状态模式
定义：
允许一个对象在其内部状态改变时改变它的行为。对象看起来似乎修改了它的类。
实用性：
1、 一个对象的行为取决于它的状态，并且它必须在运行时刻根据状态改变它的行为。
2、 一个操作中含有庞大的多分支条件语句，且这些分支依赖于该对象的状态。
优点：
1、 将与特定状态相关的行为局部化，并且将不同状态的行为分割开来。
2、 使得状态转换显示化。
3、 状态对象可以被共享。
结构：
实现：
class Context;
class State
{
public:
State(string name):m_name(name)
{ }
virtual void Handle(Context *pContext)
{ }
virtual~State()
{ }
string GetStateName()
{
returnm_name;
}
protected:
voidChangeState(Context *pContext,State *pState);
private:
string m_name;
};
class Context
{
public:
voidRequest()
{
m_pCurrentState->Handle(this);
}
voidSetCurrentState(State *pState)
{
cout<<"设置当前状态为 :"<
m_pCurrentState = pState;
}
State *GetCurrentState()
{
cout<<"当前状态为 :"<
returnm_pCurrentState;
}
protected:
friend class State;
void ChangeState(State *pState)
{
cout<<"当前状态由 : "<
m_pCurrentState = pState;
}
private:
State *m_pCurrentState;
};
voidState::ChangeState(Context *pContext,State *pState)
{
pContext->ChangeState(pState);
}
//状态子类可以共享，所以用单例模式，这里就忽略内存释放的问题了
class ConcreteStateA;
class ConcreteStateC:public State
{
public:
staticState* GetStateAInstance()
{
if(m_pInstance == NULL)
{
m_pInstance = new ConcreteStateC("状态 C");
}
returnm_pInstance;
}
voidHandle(Context *pContext);
private:
ConcreteStateC(string name):State(name)
{ }
staticConcreteStateC *m_pInstance;
};
class ConcreteStateB:public State
{
public:
staticState* GetStateAInstance()
{
if(m_pInstance == NULL)
{
m_pInstance = new ConcreteStateB("状态 B");
}
returnm_pInstance;
}
voidHandle(Context *pContext)
{
cout<<"状态 B 处理的业务逻辑"<
ChangeState(pContext,ConcreteStateC::GetStateAInstance());
}
private:
ConcreteStateB(string name):State(name)
{ }
staticConcreteStateB *m_pInstance;
};
class ConcreteStateA:public State
{
public:
staticState* GetStateAInstance()
{
if(m_pInstance == NULL)
{
m_pInstance = new ConcreteStateA("状态 A");
}
returnm_pInstance;
}
voidHandle(Context *pContext)
{
cout<<"状态 A 处理的业务逻辑"<
ChangeState(pContext,ConcreteStateB::GetStateAInstance());
}
private:
ConcreteStateA(string name):State(name)
{ }
staticConcreteStateA *m_pInstance;
};
voidConcreteStateC::Handle(Context *pContext)
{
cout<<"状态 C 处理的业务逻辑"<
ChangeState(pContext,ConcreteStateA::GetStateAInstance());
}
ConcreteStateA*ConcreteStateA::m_pInstance = NULL;
ConcreteStateB*ConcreteStateB::m_pInstance = NULL;
ConcreteStateC*ConcreteStateC::m_pInstance = NULL;
Context *pContext = new Context;
pContext->SetCurrentState(ConcreteStateA::GetStateAInstance());
pContext->Request();
pContext->Request();
pContext->Request();
