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設計模式之旅：策略模式完整指南與五種實作方式

編輯：關於C#

摘要

Head First Design Patterns 以策略模式作為首個範例，陳俊杉教授授課也以此為起點，可見策略模式極適合初學者入門。

意圖

定義一族演算法，將各演算法封裝並可互換使用，且不影響外界使用者。

UML 表示

GoF 稱策略模式亦為 Policy，作者偏好稱之為 Plugin，因可動態替換演算法，如同 Eclipse 插件系統。

演進過程

原本 Grapher 只能畫正方形。
初始類別圖

需求變更：需支援三角形、圓形，未來可能擴充橢圓、菱形等。若在 Grapher 持續新增 drawTriangle()、drawCircle() 等方法，違反開放封閉原則 (OCP)。依 DP 第三守則：「識別會變的部分並將其從不變的部分分離」，將變動的繪圖行為封裝為類別，並定義介面 IShape 作為溝通標準。
策略模式應用後類別圖

此架構符合 OCP：新增形狀無需修改既有程式碼，僅需實作 IShape。.NET Framework 與 STL 大量採用策略模式。

簡述

策略模式就是將會變動的成員函數用類別封裝，變成物件「掛」在原本類別上。

五種實作方式

1. ISO C++：介面版

/**/*
 (C) OOMusou 2007 https://oomusou.cnblogs.com

Filename : DP_StrategyPattern_Classic.cpp
Compiler : Visual C++ 8.0 / BCB 6.0 / gcc 3.4.2 / ISO C++
Description : Demo how to use Strategy Pattern
Release : 03/26/2007 1.0 / 07/10/2007 2.0
*/
#include <iostream>
using namespace std;

class IDrawStrategy {
public:
 virtual void draw() const = 0;
};

class Grapher {
public:
 Grapher(IDrawStrategy* drawStrategy = 0) : _drawStrategy(drawStrategy) {}

public:
 void drawShape() const;
 void setShape(IDrawStrategy* drawStrategy);

protected:
 IDrawStrategy* _drawStrategy;
};

void Grapher::drawShape() const {
 if (_drawStrategy) _drawStrategy->draw();
}

void Grapher::setShape(IDrawStrategy* drawStrategy) {
 _drawStrategy = drawStrategy;
}

class Triangle : public IDrawStrategy {
public: void draw() const;
};
void Triangle::draw() const { cout << "Draw Triangle" << endl; }

class Circle : public IDrawStrategy {
public: void draw() const;
};
void Circle::draw() const { cout << "Draw Circle" << endl; }

class Square : public IDrawStrategy {
public: void draw() const;
};
void Square::draw() const { cout << "Draw Square" << endl; }

int main() {
 Grapher grapher(&Square());
 grapher.drawShape();
 grapher.setShape(&Circle());
 grapher.drawShape();
}

執行結果：Draw Square / Draw Circle
第 67、70 行展現策略模式之美：動態替換演算法如同 Plugin，擴充新形狀只需新增實作 IDrawStrategy 的類別，符合 OCP。

2. C#：介面版

/**/*
 (C) OOMusou 2007 https://oomusou.cnblogs.com

Filename : DP_StrategyPattern_Classic.cs
Compiler : Visual Studio 2005 / C# 2.0
Description : Demo how to implement Strategy Pattern by C#
Release : 07/08/2007 1.0
*/
using System;

interface IDrawStrategy { void draw(); }

class Grapher {
 private IDrawStrategy _drawStrategy = null;
 public Grapher() {}
 public Grapher(IDrawStrategy drawStrategy) { _drawStrategy = drawStrategy; }
 public void drawShape() { if (_drawStrategy != null) _drawStrategy.draw(); }
 public void setShape(IDrawStrategy drawStrategy) { _drawStrategy = drawStrategy; }
}

class Triangle : IDrawStrategy { public void draw() { Console.WriteLine("Draw Triangle"); } }
class Circle : IDrawStrategy { public void draw() { Console.WriteLine("Draw Circle"); } }
class Square : IDrawStrategy { public void draw() { Console.WriteLine("Draw Square"); } }

class Program {
 public static void Main() {
 Grapher grapher = new Grapher(new Square());
 grapher.drawShape();
 grapher.setShape(new Circle());
 grapher.drawShape();
 }
}

執行結果：Draw Square / Draw Circle

3. ISO C++：函數指標版

Effective C++ Item 35 亦使用函數指標實作策略模式，為 C/C++ 獨有寫法。

/**/*
 (C) OOMusou 2007 https://oomusou.cnblogs.com

Filename : DP_StrategyPattern_FunctionPointer.cpp
Compiler : Visual C++ 8.0 / BCB 6.0 / gcc 3.4.2 / ISO C++
Description : Demo how to use Strategy Pattern by function pointer
Release : 03/31/2007 1.0 / 07/08/2007 2.0 / 07/10/2007 3.0
*/
#include <iostream>
using namespace std;

class Grapher {
public:
 typedef void (*pfDraw)();
 Grapher(pfDraw draw = 0) : _draw(draw) {}

public:
 void drawShape() const;
 void setShape(pfDraw draw);

protected:
 pfDraw _draw;
};

void Grapher::drawShape() const { if (_draw) _draw(); }
void Grapher::setShape(pfDraw draw) { _draw = draw; }

class Triangle { public: static void draw(); };
void Triangle::draw() { cout << "Draw Triangle" << endl; }
class Circle { public: static void draw(); };
void Circle::draw() { cout << "Draw Circle" << endl; }
class Square { public: static void draw(); };
void Square::draw() { cout << "Draw Square" << endl; }

int main() {
 Grapher grapher(Square::draw);
 grapher.drawShape();
 grapher.setShape(Circle::draw);
 grapher.drawShape();
}

本質上將介面定義函數簽名，改由 typedef void (*pfDraw)() 定義函數指標型別。

4. C#：委派版

委派是 C# 對函數指標與觀察者模式的實作，理應可用於策略模式。

/**/*
 (C) OOMusou 2007 https://oomusou.cnblogs.com

Filename : DP_StrategyPattern_Delegate.cs
Compiler : Visual Studio 2005 / C# 2.0
Description : Demo how to implement Strategy Pattern by C# delegate
Release : 07/08/2007 1.0
*/
using System;

class Grapher {
 private DrawDelegate _drawDelegate = null;
 public delegate void DrawDelegate();
 public Grapher() {}
 public Grapher(DrawDelegate drawDelegate) { _drawDelegate = drawDelegate; }
 public void drawShape() { if (_drawDelegate != null) _drawDelegate(); }
 public void setShape(DrawDelegate drawDelegate) { _drawDelegate = drawDelegate; }
}

class Triangle { public static void draw() { Console.WriteLine("Draw Triangle"); } }
class Circle { public static void draw() { Console.WriteLine("Draw Circle"); } }
class Square { public static void draw() { Console.WriteLine("Draw Square"); } }

class Program {
 public static void Main() {
 Grapher grapher = new Grapher(Square.draw);
 grapher.drawShape();
 grapher.setShape(Circle.draw);
 grapher.drawShape();
 }
}

執行結果：Draw Square / Draw Circle

5. ISO C++：Template 版

GoF 《Design Patterns》亦展示 Template 實作策略模式。

/**/*
 (C) OOMusou 2007 https://oomusou.cnblogs.com

Filename : DP_StrategyPattern_template.cpp
Compiler : Visual C++ 8.0 / BCB 6.0 / gcc 3.4.2 / ISO C++
Description : Demo how to use Strategy Pattern by template
Release : 03/31/2007 1.0
*/
#include <iostream>
using namespace std;

template <typename T>
class Grapher {
private: T _drawTemplate;
public: void drawShape() const;
};

template<typename T>
void Grapher<T>::drawShape() const { _drawTemplate.draw(); }

class Triangle { public: void draw() const; };
void Triangle::draw() const { cout << "Draw Triangle" << endl; }
class Circle { public: void draw() const; };
void Circle::draw() const { cout << "Draw Circle" << endl; }
class Square { public: void draw() const; };
void Square::draw() const { cout << "Draw Square" << endl; }

int main() {
 Grapher<Square> grapher; grapher.drawShape();
 Grapher<Circle> grapher2; grapher2.drawShape();
}

執行結果：Draw Square / Draw Circle
差異：OOP 介面 + 動態繫結屬執行期完成，優點可動態改變，缺點速度較慢；GP Template 屬編譯期完成，優點速度快，缺點無法執行期動態改變，故無 setShape()，策略需在實例化時指定。

6. C#：Generics 版

C# 2.0 提供泛型，同樣可實作策略模式。

/**/*
 (C) OOMusou 2007 https://oomusou.cnblogs.com

Filename : DP_StrategyPattern_Generics.cs
Compiler : Visual Studio 2005 / C# 2.0
Description : Demo how to implement Strategy Pattern by C# Generics
Release : 07/08/2007 1.0
*/
using System;

interface IDrawStrategy { void draw(); }

class Grapher<T> where T : class, IDrawStrategy, new() {
 private T _drawStrategy = default(T);
 public Grapher() { _drawStrategy = new T(); }
 public void drawShape() { if (_drawStrategy != null) _drawStrategy.draw(); }
 public void setShape(T drawStrategy) { _drawStrategy = drawStrategy; }
}

class Triangle : IDrawStrategy { public void draw() { Console.WriteLine("Draw Triangle"); } }
class Circle : IDrawStrategy { public void draw() { Console.WriteLine("Draw Circle"); } }
class Square : IDrawStrategy { public void draw() { Console.WriteLine("Draw Square"); } }

class Program {
 public static void Main() {
 Grapher<Square> grapher = new Grapher<Square>(); grapher.drawShape();
 Grapher<Circle> grapher2 = new Grapher<Circle>(); grapher2.drawShape();
 }
}

執行結果：Draw Square / Draw Circle

備註：策略 vs 模板方法

兩者目的相同，手法不同：策略用物件委派（執行期可變），模板方法用類別繼承（編譯期決定）。
GoF 原文：Template methods use inheritance to vary part of an algorithm. Strategies use delegation to vary the entire algorithm.