State模式允許一個(gè)對(duì)象在其內(nèi)部狀態(tài)改變時(shí)改變它的行為。對(duì)象看起來(lái)似乎修改了它的類(lèi)。
在面向?qū)ο笙到y(tǒng)的開(kāi)發(fā)和設(shè)計(jì)過(guò)程，經(jīng)常會(huì)遇到一種情況就是需求變更（Requirement Changing），經(jīng)常我們做好的一個(gè)設(shè)計(jì)、實(shí)現(xiàn)了一個(gè)系統(tǒng)原型，咱們的客戶又會(huì)有了新的需求。我們又因此不得不去修改已有的設(shè)計(jì)，最常見(jiàn)就是解決方案就是給已經(jīng)設(shè)計(jì)、實(shí)現(xiàn)好的類(lèi)添加新的方法去實(shí)現(xiàn)客戶新的需求，這樣就陷入了設(shè)計(jì)變更的夢(mèng)魘：不停地打補(bǔ)丁，其帶來(lái)的后果就是設(shè)計(jì)根本就不可能封閉、編譯永遠(yuǎn)都是整個(gè)系統(tǒng)代碼。
訪問(wèn)者模式則提供了一種解決方案：將更新（變更）封裝到一個(gè)類(lèi)中（訪問(wèn)操作），并由待更改類(lèi)提供一個(gè)接收接口，則可達(dá)到效果。
結(jié)構(gòu)圖：
實(shí)例：
context.h
#ifndef _CONTEXT_H_#define _CONTEXT_H_class State;/*****/class Context{ public: Context(); Context(State* state); ~Context(); void Handle(); void OperationForStateA(); void OperationForStateB(); protected: private: friend class State; //表明在 State 類(lèi)中可以訪問(wèn) Context 類(lèi)的 private 字段，重要是訪問(wèn) ChangeState void ChangeState(State* state); private: State* _state;};#endif //~_CONTEXT_H_
context.cpp
#include "Context.h"#include "State.h"#include <iostream>using namespace std;Context::Context(){}Context::Context(State* state){ this->_state = state;}Context::~Context(){ delete _state;}void Context::Handle(){ _state->Handle(this);}void Context::ChangeState(State* state){ ///_state->ChangeState(this,state); this->_state = state;}void Context::OperationForStateA(){ cout<<"Do operation in State A ";}void Context::OperationForStateB(){ cout<<"Do operation in State B ";}
state.h
#ifndef _STATE_H_#define _STATE_H_class Context; //前置聲明class State{ public: State(); virtual ~State(); virtual void Handle(Context* con) = 0; protected: void ChangeState(Context* con,State* st); private: //bool ChangeState(Context* con,State* st);};class ConcreteStateA:public State{ public: ConcreteStateA(); virtual ~ConcreteStateA(); void Handle(Context* con); protected: private:};class ConcreteStateB:public State{ public: ConcreteStateB(); virtual ~ConcreteStateB(); void Handle(Context* con); protected: private:};#endif //~_STATE_H_
State.cpp
#include "State.h"#include "Context.h"#include <iostream>using namespace std;State::State(){}State::~State(){}void State::ChangeState(Context* con,State* st){ con->ChangeState(st);}///ConcreteStateA::ConcreteStateA(){}ConcreteStateA::~ConcreteStateA(){}void ConcreteStateA::Handle(Context* con){ con->OperationForStateA(); cout<<":: State change from A to B"<<endl; State::ChangeState(con,new ConcreteStateB());}///ConcreteStateB::ConcreteStateB(){}ConcreteStateB::~ConcreteStateB(){}void ConcreteStateB::Handle(Context* con){ con->OperationForStateB(); cout<<":: State change from B to A"<<endl; State::ChangeState(con,new ConcreteStateA());}
main.cpp
#include "Context.h"#include "State.h"#include <iostream>using namespace std;int main(int argc,char* argv[]){ State* st = new ConcreteStateA(); Context* con = new Context(st); con->Handle(); con->Handle(); con->Handle(); if (con != NULL) delete con; if (st != NULL) st = NULL; return 0;} 可以看到在測(cè)試程序中，三次調(diào)用 con->Handle()，因?yàn)?con 狀態(tài)的不同，可以得到以下的輸出：
Do operation in State A :: State change from A to BDo operation in State B :: State change from B to ADo operation in State A :: State change from A to B
適用性
一個(gè)對(duì)象的行為取決于它的狀態(tài), 并且它必須在運(yùn)行時(shí)刻根據(jù)狀態(tài)改變它的行為。
一個(gè)操作中含有龐大的多分支的條件語(yǔ)句，且這些分支依賴于該對(duì)象的狀態(tài)。這個(gè)狀態(tài)通常用一個(gè)或多個(gè)枚舉常量表示。通常, 有多個(gè)操作包含這一相同的條件結(jié)構(gòu)。S t a t e模式將每一個(gè)條件分支放入一個(gè)獨(dú)立的類(lèi)中。這使得你可以根據(jù)對(duì)象自身的情況將對(duì)象的狀態(tài)作為一個(gè)對(duì)象，這一對(duì)象可以不依賴于其他對(duì)象而獨(dú)立變化。
