16_设计模式之状态模式

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。从而时对象看起来似乎改了其行为。

使用动机：

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改变，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。

enum NetworkState
{
	OPEN,
	CLOSE,
	CONNECT
};

class NetworkProcessor
{
private:
	NetworkState m_state;

public:
	void Operation1()
	{
		if (OPEN == m_state)
		{
			//...
			m_state = CLOSE;
		}
		else if (CLOSE == m_state)
		{
			//...
			m_state = CONNECT;
		}
		else if (CONNECT == m_state)
		{
			//...
			m_state = OPEN;
		}
	}
	void Operation2()
	{
		if (OPEN == m_state)
		{
			//...
			m_state = CONNECT;
		}
		else if (CLOSE == m_state)
		{
			//...
			m_state = OPEN;
		}
		else if (CONNECT == m_state)
		{
			//...
			m_state = CLOSE;
		}
	}
};

改进后：

class NetworkState
{
public:
	NetworkState *m_pNextNetworkState;

	virtual void Operation1() = 0;
	virtual void Operation2() = 0;
	virtual void Operation3() = 0;

	virtual ~NetworkState() { }
};

class OpenState : public NetworkState
{
private:
	static NetworkState *m_pInstance;

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

	virtual void Operation1() 
	{ 
		//...
		m_pNextNetworkState = CloseState::getInstance();
	}
	virtual void Operation2() 
	{//...
	}
	virtual void Operation3()
	{//...
	}
};
NetworkState *OpenState::m_pInstance = nullptr;

class CloseState : public NetworkState
{
private:
	static NetworkState *m_pInstance;

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

	virtual void Operation1()
	{
		//...
		m_pNextNetworkState = ConnectState::getInstance();
	}
	virtual void Operation2()
	{//...
	}
	virtual void Operation3()
	{//...
	}
};
NetworkState *CloseState::m_pInstance = nullptr;


class ConnectState : public NetworkState
{
private:
	static NetworkState *m_pInstance;

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

	virtual void Operation1()
	{
		//...
		m_pNextNetworkState = OpenState::getInstance();
	}
	virtual void Operation2()
	{//...
	}
	virtual void Operation3()
	{//...
	}
};
NetworkState *ConnectState::m_pInstance = nullptr;


class NetworkProcess
{
private:
	NetworkState *m_pState;

public:
	NetworkProcess(NetworkState *pState) : m_pState(pState) { }

	void Operation1()
	{
		m_pState->Operation1();
		m_pState = m_pState->m_pNextNetworkState;
	}
	void Operation2()
	{
		m_pState->Operation2();
		m_pState = m_pState->m_pNextNetworkState;
	}
	void Operation3()
	{
		m_pState->Operation3();
		m_pState = m_pState->m_pNextNetworkState;
	}
};