设计模式进阶(一) 策略模式

摘自<Design Paterns_Elements of Reusable Object-Oriented Software>

上一系列偏重于入门，从本篇开启进阶系列，着重于设计模式的适用情景。

回顾入门系列 设计模式入门(一)  策略模式

1  Intent

  Define a family of algorithms, encapsulate each one, and make them interchangeable.

  Strategy lets the algorithm vary independently from clients that use it.

2  Applicability

1)  many related classes differ only in their behavior

　　= strategies provide a way to configure a class with one of many behaviors

2)  you need different variants of an algorithm

　　= strategies can be used when these variants are implemented as a class hierarchy of algorithms

3)  an algorithm uses data that clients should not know about

　　= use strategies to avoid exposing complex, algorithm-specific data structures

4)  a class defines many behaviors, and these appear as multiple conditional statements in its operations

　　= move related conditional branches into their own strategy class

3  Sample

  many algorithms exist for breaking a stream of text into lines, and hard-wiring such algorithms into the class that require them is not desirable.

  C++ 实例：

1) class Composition

/* Composition class maintains a collection of Component instances */class Composition {public:  Composition(Compositor*);void Repair();private:  Compositor* _compositor;  Component* _components;  // the list of components  int _componentCount;  // the number of components  int _lineWidth;    // the Composition's line width  int* _lineBreaks;  // the position of linebreaks in components  int _lineCount;    // the number of lines};void Composition::Repair () {  Coord* natural;  Coord* stretchability;  Coord* shrinkability;  int componentCount;  int* breaks;  // prepare the arrays with the desired component sizes  // ...  // determine where the breaks are:  int breakCount;  breakCount = _compositor->Compose(    natural, stretchability, shrinkability,    componentCount, _lineWidth, breaks  );  // lay out components according to breaks  // ...}

2) class Compositor and its subclasses

/* Compositor is an abstract class(also interface) */class Compositor{public:  virtual int Compose(    Coord natural[], Coord stretch[],Coord shrink[],    int componentCount, int lineWidth, int breaks[]  ) = 0;protected:  Compositor();};/* Three subclasses */class SimpleCompositor : public Compositor {public:  SimpleCompositor();    virtual int Compose(    Coord natural[], Coord stretch[], Coord shrink[],    int componentCount, int lineWidth, int breaks[]  );  // ...};class TeXCompositor : public Compositor {public:  TeXCompositor();  virtual int Compose(    Coord natural[], Coord stretch[], Coord shrink[],    int componentCount, int lineWidth, int breaks[]  );  // ...};class ArrayCompositor : public Compositor{public:  ArrayCompositor(int interval);    virtual int Compose(    Coord natural[], Coord stretch[], Coord shrink[],    int componentCount, int lineWidth, int breaks[]  );  // ...};

3) instantiation

Composition* quick = new Composition(new SimpleCompositor);Composition* slick = new Composition(new TeXCompositor);Composition* iconic = new Composition(new ArrayCompositor(100));

原标题：设计模式进阶(一) 策略模式

关键词：设计模式