“状态变化”模式
- 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。
- 典型模式
- State
- Memento
动机（Motivation）
- 在软件构建过程中，某些对象的状态如果改变，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紧耦合？ //透明的：稳定
// 和策略模式很像。
示例
一般的写法，很多 if else， 和策略模式改造之前很像。
#include <iostream>
#include <string>
using namespace std;
enum NetworkState
{
Network_Open,
Network_Close,
Network_Connect,
};
class NetworkProcessor {
NetworkState state = NetworkState::Network_Open;
public:
/*很多 if else 和策略模式改造前很像。*/
void Operation1() {
if (state == Network_Open) {
//**********
cout << "网络" << "打开" << endl;
state = Network_Close;
}
else if (state == Network_Close) {
cout << "网络" << "关闭" << endl;
//..........
state = Network_Connect;
}
else if (state == Network_Connect) {
cout << "网络" << "连接" << endl;
//$$$$$$$$$$
state = Network_Open;
}
else;
}
void Operation2() {
if (state == Network_Open) {
//**********
cout << "网络" << "打开" << endl;
state = Network_Connect;
}
else if (state == Network_Close) {
//.....
cout << "网络" << "关闭" << endl;
state = Network_Open;
}
else if (state == Network_Connect) {
//$$$$$$$$$$
cout << "网络" << "连接" << endl;
state = Network_Close;
}
else;
}
void Operation3() {
//******
}
};
int main()
{
NetworkProcessor processor;
processor.Operation1();
processor.Operation1();
processor.Operation1();
processor.Operation1();
getchar();
return 0;
}
输出：
网络打开
网络关闭
网络连接
网络打开
状态模式：
#include <iostream>
#include <string>
using namespace std;
class NetworkState {
public:
NetworkState* pNext;
virtual void Operation1() = 0;
virtual void Operation2() = 0;
virtual void Operation3() = 0;
virtual ~NetworkState() {}
};
class CloseState :public NetworkState {
public:
CloseState() {};
~CloseState() {};
public:
static NetworkState* getInstance();
virtual void Operation1();
virtual void Operation2();
virtual void Operation3();
private:
// 这里用单例
static NetworkState* m_instance;
};
//后面扩展这个类就行
class OpenState :public NetworkState {
public:
OpenState() {};
~OpenState() {};
public:
static NetworkState* getInstance();
virtual void Operation1();
virtual void Operation2();
virtual void Operation3();
private:
// 这里用单例
static NetworkState* m_instance;
};
class ConnectState :public NetworkState {
public:
ConnectState() {};
~ConnectState() {};
public:
static NetworkState* getInstance();
virtual void Operation1();
virtual void Operation2();
virtual void Operation3();
private:
// 这里用单例
static NetworkState* m_instance;
};
NetworkState* OpenState::m_instance = nullptr;
NetworkState* OpenState::getInstance() {
if (m_instance == nullptr) {
m_instance = new OpenState();
}
return m_instance;
}
void OpenState::Operation1() {
//**********
cout << "OpenState " << "Operation1." << endl;
pNext = ConnectState::getInstance();
}
void OpenState::Operation2() {
cout << "OpenState " << "Operation2." << endl;
//..........
pNext = ConnectState::getInstance();
}
void OpenState::Operation3() {
cout << "OpenState " << "Operation3." << endl;
//$$$$$$$$$$
pNext = OpenState::getInstance();
}
NetworkState* CloseState::m_instance = nullptr;
NetworkState* CloseState::getInstance() {
if (m_instance == nullptr) {
m_instance = new CloseState();
}
return m_instance;
}
void CloseState::Operation1() {
cout << "CloseState " << "Operation1." << endl;
//**********
pNext = OpenState::getInstance();
}
void CloseState::Operation2() {
cout << "CloseState " << "Operation2." << endl;
//..........
pNext = ConnectState::getInstance();
}
void CloseState::Operation3() {
cout << "CloseState " << "Operation3." << endl;
//$$$$$$$$$$
pNext = OpenState::getInstance();
}
NetworkState* ConnectState::m_instance = nullptr;
NetworkState* ConnectState::getInstance() {
if (m_instance == nullptr) {
m_instance = new ConnectState();
}
return m_instance;
}
void ConnectState::Operation1() {
//**********
cout << "ConnectState " << "Operation1." << endl;
pNext = CloseState::getInstance();
}
void ConnectState::Operation2() {
cout << "ConnectState " << "Operation2." << endl;
//..........
pNext = ConnectState::getInstance();
}
void ConnectState::Operation3() {
cout << "ConnectState " << "Operation3." << endl;
//$$$$$$$$$$
pNext = OpenState::getInstance();
}
// 这个写好以后不用变
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;
//...
}
};
int main()
{
OpenState openState;
CloseState closeState;
ConnectState connectState;
NetworkProcessor netWorkProcessor(&openState);
netWorkProcessor.Operation1();
netWorkProcessor.Operation1();
netWorkProcessor.Operation1();
netWorkProcessor.Operation1();
getchar();
return 0;
}
输出：
OpenState Operation1.
ConnectState Operation1.
CloseState Operation1.
OpenState Operation1.
模式定义
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。——《设计模式》GoF
类图
要点总结
- 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。
- 如果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。
参考：GeekBand
