状态模式 State

“状态变化”模式

• 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。
• 典型模式

1. State
2. Memento

动机（Motivation）

• 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紧耦合？ // 透明的：稳定

// 和策略模式很像。

示例

一般的写法，很多 if else， 和策略模式改造之前很像。

#include <iostream>
#include <string>

using namespace std;

enum NetworkState
{
    Network_Open,
    Network_Close,
    Network_Connect,
};

class NetworkProcessor {
    NetworkState state = NetworkState::Network_Open;
public:
    /*很多 if else 和策略模式改造前很像。*/
    void Operation1() {
        if (state == Network_Open) {
            //**********
            cout << "网络" << "打开" << endl;
            state = Network_Close;
        }
        else if (state == Network_Close) {
            cout << "网络" << "关闭" << endl;
            //..........
            state = Network_Connect;
        }
        else if (state == Network_Connect) {
            cout << "网络" << "连接" << endl;
            //$$$$$$$$$$
            state = Network_Open;
        }
        else;
    }

     void Operation2() {
        if (state == Network_Open) {
            //**********
            cout << "网络" << "打开" << endl;
            state = Network_Connect;
        }
        else if (state == Network_Close) {
            //.....
            cout << "网络" << "关闭" << endl;
            state = Network_Open;
        }
        else if (state == Network_Connect) {
            //$$$$$$$$$$
            cout << "网络" << "连接" << endl;
            state = Network_Close;
        }
        else;
    }

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

int main()
{
    NetworkProcessor processor;
    processor.Operation1();
    processor.Operation1();
    processor.Operation1();
    processor.Operation1();

    getchar();
    return 0;
}

输出：

网络打开
网络关闭
网络连接
网络打开

状态模式：

#include <iostream>
#include <string>

using namespace std;

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

class CloseState :public NetworkState {
public:
    CloseState() {};
    ~CloseState() {};
public:
    static NetworkState* getInstance();
    virtual void Operation1();
    virtual void Operation2();
    virtual void Operation3();
private:
    // 这里用单例
    static NetworkState* m_instance;
};

//后面扩展这个类就行
class OpenState :public NetworkState {
public:
    OpenState() {};
    ~OpenState() {};
public:
    static NetworkState* getInstance();
    virtual void Operation1();
    virtual void Operation2();
    virtual void Operation3();
private:
    // 这里用单例
    static NetworkState* m_instance;
};

class ConnectState :public NetworkState {
public:
    ConnectState() {};
    ~ConnectState() {};
public:
    static NetworkState* getInstance();
    virtual void Operation1();
    virtual void Operation2();
    virtual void Operation3();
private:
    // 这里用单例
    static NetworkState* m_instance;
};

NetworkState* OpenState::m_instance = nullptr;

 NetworkState* OpenState::getInstance() {
    if (m_instance == nullptr) {
        m_instance = new OpenState();
    }
    return m_instance;
}

void OpenState::Operation1() {
    //**********
    cout << "OpenState " << "Operation1." << endl;
    pNext = ConnectState::getInstance();
}

void OpenState::Operation2() {
    cout << "OpenState " << "Operation2." << endl;
    //..........
    pNext = ConnectState::getInstance();
}

void OpenState::Operation3() {
    cout << "OpenState " << "Operation3." << endl;
    //$$$$$$$$$$
    pNext = OpenState::getInstance();
}


NetworkState* CloseState::m_instance = nullptr;

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

void CloseState::Operation1() {
    cout << "CloseState " << "Operation1." << endl;
    //**********
    pNext = OpenState::getInstance();
}

void CloseState::Operation2() {
    cout << "CloseState " << "Operation2." << endl;
    //..........
    pNext = ConnectState::getInstance();
}

void CloseState::Operation3() {
    cout << "CloseState " << "Operation3." << endl;
    //$$$$$$$$$$
    pNext = OpenState::getInstance();
}


NetworkState* ConnectState::m_instance = nullptr;

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

void ConnectState::Operation1() {
    //**********
    cout << "ConnectState " << "Operation1." << endl;
    pNext = CloseState::getInstance();
}

void ConnectState::Operation2() {
    cout << "ConnectState " << "Operation2." << endl;
    //..........
    pNext = ConnectState::getInstance();
}

void ConnectState::Operation3() {
    cout << "ConnectState " << "Operation3." << endl;
    //$$$$$$$$$$
    pNext = OpenState::getInstance();
}

// 这个写好以后不用变
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;
        //...
    }
};

int main()
{
    OpenState openState;
    CloseState closeState;
    ConnectState connectState;

    NetworkProcessor netWorkProcessor(&openState);
    netWorkProcessor.Operation1();
    netWorkProcessor.Operation1();
    netWorkProcessor.Operation1();
    netWorkProcessor.Operation1();

    getchar();
    return 0;
}

输出：

OpenState Operation1.
ConnectState Operation1.
CloseState Operation1.
OpenState Operation1.

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。——《设计模式》GoF

类图

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。
• 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。

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参考：GeekBand