动机

在软件构建过程中,某些对象的状态如果改变,其行为也会随之而发生变化,比如文档处于只读状态,其支持的行为和读写状态支持的行为就可能不同.

如何在运行时根据对象的状态来透明地更改对象的行为?而不会为对象操作和状态转换之间引入紧耦合

模式定义

允许一个对象在其内部状态改变时改变它的行为.从而使对象看起来似乎修改了其行为.从而使对象看起来似乎修改了其行为.从而使对象看起来似乎修改了其行为.

实例

朴素
根据状态来执行下一步

enum NetworkState{ 
   
    Network_Open,
    Network_Close,
    Network_Connect
}

class NetworkProcessor{ 
   
    NetworkState state;
public:
    void Opration1(){ 
   
        if(state == Network_Open){ 
   
            //******
            state = Network_Close;
        }
        else if(state == Network_Close){ 
   
            //.....
            state = Network_Connect;
        }
        else if(state == Network_Connect){ 
   
            //$$$$$
            state = Network_Open;
        }
    }
    void Opration2(){ 
   
        if(state == Network_Open){ 
   
            //******
            state = Network_Connect;
        }
        else if(state == Network_Close){ 
   
            //.....
            state = Network_Open;
        }
        else if(state == Network_Connect){ 
   
            //$$$$$
            state = Network_Close;
        }
    }
    void process3(){ 
   //...}
}

状态模式State

enum NetworkState{ 
   
public:
    NetworkState *pNext;
    virtual void Operation1() = 0;
    virtual void Operation2() = 0;
    virtual void Operatino3() = 0;

    virtual ~NetworkState(){ 
   }
}

class OpenState : public NetworkState{ 
   
    static NetworkState* m_instance;
public:
    static NetworkState* getInstance(){ 
   
        if(m_instance == nullptr){ 
   
            return m_instance;
        }
    }
    void Operation1(){ 
   
        //*******
        pNext = CloseState::getInstance();
    }
    void Operation2(){ 
   
        //........
        pNext = ConnectState::getInstance();
    }
    void Operation3(){ 
   
        //$$$$$$$$
        pNext = NextworkOpen::getInstance();
    }
}
class CloseState : public NetworkState{ 
   //...}

//扩展状态
class WaitState : public NetworkState{ 
   //...}
class NetworkProcessor{ 
   
    NetworkState* pState;
public:
    NetworkProcessor(NetworkState *pstate){ 
   
        this->pState;
    }
    void Opration1(){ 
   
        //....
        pState->Operation1();
        pState = pState->pNext();
        //...
    }
    void Opration2(){ 
   
        //....
        pState->Operation2();
        pState = pState->pNext();
        //..
    }
    void process3(){ 
   
        //....
        pState->Operation3();
        pState = pState->pNext();
        //..
    }
}