19_设计模式之职责链模式

模式定义：

使多个对象都有机会处理请求，从而避免请求的发送者和接收者之间的耦合关系。将这些对象连成一条链，并沿着这条链传递请求，直到有一个对象处理它为止。

使用动机：

软件构建过程中，一个请求可能被多个对象处理，但是每个请求在运行时只能有一个接受者，如果显示指定，将必不可少的带来请求发送者与接受者的紧耦合。

enum RequestType
{REQ_HANDLER1,REQ_HANDLER2,REQ_HANDLER3
};class Request
{
private:string m_desc;RequestType m_reqType;public:Request(const string &desc, const RequestType &reqType) : m_desc(desc), m_reqType(reqType) { }string getDesc() const { return m_desc; }RequestType getReqType() const { return m_reqType;  }
};class ChainHandler
{
public:ChainHandler *m_pNextChainHandler;void sendRequestToNextHandler(const Request &req){if (m_pNextChainHandler != nullptr){m_pNextChainHandler->handle(req);}}
protected:virtual bool canHandleRequest(const Request &req) = 0;virtual void processRequest(const Request &req) = 0;public:ChainHandler() : m_pNextChainHandler(nullptr) { }void setNextChain(ChainHandler *pNext){m_pNextChainHandler = pNext;}void handle(const Request &req){if (canHandleRequest(req)){processRequest(req);}else{sendRequestToNextHandler(req);}}
};class Handler1 : public ChainHandler
{
protected:bool canHandleRequest(const Request &req) override{return req.getReqType() == RequestType::REQ_HANDLER1;}void processRequest(const Request &req) override{}
};class Handler2 : public ChainHandler
{
protected:bool canHandleRequest(const Request &req) override{return req.getReqType() == RequestType::REQ_HANDLER2;}void processRequest(const Request &req) override { }
};class Handler3 : public ChainHandler
{
protected:bool canHandleRequest(const Request &req) override{return req.getReqType() == RequestType::REQ_HANDLER3;}void processRequest(const Request &req) override { }
};int main(array<System::String ^> ^args)
{Handler1 h1;Handler2 h2;Handler3 h3;h1.setNextChain(&h2);h2.setNextChain(&h3);Request req("test", RequestType::REQ_HANDLER3);h1.handle(req);return 0;
}