<QuackBehavior>(new Quack());flyBehavior = std::unique_ptr<FlyBehavior>(new FlyWithWings());}void display() const {std::cout << "I'm a real Mallard duck" << std::endl;}
};****************************************************************************
// FlyBehavior.h
#include <iostream>
#include <memory>class FlyBehavior {public:virtual void fly () const = 0;
};class FlyWithWings : public FlyBehavior {public:void fly () const {std::cout << "I'm flying!!" << std::endl;}
};class FlyNoWay : public FlyBehavior {public:void fly () const {std::cout << "I can't fly" << std::endl;}
};****************************************************************************
// QuackBehavior.h
#include <iostream>
#include <memory>class QuackBehavior {public:virtual void quack() const = 0;
};class Quack : public QuackBehavior {public:void quack () const {std::cout << "Quack" << std::endl;}
};class MuteQuack : public QuackBehavior {public:void quack () const {std::cout << "<< Silence >>" << std::endl;}
};class Squeak : public QuackBehavior {public:void quack () const {std::cout << "Squeak" << std::endl;}
};****************************************************************************
// main.cpp
#include <iostream>
#include <memory>
#include "Duck.h"
#include "FlyBehavior.h"
#include "QuackBehavior.h"int main() {std::unique_ptr<MallardDuck> mallard(new MallardDuck());mallard->performQuack();mallard->performFly();system("pause");return 0;
}
运行代码：
动态设定行为
可以通过鸭子子类上的设定方法来设定鸭子的行为类型，而不只是通过在鸭子的构造函数中实例化鸭子的行为类型。
- 在 Duck 类中，添加两个方法。
可以随时调用下面两个方法改变鸭子的行为。
public: void setFlyBehavior(FlyBehavior* fb) {flyBehavior = std::unique_ptr<FlyBehavior>(fb);}public: void setQuackBehavior(QuackBehavior* qb) {quackBehavior = std::unique_ptr<QuackBehavior>(qb);}
- 创建一个新的 Duck 类型 ModelDuck。
class ModelDuck : public Duck {public:ModelDuck() {quackBehavior = std::unique_ptr<QuackBehavior>(new Quack());flyBehavior = std::unique_ptr<FlyBehavior>(new FlyNoWay()); // 一开始，模型鸭是不会飞的}void display() const {std::cout << "I'm a model duck" << std::endl;}
};
- 创建一个新的 FlyBehavior 类型 FlyRocketPowered。
// 创建一个利用火箭动力飞行的行为
class FlyRocketPowered : public FlyBehavior {public:void fly () const {std::cout << "I'm flying with a rocket" << std::endl;}
};
- 在 main 函数中增加 ModelDuck，并使模型鸭具有火箭动力。
std::unique_ptr<ModelDuck> model(new ModelDuck());model->performFly(); model->setFlyBehavior(new FlyRocketPowered());model->performFly();
运行程序：
封装行为的大局观
现在看看整体的格局。下面是整个重新设计的类结构。
注意，现在开始对事物的描述有所不同。不再将鸭子行为视为一组行为 (set of behaviors)，而是将它们视为一族算法 (family of behaviors)。想想看：在 SimUDuck 设计中，算法代表鸭子会做的事情 (呱呱叫或飞行的不同方式)，这样的技术可以很容易地用于一组类，用来实现计算不同州的销售税金。
注意类之间的关系 (relationships)。
在类图中的每个箭头上标上适当的关系：IS-A (是一个)、HAS-A (有一个) 和 IMPLEMENTS (实现)。
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设计原则三
HAS-A 可能比 IS-A 更好
HAS-A 关系：每个鸭子有一个 FlyBehavior 和 QuackBehavior，可以将飞行和呱呱叫行为委托给它们。
将两个类像这样结合起来，就是组合 (composition)。鸭子获取行为不是通过继承行为，而是通过与适当的行为对象进行组合。
这是一个很重要的技术，使用了下面的设计原则。
设计原则3：优先考虑组合，而非继承。
使用组合建立系统可以带来更大的灵活性。它不仅可以将算法族封装到它们自己的类的集合中，而且还可以在运行时改变行为，只要组合的对象实现了正确的行为接口即可。
策略模式
上面的应用程序使用了策略模式 (Strategy Pattern)。
策略模式定义了一族算法，将每个算法封装起来，并使它们之间可互相替换。策略模式使算法的变化可以独立于使用该算法的客户。
共享模式词汇的力量
设计模式为你提供了与其他开发人员共享的词汇表。掌握了这些词汇后，你可以更轻松地与其他开发人员进行交流。通过在模式级别进行思考，而不是在具体细节的对象级别进行思考，还可以提高你对架构的思考。
当你使用模式与他人沟通时，你做的不只是在与他人共享“行话”。
- 共享的模式词汇是强有力的。当你使用模式与其他开发人员或团队交流时，你们交流的不只是模式名称，还是模式所代表的一整套质量、特性和约束。
“使用策略模式来实现鸭子的各种行为。” 这告诉你，鸭子行为已封装到它自己的一组类中，可以被轻松地扩展和更改，甚至在需要时也可以在运行时进行更改。 - 模式让你使用更少的话语表述更多的内容。当你在描述中使用模式时，其他开发人员会快速准确地了解你头脑中的设计。
- 在模式级别进行交谈可以使你待“在设计中”的时间更长。讨论使用模式的软件系统使你可以将讨论保持在设计级别，而不必深入了解实现对象和类的具体细节。
- 共享的词汇可以为你的开发团队提供强大的动力。精通设计模式的团队可以更快地行动，减少误解的空间。
- 共享的词汇表鼓励更多的初级开发人员快速上手。初级开发人员仰赖经验丰富的开发人员。当高级开发人员使用设计模式时，初级开发人员也会开始学习它们。在你的组织中建立模式使用者的社区。
如何使用设计模式
我们都使用过现成的库和框架。我们使用它们，针对它们的API编写一些代码，将它们编译到我们的程序中。库和框架对开发模型大有帮助，我们可以选择组件并直接将其放入适当的位置。但是，它们不能帮助我们以更易于理解、更易于维护和更灵活的方式来构建应用程序。这就是设计模式的用武之地。
设计模式不会直接进入你的代码，而是会首先进入你的大脑。一旦你在大脑中装载了许多关于模式的运用知识，就可以开始将其应用于新设计，并在发现旧代码变得混乱没有灵活性时对其进行重新处理。
设计模式比库的等级更高。设计模式告诉我们如何构建类和对象来解决某个问题。
框架和库不是设计模式；它们提供特定的实现，可以应用到我们的代码中。有时，库和框架会在实现中使用模式设计。一旦理解了设计模式，你会更快地理解围绕着设计模式构建的API。
为什么要使用设计模式
知道抽象、继承和多态这些概念不会让你成为一个好的面向对象的设计者。设计大师考虑如何创建灵活的设计，它是可维护的，可以应对变化。
如果找不到设计模式怎么办？
构成模式的基础是一些面向对象的原理，当找不到适合的模式解决问题时，这些原理会为你提供帮助。
创建可维护的OO系统的秘诀之一就是思考它们将来如何变化，这些原则解决了这些问题。
设计工具箱内的工具
OO基础：抽象、封装、多态、继承
OO原则：
- 将变化的部分封装起来
- 多用组合，少用继承
- 针对接口编程，不针对实现编程
OO模式：
- 策略模式
要点：
- 好的OO设计是可复用的、可扩充的、可维护的。
- 模式是经过验证的面向对象的经验。
- 模式不会为你提供代码，而是为你提供设计问题的通用解决方案。
- 大多数模式和原则都应对软件变化问题。
- 大多数模式都允许系统的某些部分的变化独立于所有其他部分。
- 我们经常尝试取出系统中的变化部分，并将其封装。
【Head First 模式设计】目录
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