状态模式State

在组件构建过程中，某些对象的状态经常面临变化，如何对这些变化进行有效的管理？同时又维持高层模块的稳定？“状态变化”模式为这一问题提供了一种解决方案。

经典模式：State、Memento

动机（Motivation）

在软件构建过程中，某些对象的状态如何改变，其行为也随之而发生变化，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以完全不同。

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化之间引入耦合？

示例1

enum NetworkState
{
    Network_Open,
    Network_Close,
    Network_Connect,
    NetWork_Wait    // 新增状态
};

class NetworkProcessor {
    NetworkState state_;

public:
    void Operation1() {
        if (Network_Open == state_) {
            //***
            state_ = Network_Close;
        }
        else if(Network_Close == state_)
        {
            //...
            state_ = Network_Connect;
        }
        else if (Network_Connect == state_) {
            //$$$
            state_ = Network_Open;
        }
        // 变化
        else if (NetWork_Wait == state_) {
            //---
        }
    }

    void Operation2() {
        if (Network_Open == state_) {
            //***
            state_ = Network_Connect;
        }
        else if (Network_Close == state_)
        {
            //...
            state_ = Network_Open;
        }
        else if (Network_Connect == state_) {
            //$$$
            state_ = Network_Close;
        }
        // 变化
        else if (NetWork_Wait == state_) {
            //---
        }
    }

    void Operation3() {
        //...
    }
};

示例2

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 (nullptr == m_instance) {
            m_instance = new OpenState();
        }

        return m_instance;
    }

    virtual void Operation1() {
        //***
        pNext = CloseState::getInstance();
    }

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

class CloseState : public NetworkState {
    static NetworkState* m_instance;

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

        return m_instance;
    }

    virtual void Operation1() {
        //***
        pNext = ConnectState::getInstance();
    }

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

class ConnectState : public NetworkState {
    static NetworkState* m_instance;

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

        return m_instance;
    }

    virtual void Operation1() {
        //***
        pNext = OpenState::getInstance();
    }

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

// 扩展
class WaitState : public NetworkState {
    //...
};

class NetworkProcessor {
    NetworkState* state_;

public:
    NetworkProcessor(NetworkState* state) {
        this->state_ = state;
    }

    void Operation1() {
        //...
        state_->Operation1();
        state_ = state_->pNext;
        //...
    }

    void Operation2() {
        //...
        state_->Operation2();
        state_ = state_->pNext;
        //...
    }

    void Operation3() {
        //...
        state_->Operation3();
        state_ = state_->pNext;
        //...
    }

};

要点总结

State模式将所有与一个特定状态的相关的行为都放入一个State的子类对象中，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，而且可怀疑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。

如果State对象没有实例变量，那么各个上下文可怀疑共享同一个State对象，从而节省对象开销。