设计模式学习笔记（二十三）——Strategy策略

 

二十一、Strategy（策略）

情景举例：

定义一系列的算法,把它们一个个封装起来, 并且使它们可相互替换。本模式使得算法可独立于使用它的客户而变化。

代码示例：

/* 使用Strategy的类（ok，客户）

*/

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

};

/* Strategy的抽象父类

*/

class Compositor {

public:

    virtual int Compose(

        Coord natural[], Coord stretch[], Coord shrink[],

        int componentCount, int lineWidth, int breaks[]

    ) = 0;

protected:

    Compositor();

};

/*

*/

void Composition::Repair () {

    Coord* natural;

    Coord* stretchability;

    Coord* shrinkability;

    int componentCount;

    int* breaks;

    // prepare the arrays with the desired component sizes

    // ...

/*

*/

    // kills comiler warnings

    natural = coords;

    stretchability = coords;

    shrinkability = coords;

    componentCount = 1;

    breaks = b;

/* 注意这里：使用了Strategy模式。调用了多态的Compose算法

*/

    // determine where the breaks are:

    int breakCount;

    breakCount = _compositor->Compose(

        natural, stretchability, shrinkability,

        componentCount, _lineWidth, breaks

    );

/*

*/

    // lay out components according to breaks

    // ...

}

/* 下面是Compositor的子类，他们都分别实现了Compose算法

*/

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[]

    );

    // ...

};

/*

*/

Composition* quick = new Composition(new SimpleCompositor);

Composition* slick = new Composition(new TeXCompositor);

Composition* iconic = new Composition(new ArrayCompositor(100));

/*

*/

个人理解：

Strategy模式感觉上要比State模式更简单。它们都是在不更改客户类的继承层次的前提下，达到了多态的目的。Strategy中需要注意的是，定义的算法抽象类函数（例子中的Compose函数）的申明一定要适用于所有子类。