一、模式动机

　　在很多情况下，一个对象的行为取决于一个或多个动态变化的属性，这样的属性叫做状态，这样的对象叫做有状态的对象，对象的状态是从事先定义好的一系列值中取出的。当一个这样的对象与外部事件产生互动时，其内部状态就会改变，从而使得系统的行为也随之发生变化。

二、模式定义

　　状态模式（State Pattern）允许对象在内部状态改变时改变它的行为，对象看起来好像修改了它的类。UML类图如下:　　　　　 状态模式包含如下角色 ● Context: 上下文类 ● State: 抽象状态类 ● ConcreteState: 具体状态类

三、模式示例

　　万能糖果公司希望糖果机如下图一样工作。

C++代码实现

State.h

#ifndef STATE_H #define STATE_H class GumballMachine; //抽象状态类 class State { public: virtual void InsertQuarter() = 0; virtual void EjectQuarter() = 0; virtual void TurnCrank() = 0; virtual void Dispense() = 0; }; /*具体状态类*/ class NoQuarterState : public State { public: NoQuarterState(GumballMachine *gumballMachine); void InsertQuarter(); void EjectQuarter(); void TurnCrank(); void Dispense(); private: GumballMachine* gumballMachine; }; class HasQuarterState : public State { public: HasQuarterState(GumballMachine* gumballMachine); void InsertQuarter(); void EjectQuarter(); void TurnCrank(); void Dispense(); private: GumballMachine* gumballMachine; }; class SoldState : public State { public: SoldState(GumballMachine* gumballMachine); void InsertQuarter(); void EjectQuarter(); void TurnCrank(); void Dispense(); private: GumballMachine* gumballMachine; }; class SoldOutState : public State { public: SoldOutState(GumballMachine* gumballMachine); void InsertQuarter(); void EjectQuarter(); void TurnCrank(); void Dispense(); private: GumballMachine* gumballMachine; }; #endif

State.cpp

#include <iostream> #include "GumballMachine.h" using namespace std; NoQuarterState::NoQuarterState(GumballMachine* gumballMachine) { this->gumballMachine = gumballMachine; } void NoQuarterState::InsertQuarter() { cout << "You inserted a quarter" << endl; gumballMachine->SetState(gumballMachine->GetHasQuarterState()); } void NoQuarterState::EjectQuarter() { cout << "You haven't inserted a quarter" << endl; } void NoQuarterState::TurnCrank() { cout << "You turned, but there's no quarter" << endl; } void NoQuarterState::Dispense() { cout << "You need to pay first" << endl; } HasQuarterState::HasQuarterState(GumballMachine* gumballMachine) { this->gumballMachine = gumballMachine; } void HasQuarterState::InsertQuarter() { cout << "You can't insert another quarter" << endl; } void HasQuarterState::EjectQuarter() { cout << "Quarter returned" << endl; gumballMachine->SetState(gumballMachine->GetNoQuarterState()); } void HasQuarterState::TurnCrank() { cout << "You turned..." << endl; gumballMachine->SetState(gumballMachine->GetSoldState()); } void HasQuarterState::Dispense() { cout << "No gumball dispensed" << endl; } SoldState::SoldState(GumballMachine* gumballMachine) { this->gumballMachine = gumballMachine; } void SoldState::InsertQuarter() { cout << "Please wait, we're already giving you a gumball" << endl; } void SoldState::EjectQuarter() { cout << "Sorry, you already turned the crank" << endl; } void SoldState::TurnCrank() { cout << "Turning twice doesn't get you another gumball" << endl; } void SoldState::Dispense() { gumballMachine->ReleaseBall(); if (gumballMachine->GetCount() > 0) { gumballMachine->SetState(gumballMachine->GetNoQuarterState()); } else { gumballMachine->SetState(gumballMachine->GetSoldOutState()); } } SoldOutState::SoldOutState(GumballMachine* gumballMachine) { this->gumballMachine = gumballMachine; } void SoldOutState::InsertQuarter() { cout << "You can't insert a quarter, the machine is sold out" << endl; } void SoldOutState::EjectQuarter() { cout << "You can't eject, you haven't inserted a quarter yet" << endl; } void SoldOutState::TurnCrank() { cout << "You turned, but there are no gumballs" << endl; } void SoldOutState::Dispense() { cout << "No gumball dispensed" << endl; }

GumballMachine.h

#ifndef GUMBALL_MACHINE_H #define GUMBALL_MACHINE_H class State; class GumballMachine { public: GumballMachine(int numberGumballs); void InsertQuarter(); void EjectQuarter(); void TurnCrank(); State* GetHasQuarterState(); State* GetNoQuarterState(); State* GetSoldState(); State* GetSoldOutState(); void SetState(State* state); int GetCount() { return count; } void ReleaseBall() { if (count != 0) { count--; } } private: State* state; State* soldOutState; State* noQuarterState; State* hasQuarterState; State* soldState; int count = 0; }; #endif

GumballMachine.cpp

#include "GumballMachine.h" #include "State.h" GumballMachine::GumballMachine(int numberGumballs) { soldOutState = new SoldOutState(this); noQuarterState = new NoQuarterState(this); hasQuarterState = new HasQuarterState(this); soldState = new SoldState(this); count = numberGumballs; if (count > 0) { state = noQuarterState; } } void GumballMachine::InsertQuarter() { state->InsertQuarter(); } void GumballMachine::EjectQuarter() { state->EjectQuarter(); } void GumballMachine::TurnCrank() { state->TurnCrank(); state->Dispense(); } State* GumballMachine::GetHasQuarterState() { return hasQuarterState; } State* GumballMachine::GetNoQuarterState() { return noQuarterState; } State* GumballMachine::GetSoldState() { return soldState; } State* GumballMachine::GetSoldOutState() { return soldOutState; } void GumballMachine::SetState(State* state) { this->state = state; }

main.cpp

#include "stdlib.h" #include "GumballMachine.h" int _tmain(int argc, _TCHAR* argv[]) { GumballMachine gumballMachine = GumballMachine(2); gumballMachine.InsertQuarter(); gumballMachine.TurnCrank(); gumballMachine.InsertQuarter(); gumballMachine.TurnCrank(); gumballMachine.InsertQuarter(); gumballMachine.TurnCrank(); system("pause"); return 0; }

运行结果：

四、分析总结

　　状态模式的关键是引入了一个抽象类来专门表示对象的状态，这个类我们叫做抽象状态类，而对象的每一种具体状态类都继承了该类，并在不同具体状态类中实现了不同状态的行为，包括各种状态之间的转换。这使得环境类对象在其内部状态改变时可以改变它的行为，对象看起来似乎修改了它的类，而实际上是由于切换到不同的具体状态类实现的。一句话：将状态封装成为独立的类，Context会将行为委托到代表当前状态的对象。 　　策略模式通常会用行为或算法来配置Context类，状态模式允许Context随着状态的改变而改变行为。

优点

封装了转换规则。枚举可能的状态，在枚举状态之前需要确定状态种类。将所有与某个状态有关的行为放到一个类中，并且可以方便地增加新的状态，只需要改变对象状态即可改变对象的行为。允许状态转换逻辑与状态对象合成一体，而不是某一个巨大的条件语句块。可以让多个环境对象共享一个状态对象，从而减少系统中对象的个数。

缺点

状态模式的使用必然会增加系统类和对象的个数。状态模式的结构与实现都较为复杂，如果使用不当将导致程序结构和代码的混乱。状态模式对“开闭原则”的支持并不太好，对于可以切换状态的状态模式，增加新的状态类需要修改那些负责状态转换的源代码，否则无法切换到新增状态；而且修改某个状态类的行为也需修改对应类的源代码。