作用:
允許一個對象在其內部狀態改變時改變它的行為.
UML結構圖:
解析:
State模式主要解決的是在開發中時常遇到的根據不同的狀態需要進行不同的處理操作的問題,而這樣的問題,大部分人是采用switch-case語句進行處理的,這樣會造成一個問題:分支過多,而且如果加入一個新的狀態就需要對原來的代碼進行編譯.State模式采用了對這些不同的狀態進行封裝的方式處理這類問題,當狀態改變的時候進行處理然后再切換到另一種狀態,也就是說把狀態的切換責任交給了具體的狀態類去負責.同時,State模式和Strategy模式在圖示上有很多相似的地方,需要說明的是兩者的思想都是一致的,只不過封裝的東西不同:State模式封裝的是不同的狀態,而Stategy模式封裝的是不同的算法.
實現:
1)State.h
/**//********************************************************************
????created:????2006/08/05
????filename:?????State.h
????author:????????李創
????????????????http://www.shnenglu.com/converse/
????purpose:????State模式的演示代碼
*********************************************************************/
#ifndef?STATE_H
#define?STATE_H
class?State;
class?Context
{
public:
????Context(State*?pState);
????~Context();
????void?Request();
????void?ChangeState(State?*pState);
private:
????State?*m_pState;
};
class?State
{
public:
????virtual?~State(){}
????virtual?void?Handle(Context*?pContext)?=?0;
};
class?ConcreateStateA
????:?public?State
{
public:
????void?Handle(Context*?pContext);
};
class?ConcreateStateB
????:?public?State
{
public:
????void?Handle(Context*?pContext);
};
#endif
2)State.cpp
/**//********************************************************************
????created:????2006/08/05
????filename:?????State.cpp
????author:????????李創
????????????????http://www.shnenglu.com/converse/
????purpose:????State模式的演示代碼
*********************************************************************/
#include?"State.h"
#include?<iostream>
Context::Context(State*?pState)
????:?m_pState(pState)
{
}
Context::~Context()
{
????delete?m_pState;
????m_pState?=?NULL;
}
void?Context::Request()
{
????if?(NULL?!=?m_pState)
????{
????????m_pState->Handle(this);
????}
}
void?Context::ChangeState(State?*pState)
{
????if?(NULL?!=?m_pState)
????{
????????delete?m_pState;
????????m_pState?=?NULL;
????}
????
????m_pState?=?pState;
}
void?ConcreateStateA::Handle(Context*?pContext)
{
????std::cout?<<?"Handle?by?ConcreateStateA\n";
????
????if?(NULL?!=?pContext)
????{
????????pContext->ChangeState(new?ConcreateStateB());
????}
}
void?ConcreateStateB::Handle(Context*?pContext)
{
????std::cout?<<?"Handle?by?ConcreateStateB\n";
????if?(NULL?!=?pContext)
????{
????????pContext->ChangeState(new?ConcreateStateA());
????}
}
3)Main.cpp
/**//********************************************************************
????created:????2006/08/05
????filename:?????Main.cpp
????author:????????李創
????????????????http://www.shnenglu.com/converse/
????purpose:????State模式的測試代碼
*********************************************************************/
#include?"State.h"
int?main()
{
????State?*pState?=?new?ConcreateStateA();
????Context?*pContext?=?new?Context(pState);
????pContext->Request();
????pContext->Request();
????pContext->Request();
????delete?pContext;
????return?0;
}