Flyweight(享元）--對象結構型模式

1.意圖

    運用共用技術有效地支持大量細粒度的對象。

2.動機

    Flyweight模式描述瞭如何共用對象，使得可以細粒度地使用它們，而無需高昂的代價。flyweight是一個共用對象，它可以同時在多個場景（context）中使用，並且在每個場景中flyweight都可以作為一個獨立的對象---這一點與非共用對象的實例沒有區別。

3.適用性

• 一個應用程式使用了大量的對象。
• 完全由於使用大量的對象，造成很大的存儲開銷。
• 對象的大多數狀態都可變為外部狀態。
• 如果刪除對象的外部狀態，那麼可以用相對較少的共用對象取代很多組對象。
• 應用程式不依賴於對象標識。由於Flyweight對象可以被共用，對於概念上明顯有別的對象，標識測試將返回真值。

4.結構圖

5.代碼實例

//Flyweight.h



#include <string>

class FlweightGraphicBase
{
public:
    virtual void PrintColor()=0;
protected:
    std::string m_Color;
};

class FlweightWhiteGraphic : public FlweightGraphicBase
{
public:
    FlweightWhiteGraphic();
    void PrintColor();
};

class FlweightBlackGraphic : public FlweightGraphicBase
{
public:
    FlweightBlackGraphic();
    void PrintColor();
};

//FlyweigtFactory.h



#include <string>
#include <memory>
#include <map>

class FlweightGraphicBase;

class FlyweightFactory
{
public:
    std::shared_ptr<FlweightGraphicBase> GetFlyweight(std::string sColor);
    void SetFlyweight(std::string sColor,
        std::shared_ptr<FlweightGraphicBase> pFlyweight);

private:
    std::map <std::string,std::shared_ptr<FlweightGraphicBase>> m_mapFlyweight;
};

//Flyweight.cpp

#include "Flyweight.h"
#include <iostream>

FlweightWhiteGraphic::FlweightWhiteGraphic()
{
    m_Color = "White";
}


void FlweightWhiteGraphic :: PrintColor()
{
    std::cout<<"Color is :" << m_Color << std::endl;
}


FlweightBlackGraphic::FlweightBlackGraphic()
{
    m_Color = "Black";
}


void FlweightBlackGraphic :: PrintColor()
{
    std::cout<<"Color is :" << m_Color << std::endl;
}

//FlyweightFactory.cpp


#include "FlyweightFactory.h"


std::shared_ptr<FlweightGraphicBase> FlyweightFactory::GetFlyweight(std::string sColor)
{
    auto iter = m_mapFlyweight.find(sColor);
    if(iter == m_mapFlyweight.end())
    {
        return nullptr;
    }

    return iter->second;
}

void FlyweightFactory::SetFlyweight(std::string sColor,
        std::shared_ptr<FlweightGraphicBase> pFlyweight)
{
    m_mapFlyweight[sColor] = pFlyweight;
}

//Client.cpp



#include "Flyweight.h"
#include "FlyweightFactory.h"
#include <iostream>

void Insert(std::string sColor,
    int index,std::shared_ptr<FlweightGraphicBase> pFlweightGraphic)
{
    std::cout<<"Posion " << index << " Insert: " << sColor << std::endl ;
}

int main()
{
    std::shared_ptr<FlyweightFactory> pFlyweightFactory(new FlyweightFactory);
    std::shared_ptr<FlweightWhiteGraphic> pFlweightWhiteGraphic(new FlweightWhiteGraphic);
    std::shared_ptr<FlweightBlackGraphic> pFlweightBlackGraphic(new FlweightBlackGraphic);

    pFlyweightFactory->SetFlyweight("White",pFlweightWhiteGraphic);
    pFlyweightFactory->SetFlyweight("Black",pFlweightBlackGraphic);

    auto pFlyweight = pFlyweightFactory->GetFlyweight("White");
    Insert("White",0,pFlyweight);

    pFlyweight = pFlyweightFactory->GetFlyweight("Black");
    Insert("Black",1,pFlyweight);

    pFlyweight = pFlyweightFactory->GetFlyweight("White");
    Insert("White",2,pFlyweight);

    pFlyweight = pFlyweightFactory->GetFlyweight("Black");
    Insert("Black",3,pFlyweight);

    while(1);

}

6.測試結果 

     實例代碼中 所有插入的Graphic 共用黑白兩種顏色，位置信息存儲在外部。

7.效果

• 存儲節約，和以下因素有關：
• 1.因為共用，實例總數減少的數目。
• 2.對象內部狀態的平均數目。
• 3.外部狀態是計算的還是存儲的。