设计方式(3)-策略模式(Strategy)

设计模式(3)-策略模式(Strategy)

【描述】策略模式本质上利用的是面向对象的多态特性，构建者不必自身包含实现的逻辑，而是根据需要利用其他对象中的算法。

【UML图】

图1 UML图

(1) 定义了一个Methods基类，包含一个策略的接口MethodsInterface。

(2) 定义了MethodsStrategyA、MethodsStrategyB两种策略。

(3) 定义了一个策略构建者Context，包含ContextInterface方法。

 

【示例代码】

methods.h

#ifndef METHODS_H #define METHODS_H class Methods { public: Methods(); public: virtual void MethodsInterface(); }; #endif // METHODS_H

methods.cpp

#include <QDebug> #include "methods.h" Methods::Methods() { qDebug()<<"construct Methods"; } void Methods::MethodsInterface() { qDebug()<<"Methods::MethodsInterface"; }

methodstrategya.h

#ifndef METHODSTRATEGYA_H #define METHODSTRATEGYA_H #include "methods.h" class MethodStrategyA : public Methods { public: MethodStrategyA(); public: void MethodsInterface(); }; #endif // METHODSTRATEGYA_H

methodstrategya.cpp

#include <QDebug> #include "methodstrategya.h" MethodStrategyA::MethodStrategyA() { qDebug()<<"construct MethodStrategyA"; } void MethodStrategyA::MethodsInterface() { qDebug()<<"Strategy A"; }

methodstrategyb.h

#ifndef METHODSTRATEGYB_H #define METHODSTRATEGYB_H #include "methods.h" class MethodStrategyB : public Methods { public: MethodStrategyB(); public: void MethodsInterface(); }; #endif // METHODSTRATEGYB_H

methodstrategyb.cpp

#include <QDebug> #include "methodstrategyb.h" MethodStrategyB::MethodStrategyB() { qDebug()<<"construct MethodStrategyB"; } void MethodStrategyB::MethodsInterface() { qDebug()<<"Strategy B"; }

context.h

#ifndef CONTEXT_H #define CONTEXT_H #include "methods.h" class Context { public: Context(Methods *method); private: Methods *_method; public: void ContextInterface(); }; #endif // CONTEXT_H

context.cpp

#include "context.h" Context::Context(Methods *method) { _method = method; } void Context::ContextInterface() { _method->MethodsInterface(); }

main.cpp

#include <QDebug> #include "context.h" #include "methods.h" #include "methodstrategya.h" #include "methodstrategyb.h" int main() { Context *context; context = new Context(new MethodStrategyA); context->ContextInterface(); context = new Context(new MethodStrategyB); context->ContextInterface(); return 0; }

 

【运行结果】

construct Methods construct MethodStrategyA Strategy A construct Methods construct MethodStrategyB Strategy B

【结果分析】

利用多态特性，实现了不同策略的调用，由于是测试代码，本身很简单，省略了析构函数。

 

【实例剖析】

*本例来源于《重构-改善既有代码的设计》第1章。原例采用Java语言编写，本例利用Qt C++进行了改写。

看下面一个关于电影租赁计价的例子。

movie.h

#ifndef MOVIE_H #define MOVIE_H #include <QString> enum {REGULAR = 0,NEW_RELEASE = 1,DISCOUNT = 2}; class Movie { public: Movie(); Movie(QString title, int priceCode); private: QString _title; int _priceCode; public: QString getTitle(); int getPriceCode(); void setPriceCode(int arg); }; #endif // MOVIE_H

movie.cpp

#include <QDebug> #include "movie.h" Movie::Movie() { qDebug()<<"construct Movie::Movie()"; } Movie::Movie(QString title, int priceCode) { qDebug()<<QString("construct Movie::Movie(%1,%2)").arg(title).arg(priceCode); _title = title; _priceCode = priceCode; } QString Movie::getTitle() { return _title; } int Movie::getPriceCode() { return _priceCode; } void Movie::setPriceCode(int arg) { _priceCode = arg; }

rental.h

#ifndef RENTAL_H #define RENTAL_H #include "movie.h" class Rental { public: Rental(Movie movie, int daysRented); private: Movie _movie; int _daysRented; public: Movie getMovie(); int getDaysRented(); double getCharge(int daysRented); }; #endif // RENTAL_H

rental.cpp

#include <QDebug> #include "rental.h" Rental::Rental(Movie movie, int daysRented) { qDebug()<<"construct Rental"; _movie = movie; _daysRented = daysRented; } Movie Rental::getMovie() { return _movie; } int Rental::getDaysRented() { return _daysRented; } double Rental::getCharge(int daysRented) { double result = 0; qDebug()<<getMovie().getPriceCode(); switch(getMovie().getPriceCode()) { case REGULAR: result +=2; if(daysRented>2) { result += (daysRented-2)*1.5; } break; case NEW_RELEASE: result += daysRented*3; break; case DISCOUNT: result +=2; if(daysRented>2) { result += (daysRented-2)*1; } break; } qDebug()<<result; return result; }

main.cpp

#include "movie.h" #include "rental.h" int main() { int daysRented = 30; Movie movie("Book1",NEW_RELEASE); Rental rental(movie,daysRented); movie.setPriceCode(NEW_RELEASE); rental.getCharge(daysRented); return 0; }

运行结果如下：

"construct Movie::Movie(Book1,1)" construct Movie::Movie() construct Rental 1 90

示例调用了getCharge，并返回了priceCode相应的计价。相对于《重构》一书最原始代码，这段代码经过了一定的重构处理。

先看下述代码，

double Rental::getCharge(int daysRented) { double result = 0; qDebug()<<getMovie().getPriceCode(); switch(getMovie().getPriceCode()) { case REGULAR: result +=2; if(daysRented>2) { result += (daysRented-2)*1.5; } break; case NEW_RELEASE: result += daysRented*3; break; case DISCOUNT: result +=2; if(daysRented>2) { result += (daysRented-2)*1; } break; } qDebug()<<result; return result; }

这段代码，有一些不好的地方：

(1) 这段代码通过priceCode判断电影的租金，采用了switch-case语言，实质上还是面向过程的范畴，代码可重用性很低；

(2) 如果情形较多，switch-case会变得十分庞大。

 

有什么办法能去掉switch-case吗？下面通过引入策略模式实现这一目的。

先看UML图，

图2

(1) 引入了一个Price基类，提供了getCharge()接口供子类实现；

(2) 将switch-case语句中的3种情形抽取为3个Price子类；

(3) Movie调用了Price提供的接口。

 

【代码清单】

price.h

#ifndef PRICE_H #define PRICE_H class Price { public: Price(); private: int _priceCode; public: int getPriceCode(); void setPriceCode(int arg); virtual double getCharge(int daysRented); }; #endif // PRICE_H

price.cpp

#include <QDebug> #include "price.h" Price::Price() { qDebug()<<"construct Price"; _priceCode = 0; } int Price::getPriceCode() { qDebug()<<_priceCode; return _priceCode; } void Price::setPriceCode(int arg) { _priceCode = arg; } double Price::getCharge(int daysRented) { qDebug()<<"Price::getCharge"; return daysRented; }

regularprice.h

#ifndef REGULARPRICE_H #define REGULARPRICE_H #include "price.h" class RegularPrice : public Price { public: RegularPrice(); public: double getCharge(int daysRented); }; #endif // REGULARPRICE_H

regularprice.cpp

#include <QDebug> #include "regularprice.h" RegularPrice::RegularPrice() { qDebug()<<"construct RegularPrice"; } double RegularPrice::getCharge(int daysRented) { qDebug()<<"RegularPrice::getCharge"; double result = 2; if(daysRented>2) { result += (daysRented-2)*1.5; } return result; }

newreleaseprice.h

#ifndef NEWRELEASEPRICE_H #define NEWRELEASEPRICE_H #include "price.h" class NewReleasePrice : public Price { public: NewReleasePrice(); public: double getCharge(int daysRented); }; #endif // NEWRELEASEPRICE_H

newreleaseprice.cpp

#include <QDebug> #include "newreleaseprice.h" NewReleasePrice::NewReleasePrice() { qDebug()<<"construct NewReleasePrice"; } double NewReleasePrice::getCharge(int daysRented) { qDebug()<<"NewReleasePrice::getCharge"; return daysRented*3;; }

discountprice.h

#ifndef DISCOUNTPRICE_H #define DISCOUNTPRICE_H #include "price.h" class DiscountPrice : public Price { public: DiscountPrice(); public: double getCharge(int daysRented); }; #endif // DISCOUNTPRICE_H

discountprice.cpp

#include <QDebug> #include "discountprice.h" DiscountPrice::DiscountPrice() { qDebug()<<"construct DiscountPrice"; } double DiscountPrice::getCharge(int daysRented) { qDebug()<<"DiscountPrice::getCharge"; double result = 2; if(daysRented>2) { result += daysRented; } return result; }

movie.h

#ifndef MOVIE_H #define MOVIE_H #include <QString> #include "price.h" enum {REGULAR = 0,NEW_RELEASE = 1,DISCOUNT = 2}; class Movie { public: Movie(); Movie(QString title, int priceCode); ~Movie(); private: QString _title; int _priceCode; Price *_price; public: QString getTitle(); int getPriceCode(); void setPriceCode(int arg, Price *_price); double getCharge(int daysRented); }; #endif // MOVIE_H

movie.cpp

#include <QDebug> #include "movie.h" #include "regularprice.h" #include "newreleaseprice.h" #include "discountprice.h" Movie::Movie() { qDebug()<<"construct Movie::Movie()"; } Movie::Movie(QString title, int priceCode) { qDebug()<<QString("construct Movie::Movie(%1,%2)").arg(title).arg(priceCode); _price = new Price; _title = title; _priceCode = priceCode; } Movie::~Movie() { delete _price; } QString Movie::getTitle() { return _title; } int Movie::getPriceCode() { return _price->getPriceCode(); } void Movie::setPriceCode(int arg, Price *price) { _price = price; _price->setPriceCode(arg); } double Movie::getCharge(int daysRented) { return _price->getCharge(daysRented); }

rental.h

#ifndef RENTAL_H #define RENTAL_H #include "movie.h" class Rental { public: Rental(Movie movie, int daysRented); private: Movie _movie; int _daysRented; public: Movie getMovie(); int getDaysRented(); double getCharge(int daysRented); }; #endif // RENTAL_H

rental.cpp

#include <QDebug> #include "rental.h" Rental::Rental(Movie movie, int daysRented) { qDebug()<<"construct Rental"; _movie = movie; _daysRented = daysRented; } Movie Rental::getMovie() { return _movie; } int Rental::getDaysRented() { return _daysRented; } double Rental::getCharge(int daysRented) { return _movie.getCharge(daysRented); }

main.cpp

#include <QDebug> #include "movie.h" #include "rental.h" #include "regularprice.h" #include "newreleaseprice.h" #include "discountprice.h" int main() { int daysRented = 30; Movie movie("Book1",NEW_RELEASE); Rental rental(movie,daysRented); movie.setPriceCode(NEW_RELEASE, new NewReleasePrice); rental.getCharge(daysRented); return 0; }

【运行结果】

"construct Movie::Movie(Book1,1)" construct Price construct Movie::Movie() construct Rental construct Price construct NewReleasePrice NewReleasePrice::getCharge

【结果分析】

movie.setPriceCode(NEW_RELEASE, new NewReleasePrice);

传入priceCode和Price子对象作为参数，从运行结果可以看出，调用了NewReleasePrice的getCharge方法。

应用策略模式后，Rental::getCharge中仅有一行代码。

double Rental::getCharge(int daysRented) { return _movie.getCharge(daysRented); }

将原来的getCharge做了搬移处理，而Movie::getCharge也很简短：

double Movie::getCharge(int daysRented) { return _price->getCharge(daysRented); }

 

*应用策略模式后，消除了switch-case语句，函数更为简洁。将Price单独抽出，成为一个类组，提高了代码的可重用性。

 

【学习参考】

(1) 设计模式之三 --- 策略模式(Strategy Pattern)

 

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