《headfirst设计模式》读书笔记10-状态模式

1. 糖果机

这是一个状态图，糖果机有“糖果售完”，“有25分钱”，“没有25分钱”，“售出糖果”四个状态（其实还有一个检测是否售完的状态），我们也有四个动作“投入25分钱”，“退回25分钱”，“转动曲柄”，“发放糖果”。但是对于任意一个可能的动作，我们都要检查，看看我们所处的状态和动作是否合适（所以每种状态下都可能做这些动作）。

2. 看图写代码

这是一个照着图写的代码，只要一变更需求则需要重构代码

2.1 GumballMachine.h

#include <string>
#include <iostream>
using std::cout;
using std::endl;
using std::string;
enum{SOLD_OUT,NO_QUARTER,HAS_QUARTER,SOLD};
class GumballMachine{
public:GumballMachine(int count);void insertQuarter();void ejectQuarter();void turnCrank();void dispense();void refill(int numGumBalls);string toString();
private:int state=SOLD_OUT;int count=0;
};

2.2 GumballMachine.cpp

#include "GumballMachine.h"GumballMachine::GumballMachine(int count)
{this->count=count;if(count>0){state=NO_QUARTER;}
}void GumballMachine::insertQuarter()
{if (state == HAS_QUARTER) {cout<<"You can't insert another quarter"<<endl;} else if (state == NO_QUARTER) {state = HAS_QUARTER;cout<<"You inserted a quarter"<<endl;} else if (state == SOLD_OUT) {cout<<"You can't insert a quarter, the machine is sold out"<<endl;} else if (state == SOLD) {cout<<"Please wait, we're already giving you a gumball"<<endl;}
}void GumballMachine::ejectQuarter()
{if (state == HAS_QUARTER) {cout<<"Quarter returned"<<endl;state = NO_QUARTER;} else if (state == NO_QUARTER) {cout<<"You haven't inserted a quarter"<<endl;} else if (state == SOLD) {cout<<"Sorry, you already turned the crank"<<endl;} else if (state == SOLD_OUT) {cout<<"You can't eject, you haven't inserted a quarter yet"<<endl;}
}void GumballMachine::turnCrank()
{if (state == SOLD) {cout<<"Turning twice doesn't get you another gumball!"<<endl;} else if (state == NO_QUARTER) {cout<<"You turned but there's no quarter"<<endl;} else if (state == SOLD_OUT) {cout<<"You turned, but there are no gumballs"<<endl;} else if (state == HAS_QUARTER) {cout<<"You turned..."<<endl;state = SOLD;dispense();}
}void GumballMachine::dispense()
{if (state == SOLD) {cout<<"A gumball comes rolling out the slot"<<endl;count = count - 1;if (count == 0) {cout<<"Oops, out of gumballs!"<<endl;state = SOLD_OUT;} else {state = NO_QUARTER;}} else if (state == NO_QUARTER) {cout<<"You need to pay first"<<endl;} else if (state == SOLD_OUT) {cout<<"No gumball dispensed"<<endl;} else if (state == HAS_QUARTER) {cout<<"No gumball dispensed"<<endl;}
}void GumballMachine::refill(int numGumBalls)
{this->count = numGumBalls;state = NO_QUARTER;
}string GumballMachine::toString()
{string result="";result.append("\nMighty Gumball, Inc.");result.append("\nJava-enabled Standing Gumball Model #2004\n");result.append("Inventory: " + std::to_string(count) + " gumball");if (count != 1) {result.append("s");}result.append("\nMachine is ");if (state == SOLD_OUT) {result.append("sold out");} else if (state == NO_QUARTER) {result.append("waiting for quarter");} else if (state == HAS_QUARTER) {result.append("waiting for turn of crank");} else if (state == SOLD) {result.append("delivering a gumball");}result.append("\n");return result;
}

3. 定义状态模式

状态模式就是将状态封装成独立的类（通过抽象出状态类，实现共同的方法），通过组合的改变关联的类（组合可以随意改变关联的类），然后委托关联的类进行方法的处理（不同的状态就会关联不同的类，从而调用不同的方法）

状态模式：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

4. 部分源码

4.1 State.h

#include <iostream>
#include <string>
using std::string;
using std::cout;
using std::endl;
class State{
public:virtual void insertQuarter()=0;virtual void ejectQuarter()=0;virtual void turnCrank()=0;virtual void dispense()=0;virtual void refill()=0;virtual string toString()=0;virtual ~State(){};
};

4.2 HasQuarterState.h

#include "State.h"
class GumballMachine;
class HasQuarterState:public State{
public:HasQuarterState(GumballMachine *gumballMachine);virtual void insertQuarter()override final;virtual void ejectQuarter()override final;virtual void turnCrank()override final;virtual void dispense()override final;virtual void refill()override final;virtual string toString()override final;virtual ~HasQuarterState();
private:GumballMachine *gumballMachine;
};

4.3 HasQuarterState.cpp

#include "HasQuarterState.h"
#include "GumballMachine.h"
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;
}void HasQuarterState::refill()
{}string HasQuarterState::toString()
{return "waiting for turn of crank";
}HasQuarterState::~HasQuarterState()
{}

4.4 GumballMachine.h

#include "State.h"
#include <string>
using std::string;
class GumballMachine{
public:GumballMachine(int numberGumballs);void insertQuarter();void ejectQuarter();void turnCrank();void releaseBall();int getCount();void refill(int count);void setState(State *state);State *getState();State *getSoldOutState();State *getNoQuarterState();State *getHasQuarterState();State *getSoldState();string toString();~GumballMachine();
private:State *state;int count=0;State *soldOutState;State *noQuarterState;State *hasQuarterState;State *soldState;
};

5.策略模式和状态模式的区别

策略模式和状态模式的类图相同，但是它们的意图不同：
1.状态模式不想让客户知道状态对象的改变，策略模式是客户主动组合的对象
2.状态模式内部状态对象变化很快，策略模式内部状态对象一般是不咋改变
3.状态模式是不用在context中放置许多条件判断的替换方案，策略模式是想除了继承之外的一种弹性替换方案。