状态模式

在软件构建过程中,某些对象的状态如何改变,其行为也会随之而发生变化,比如文档处于只读状态,其支持的行为和读写状态支持的行为就可能有些不同。
如何在运行时根据对象的状态来透明地更改对象的行为?而不会为对象操作和状态转换之间引入紧耦合?


Defination

允许一个对象在其内部状态改变时改变它的行为。从而使对象看起来似乎修改了其行为。


Structure


Impletation

例如实现一个网络模块处理类。

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enum NetworkState
{
Network_Open,
Network_Close,
Network_Connect,
};
class NetworkProcessor{
NetworkState state;
public:
void Operation1(){
if (state == Network_Open){
//**********
state = Network_Close;
}
else if (state == Network_Close){
//..........
state = Network_Connect;
}
else if (state == Network_Connect){
//$$$$$$$$$$
state = Network_Open;
}
}
public void Operation2(){
if (state == Network_Open){
//**********
state = Network_Connect;
}
else if (state == Network_Close){
//.....
state = Network_Open;
}
else if (state == Network_Connect){
//$$$$$$$$$$
state = Network_Close;
}
}
public void Operation3(){
}
};

上述代码的主要问题在于添加一个状态时,需要更改对应的行为函数。

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class NetworkState{
public:
NetworkState* pNext;
virtual void Operation1()=0;
virtual void Operation2()=0;
virtual void Operation3()=0;
virtual ~NetworkState(){}
};
class OpenState :public NetworkState{
private:
OpenState() {}
OpenState(const Openstate &) {}
static violate NetworkState* m_instance;
public:
static violate NetworkState* violategetInstance(){
if (m_instance == nullptr) {
Lock lock;
if (m_instance == nullptr) {
m_instance = new OpenState();
}
}
return m_instance;
}
void Operation1(){
//**********
pNext = CloseState::getInstance();
}
void Operation2(){
//..........
pNext = ConnectState::getInstance();
}
void Operation3(){
//$$$$$$$$$$
pNext = OpenState::getInstance();
}
};
class CloseState:public NetworkState{ }
//...
class NetworkProcessor{
NetworkState* pState;
public:
NetworkProcessor(NetworkState* pState){
this->pState = pState;
}
void Operation1(){
//...
pState->Operation1();
pState = pState->pNext;
//...
}
void Operation2(){
//...
pState->Operation2();
pState = pState->pNext;
//...
}
void Operation3(){
//...
pState->Operation3();
pState = pState->pNext;
//...
}
};


Summary

  • state模式将所有与一个特定状态相关的行为都放入一个State的子类对象中,在对象状态切换时,切换对象的对象;但同时维持State的接口,这样实现了具体操作与状态转换之间的解耦。
  • 为不同的状态引入不同的对象使得状态转换变得更加明确,而且可以保证不会出现状态不一致的情况,因为转换是原子性的——即要么彻底转换过来,要么不转换。
  • 如何State对象没有实例变量,那么各个上下文可以共享同一个State对象,从而节省对象开销。