设计模式——状态模式（State）

• 定义：允许一个对象在其状态改变时，改变它的行为。看起来对象似乎修改了它的类。
• 解决根据不同的对象状态进行不同的操作的问题
• UML

  

• 代码示例：

  //==========================================
  //Context.h
  //==========================================
  #pragma once
  
  class State;
  class Context
  {
  public:
      Context(State* pState);
      ~Context();
      void Request();
  private:
      void UpdateState(State* pState);
      friend class ConcreteStateA;
      friend class ConcreteStateB;
  private:
      State* m_pState;
  };
  
  //==========================================
  //Context.cpp
  //==========================================
  #include "Context.h"
  #include <stdio.h>
  #include "State.h"
  
  Context::Context(State* pState)
      :m_pState(pState)
  {
  }
  
  Context::~Context()
  {
      delete m_pState;
  }
  
  void Context::Request()
  {
      if (NULL != m_pState)
      {
          m_pState->Handle(this);
      }
  }
  
  void Context::UpdateState(State* pState)
  {
      delete m_pState;
      m_pState = pState;
  }
  
  //==========================================
  //State.h
  //==========================================
  #pragma once
  
  class Context;
  class State
  {
  public:
      virtual ~State(){}
      virtual void Handle(Context* pContex) = 0;
  };
  
  class ConcreteStateA : public State
  {
  public:
      void Handle(Context* pContex);
  };
  
  class ConcreteStateB : public State
  {
  public:
      void Handle(Context* pContex);
  };
  
  //==========================================
  //State.cpp
  //==========================================
  #include "State.h"
  #include <iostream>
  #include "Context.h"
  using namespace std;
  
  //处理、更新状态
  void ConcreteStateA::Handle(Context* pContex)
  {
      cout<<"ConcreteStateA :: Handle"<<endl;
      pContex->UpdateState(new ConcreteStateB);
  }
  
  void ConcreteStateB::Handle(Context* pContex)
  {
      cout<<"ConcreteStateB :: Handle"<<endl;
      pContex->UpdateState(new ConcreteStateA);
  }
  
  //==========================================
  //main.cpp
  //==========================================
  #include "Context.h"
  #include "State.h"
  
  int main()
  {
      State *pState = new ConcreteStateA();
      Context *pContext = new Context(pState);
      pContext->Request();
      pContext->Request();
      pContext->Request();
  
      delete pContext;
  
      return 0;
  }