State模式

一个事物，在不同的状态下会有不同的动作，还可以变化到另一个状态，在开发时有时会遇到这样的事物．有限状态机就是这样的一个事物，在实现时用到了switch case来解决，在状态不多时，这样可以解决，如何状态很多，switch case就会变得难以维护．switch case还有一个缺点就是逻辑和实现没有分离，动作的代码直接写在了逻辑里，使得维护和扩展变得困难．

State模式就是解决这个问题的．其类图结构如下：

实现： //State.h

//State.h #ifndef _STATE_H_ #define _STATE_H_ class Context;//forward declartion class State { public: State(); virtual ~State(); virtual void OperationInterface(Context*) = 0; virtual void OperationChangeState(Context*) = 0; protected: bool ChangeState(Context* con, State* st); }; class ConcreteStateA :public State { public: ConcreteStateA(); virtual ~ConcreteStateA(); virtual void OperationInterface(Context*) ; virtual void OperationChangeState(Context*); }; class ConcreteStateB :public State { public: ConcreteStateB(); virtual ~ConcreteStateB(); virtual void OperationInterface(Context*) ; virtual void OperationChangeState(Context*); }; #endif

//State.cpp

//State.cpp #include"State.h" #include"Context.h" #include<iostream> using namespace::std; State::State() {} State::~State() {} bool State::ChangeState(Context* con, State* st) { con->ChangeState(st); return true; } ConcreteStateA::ConcreteStateA() {} ConcreteStateA::~ConcreteStateA() {} void ConcreteStateA::OperationInterface(Context* con) { cout << "ConcreteStateA::OperationInterface..." << endl; } void ConcreteStateA::OperationChangeState(Context* con) { OperationInterface(con); ChangeState(con, new ConcreteStateB()); } ConcreteStateB::ConcreteStateB() {} ConcreteStateB::~ConcreteStateB() {} void ConcreteStateB::OperationInterface(Context* con) { cout << "ConcreteStateB::OperationInterface..." << endl; } void ConcreteStateB::OperationChangeState(Context* con) { OperationInterface(con); ChangeState(con, new ConcreteStateA()); }

//Context.h

//Context.h #ifndef _CONTEXT_H_ #define _CONTEXT_H_ class State; class Context { public: Context(); Context(State* st); ~Context(); void OperationInterface(); void OperationChangeState(); private: friend class State;//在State实例中可以访问ChangeState bool ChangeState(State* st); State* state_; }; #endif

//Context.cpp

//Context.cpp

//main.cpp

//main.cpp #include"State.h" #include"Context.h" #include<iostream> int main() { State* st = new ConcreteStateA(); Context* con = new Context(st); con->OperationInterface(); con->OperationChangeState(); con->OperationInterface(); if (con != NULL) delete con; return 0; }

在上面的实现中，State声明为Context的友元，方便State访问Context的protected方法．State和它的子类将Context *作为方法的参数，通过指针调用Context的方法，这正是区分Strategy模式的地方．