实战设计模式系列-State(状态)

【需求分析】

 

        state模式中，我们将状态逻辑和动作实现进行分离，当一个操作中要维护大量的case分支语句的时候，并且这些分支也都要依赖于对象的状态时，state模式将每一个分支都封装到独立的类中实现。不同的事物在不同的状态下会有不同的动作，在一个状态下又会转移到下一个状态。

 

        以TCP状态处理为例，TcpConnection只负责独立的业务逻辑，而TcpState负责业务逻辑转换和组合的业务逻辑，如TcpConnection会提供创建socket、监听接口，而TcpState业务从Close状态转换到Listen转换时，会调用TcpConnection类的创建socket和监听接口，这一点，也是通过在TcpConnection中声明TcpState为友元实例的。

 

        下面的例子给出了代码实例，在main函数中，只要接收到请求，直接调用HandleRequest即可，而不用关心当前的TCP连接状态。

 

【代码示例】

 

/* * state.h *//* * state.h */ #include <stdio.h> #include <stdlib.h> class TcpState; class TcpConnection { public: TcpConnection(TcpState* instance); ~TcpConnection(); public: void HandleRequest(); void ChangeState(TcpState* instance); public: void CreateSocket() { printf("CreateSocket/n"); } void BeginListen() { printf("BeginListen/n"); } void Accept() { printf("Accept/n"); } void Establish() { printf("Establish/n"); } void Close() { printf("Close/n"); } private: friend class TcpState; TcpState * m_instance; }; class TcpState { public: virtual ~TcpState(){}; public: virtual void Handle(TcpConnection* pTcpConnection){}; }; class TcpCloseState : public TcpState { public: void Handle(TcpConnection* pTcpConnection); }; class TcpListenState : public TcpState { public: void Handle(TcpConnection* pTcpConnection); }; class TcpEstablishState : public TcpState { public: void Handle(TcpConnection* pTcpConnection); };

 

/* * state.cpp */ #include "state.h" TcpConnection::TcpConnection(TcpState* instance) { m_instance = instance; } TcpConnection::~TcpConnection() { delete m_instance; m_instance = NULL; } void TcpConnection::HandleRequest() { if (NULL == m_instance) { return; } m_instance->Handle(this); } void TcpConnection::ChangeState(TcpState* instance) { m_instance = instance; } void TcpCloseState::Handle(TcpConnection* pTcpConnection) { //create socket and begin listen pTcpConnection->CreateSocket(); pTcpConnection->BeginListen(); //change state pTcpConnection->ChangeState(new TcpListenState()); } void TcpListenState::Handle(TcpConnection* pTcpConnection) { //accpet and set establish pTcpConnection->Accept(); pTcpConnection->Establish(); //change state pTcpConnection->ChangeState(new TcpEstablishState()); } void TcpEstablishState::Handle(TcpConnection* pTcpConnection) { //close pTcpConnection->Close(); //change state pTcpConnection->ChangeState(new TcpCloseState()); } int main(void) { TcpState * pState= new TcpCloseState(); TcpConnection * pTcpConn = new TcpConnection(pState); pTcpConn->HandleRequest(); pTcpConn->HandleRequest(); pTcpConn->HandleRequest(); delete pTcpConn; return 0; }

 

【执行结果】

 

root:/home/derrywang#g++ -o state state.cpp
root:/home/derrywang#./state
CreateSocket
BeginListen
Accept
Establish
Close