Flyweight 模式

如果一个应用程序使用了太多的对象， 就会造成很大的存储开销。 特别是对于大量轻量级 （细粒度）的对象，比如在文档编辑器的设计过程中，我们如果为每个字母创建一个对象的话，系统可能会因为大量的对象而造成存储开销的浪费。例如一个字母“a”在文档中出现了100000 次，而实际上我们可以让这一万个字母“a”共享一个对象，当然因为在不同的位置可能字母“a”有不同的显示效果（例如字体和大小等设置不同） ，在这种情况我们可以为将对象的状态分为“外部状态”和“内部状态” ，将可以被共享（不会变化）的状态作为内部状态存储在对象中，而外部对象（例如上面提到的字体、大小等）我们可以在适当的时候将外部对象最为参数传递给对象（例如在显示的时候，将字体、大小等信息传递给对象） 。

Flyweight 模式可以解决上面的问题，其典型的结构图为：

 ///////////////Flyweight.h////////////////////////
 #pragma once
 #include <string>
 using namespace std;
 class Flyweight
 {
 public:
     virtual ~Flyweight();
     virtual void Operation(const string& extrinsicState);
     string GetIntrinsicState();
 protected:
     Flyweight(string intrinsicState);
 private:
     string  _intrinsicState ;
 };

 class ConcreteFlyweight : public Flyweight
 {
 public:
     ~ConcreteFlyweight();
     ConcreteFlyweight(string intrinsicState);
     void Operation(const string& extrinsicState);
 protected:
 private:
 };

 ///////////////Flyweight.cpp//////////////////////////
 #include "Flyweight.h"
 #include <iostream>
 using namespace std;

 Flyweight::Flyweight(string intrinsicState)
 {
     _intrinsicState = intrinsicState ;
 }
 Flyweight::~Flyweight()
 {

 }

 void Flyweight::Operation(const string& extrinsicState)
 {

 }
 string Flyweight::GetIntrinsicState()
 {
     return this->_intrinsicState ;
 }

 ConcreteFlyweight::ConcreteFlyweight(string intrinsicState):Flyweight(intrinsicState)
 {
     cout<<"ConcreteFlyweight Build....."<<intrinsicState<<endl;
 }
 ConcreteFlyweight::~ConcreteFlyweight()
 {

 }
 void ConcreteFlyweight::Operation(const string& extrinsicState)
 {
     cout<<"ConcreteFlyweight: 内部["<<this->GetIntrinsicState()<<"] 外部["<<extrinsicState<<"]"<<endl;
 }

 /////////////////FlyweightFactory.h////////////////////////////
 #pragma once
 #include "Flyweight.h"
 #include <vector>
 class FlyweightFactory
 {
 public:
     FlyweightFactory();
     ~FlyweightFactory();
     Flyweight* GetFlyweight(const string& key);

 protected:
 private:
     vector<Flyweight*> _fly ;
 };

 ///////////////////FlyweightFactory.cpp//////////////////////////////
 #include "FlyweightFactory.h"
 #include <iostream>
 using namespace std;
 FlyweightFactory::FlyweightFactory()
 {

 }
 FlyweightFactory::~FlyweightFactory()
 {

 }

 Flyweight* FlyweightFactory::GetFlyweight(const string& key)
 {
     vector<Flyweight*>::iterator it = _fly.begin();
     for (;it != _fly.end() ; it++)
     {
         if ((*it)->GetIntrinsicState() == key)
         {
             cout<<"already  created  by users...."<<endl;
             return *it ;
         }
     }

     Flyweight* fn = new ConcreteFlyweight(key);
     _fly.push_back(fn);

     return fn ;

 }

 /////////////////////////main.cpp/////////////////////////////////////////////////
 #include "FlyweightFactory.h"
 #include "Flyweight.h"
 #include <iostream>
 using namespace std;
 int main()
 {
     FlyweightFactory* fc = new FlyweightFactory();
     Flyweight* fw1 = fc->GetFlyweight("hello");
     Flyweight* fw2 = fc->GetFlyweight("world");

     cout<<fw1->GetIntrinsicState()<<endl;
     fw1->Operation("red");

     cout<<fw2->GetIntrinsicState()<<endl;
     fw2->Operation("green");

     fc->GetFlyweight("hello")->Operation("black");
     getchar();
     return ;
 }