C++设计模式之二十：State（状态模式）

本文介绍了一种常用的设计模式——状态模式，通过该模式可以使对象根据内部状态的变化而改变其行为。文中详细解释了状态模式的基本概念，并提供了一个具体的实现案例，包括环境类、状态接口及具体状态子类的定义。

一、意图：

允许一个对象在其内部状态改变时改变它的行为。

二、类图：

三、组成元素：

i. Context：环境

ii. State：状态接口

ConcreteState：具体状态子类

四、实现代码：

#include "iostream"
using namespace std;

class Context;
class ConcreteStateA;
class ConcreteStateB;

class State
{
public:
	State()
	{

	}
	virtual void Handle(Context* pContext)=0;
};

class Context
{
private:
	State* m_State;
public:
	Context(State* pState)
	{
		m_State=pState;
	}
	void Request()
	{
		if (NULL!=m_State)
		{
			m_State->Handle(this);
		}
	}
	void ChangState(State* pState)
	{
		if (NULL!=m_State)
		{
			delete m_State;
			m_State=NULL;
		}
		m_State=pState;
	}
};


class ConcreteStateA:public State
{
public:
	ConcreteStateA()
	{

	}
	void Handle(Context* pContext)
	{
		cout<<"Handle by ConcreteStateA"<<endl;
		if (NULL!=pContext)
		{
			pContext->ChangState(new ConcreteStateB());
		}
	}
};


class ConcreteStateB:public State
{
public:
	ConcreteStateB()
	{

	}
	void Handle(Context* pContext)
	{
		cout<<"Handle by ConcreteStateB"<<endl;
		if (NULL!=pContext)
		{
			pContext->ChangState(new ConcreteStateA());
		}
	}
};

void main()
{
	State* pState=new ConcreteStateA();
	Context* pContext=new Context(pState);

	pContext->Request();
	pContext->Request();
	pContext->Request();

	delete pContext;
}