状态模式State
在组件构建过程中,某些对象的状态经常面临变化,如何对这些变化进行有效的管理?同时又维持高层模块的稳定?“状态变化”模式为这一问题提供了一种解决方案。
经典模式:State、Memento
动机(Motivation)
在软件构建过程中,某些对象的状态如何改变,其行为也随之而发生变化,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以完全不同。
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化之间引入耦合?
示例1
enum NetworkState
{Network_Open,Network_Close,Network_Connect,NetWork_Wait // 新增状态
};class NetworkProcessor {NetworkState state_;public:void Operation1() {if (Network_Open == state_) {//***state_ = Network_Close;}else if(Network_Close == state_){//...state_ = Network_Connect;}else if (Network_Connect == state_) {//$$$state_ = Network_Open;}// 变化else if (NetWork_Wait == state_) {//---}}void Operation2() {if (Network_Open == state_) {//***state_ = Network_Connect;}else if (Network_Close == state_){//...state_ = Network_Open;}else if (Network_Connect == state_) {//$$$state_ = Network_Close;}// 变化else if (NetWork_Wait == state_) {//---}}void Operation3() {//...}
};
示例2
class NetworkState {
public:NetworkState* pNext;virtual void Operation1() = 0;virtual void Operation2() = 0;virtual void Operation3() = 0;virtual ~NetworkState() {}
};class OpenState : public NetworkState {static NetworkState* m_instance;public:static NetworkState* getInstance() {if (nullptr == m_instance) {m_instance = new OpenState();}return m_instance;}virtual void Operation1() {//***pNext = CloseState::getInstance();}virtual void Operation2() {//...pNext = ConnectState::getInstance();}virtual void Operation3() {//$$$pNext = OpenState::getInstance();}
};class CloseState : public NetworkState {static NetworkState* m_instance;public:static NetworkState* getInstance() {if (nullptr == m_instance) {m_instance = new CloseState();}return m_instance;}virtual void Operation1() {//***pNext = ConnectState::getInstance();}virtual void Operation2() {//...pNext = OpenState::getInstance();}virtual void Operation3() {//$$$pNext = CloseState::getInstance();}
};class ConnectState : public NetworkState {static NetworkState* m_instance;public:static NetworkState* getInstance() {if (nullptr == m_instance) {m_instance = new ConnectState();}return m_instance;}virtual void Operation1() {//***pNext = OpenState::getInstance();}virtual void Operation2() {//...pNext = CloseState::getInstance();}virtual void Operation3() {//$$$pNext = ConnectState::getInstance();}
};// 扩展
class WaitState : public NetworkState {//...
};class NetworkProcessor {NetworkState* state_;public:NetworkProcessor(NetworkState* state) {this->state_ = state;}void Operation1() {//...state_->Operation1();state_ = state_->pNext;//...}void Operation2() {//...state_->Operation2();state_ = state_->pNext;//...}void Operation3() {//...state_->Operation3();state_ = state_->pNext;//...}};
要点总结
State模式将所有与一个特定状态的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,切换相应的对象;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,而且可怀疑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。
如果State对象没有实例变量,那么各个上下文可怀疑共享同一个State对象,从而节省对象开销。