[C++][设计模式][状态模式]详细讲解

1.动机

• 在软件构建过程中，某些对象的状态如果改变，其行为也会随之而放生变化
  • 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完全不同
• 如何在运行时根据对象的状态来透明地改变对象的行为？而不会为对象操作和状态转化之间引入紧耦合？

2.模式定义

• 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
• 结构
  

3.要点总结

• State模式将所有与一个特定状态相关的行为都放入一个State的子对象中，在对象状态切换时，切换相应的对象； 但同时维持State的接口，这样实现了具体操作与状态转换之间的解耦
• 为不同的状态引入不同的对象使得状态转换变得更加明确，而且可以保证不会出现状态不一致的情况，因为转换是原子性的 —— 即要么彻底转换过来，要么不转换
• 与Strategy模式类似
• 如果State对象没有实例变量，那么各个上下文可以共享同一个State对象，从而节省对象开销

4.代码感受

1.代码一

enum NetworkState
{
    Network_Open,
    Network_Close,
    Network_Connect,
};

class NetworkProcessor
{
    NetworkState state;
public:
    void Operation1()
    {
        if (state == Network_Open)
        {
            //**********
            state = Network_Close;
        }
        else if (state == Network_Close)
        {
            //..........
            state = Network_Connect;
        }
        else if (state == Network_Connect)
        {
            //$$$$$$$$$$
            state = Network_Open;
        }
    }

    void Operation2()
    {
        if (state == Network_Open)
        {
            //**********
            state = Network_Connect;
        }
        else if (state == Network_Close)
        {
            //..........
            state = Network_Open;
        }
        else if (state == Network_Connect)
        {
            //$$$$$$$$$$
            state = Network_Close;
        }
    }

    void Operation3()
    {}
};

2.代码二

class NetworkState
{
public:
    NetworkState* pNext;
    virtual void Operation1() = 0;
    virtual void Operation2() = 0;
    virtual void Operation3() = 0;

    virtual ~NetworkState(){}
};

class OpenState : public NetworkState
{
    static NetworkState* m_instance;
public:
    static NetworkState* getInstance()
    {
        if (m_instance == nullptr) 
        {
            m_instance = new OpenState();
        }

        return m_instance;
    }

    void Operation1()
    {
        //**********
        pNext = CloseState::getInstance();
    }
    
    void Operation2()
    {
        //..........
        pNext = ConnectState::getInstance();
    }
    
    void Operation3()
    {
        //$$$$$$$$$$
        pNext = OpenState::getInstance();
    }
};

class CloseState : public NetworkState
{}

class NetworkProcessor
{
    NetworkState* pState;
public:
    NetworkProcessor(NetworkState* pState)
    {
        this->pState = pState;
    }
    
    void Operation1()
    {
        //...
        pState->Operation1();
        pState = pState->pNext;
        //...
    }
    
    void Operation2()
    {
        //...
        pState->Operation2();
        pState = pState->pNext;
        //...
    }
    
    void Operation3()
    {
        //...
        pState->Operation3();
        pState = pState->pNext;
        //...
    }
};