设计模式之状态模式

情景：有一个自动投币的糖果机。它有以下几种状态（每个圆圈代表一个状态）：

如果用常量值来定义几个变量

final static int SOLD_OUT = 0;
final static int NO_QUARTER = 1;
final static int HAS_QUARTER = 2;
final static int SOLD = 3;

int state = SOLD_OUT;

这样状态变化就可以写成：

public void insertQuarter() {
    if (state == HAS_QUARTER) {
        ...
    } else if (state == ...) {
        ...
    } ...
}

每一个动作都要先判断状态，改后再决定反馈以及下一个状态是什么。

虽然可以实现要求，但是当出现新的状态的时候，很不利于扩展。

我们把状态变成一个类，把不同的状态封装在各自的类中，然后在动作发生时委托给当前状态。

首先，定义状态接口

public interface State {
    public void insertQuarter();
    public void ejectQuarter();
    public void turnCrank();
    public void dispense();
}

然后定义各种状态

public class NoQuarterState implements State {
    
    GumballMachine gumballMachine;
    
    public NoQuarterState(GumballMachine gumballMachine) {
        this.gumballMachine = gumballMachine; 
    }

    @Override
    public void insertQuarter() {
        System.out.println("You insert a quarter");
        gumballMachine.setState(gumballMachine.getHasQuarterState());
    }

    @Override
    public void ejectQuarter() {
        System.out.println("You haven't inserted a quarter");
    }

    @Override
    public void turnCrank() {
        System.out.println("You turned, but there's no quarter");
    }

    @Override
    public void dispense() {
        System.out.println("You need to pay first");
    }

}

public class HasQuarterState implements State {
    
    GumballMachine gumballMachine;
    
    public HasQuarterState(GumballMachine gumballMachine) {
        this.gumballMachine = gumballMachine;
    }

    @Override
    public void insertQuarter() {
        System.out.println("You can't insert another quater");        
    }

    @Override
    public void ejectQuarter() {
        System.out.println("Quarter returned");
        gumballMachine.setState(gumballMachine.getNoQuarterState());
    }

    @Override
    public void turnCrank() {
        System.out.println("You turned...");
        gumballMachine.setState(gumballMachine.getSoldState());
    }

    @Override
    public void dispense() {
        System.out.println("No gumball dispensed");
    }

}

public class SoldOutState implements State {
    
    GumballMachine gumballMachine;
    
    public SoldOutState(GumballMachine gumballMachine) {
        this.gumballMachine = gumballMachine;
    }

    @Override
    public void insertQuarter() {
        System.out.println("You can't insert a quarter, the machine is sold out");
    }

    @Override
    public void ejectQuarter() {
        System.out.println("You can't eject, you haven't inserted a quarter yet");
    }

    @Override
    public void turnCrank() {
        System.out.println("You turned, but there are no gumballs");
    }

    @Override
    public void dispense() {
        System.out.println("No gumball dispensed");
    }

}

public class SoldState implements State {
    
    GumballMachine gumballMachine;
    
    public SoldState(GumballMachine gumballMachine) {
        this.gumballMachine = gumballMachine;
    }

    @Override
    public void insertQuarter() {
        System.out.println("Please wait, we're already giving you a gumball");
    }

    @Override
    public void ejectQuarter() {
        System.out.println("Sorry, you already turned the crank");
    }

    @Override
    public void turnCrank() {
        System.out.println("Turning twice doesn't get you another gumball!");
    }

    @Override
    public void dispense() {
        gumballMachine.releaseBall();
        if (gumballMachine.getCount() > 0) {
            gumballMachine.setState(gumballMachine.getNoQuarterState());
        } else {
            System.out.println("Oops, out of gumballs!");
            gumballMachine.setState(gumballMachine.getSoldOutState());
        }
    }

}

最后写一下糖果机

public class GumballMachine {
    
    State soldOutState;
    State noQuarterState;
    State hasQuarterState;
    State soldState;
    
    State state = soldOutState;
    int count = 0;
    
    public GumballMachine(int numberGumballs) {
        soldOutState = new SoldOutState(this);
        noQuarterState = new NoQuarterState(this);
        hasQuarterState = new HasQuarterState(this);
        soldState = new SoldState(this);
        
        count = numberGumballs;
        if (numberGumballs > 0) {
            state = noQuarterState;
        }
    }
    
    public void insertQuarter() {
        state.insertQuarter();
    }
    
    public void ejectQuarter() {
        state.ejectQuarter();
    }
    
    public void turnCrank() {
        state.turnCrank();
        state.dispense();
    }
    
    void setState(State state) {
        this.state = state;
    }
    
    void releaseBall() {
        System.out.println("A gumball comes rolling out the slot...");
        if (count != 0) {
            count -= 1;
        }
    }
    
    public int getCount() {
        return count; 
    }
    
    public State getSoldOutState() {
        return soldOutState;
    }

    public State getNoQuarterState() {
        return noQuarterState;
    }

    public State getHasQuarterState() {
        return hasQuarterState;
    }

    public State getSoldState() {
        return soldState;
    }
    
    public String toString() {    
        return "
Mighty Gumball, Inc
" + "Inventory: " + 
                count + " gumballs
";
    }
}

测试类：

public class GumballMachineTestDriver {
    public static void main(String[] args) {
        GumballMachine gumballMachine = new GumballMachine(5);
        
        System.out.println(gumballMachine);
        
        gumballMachine.insertQuarter();
        gumballMachine.turnCrank();
        
        System.out.println(gumballMachine);
        
        gumballMachine.insertQuarter();
        gumballMachine.ejectQuarter();
        gumballMachine.turnCrank();
        
        System.out.println(gumballMachine);
        
        gumballMachine.insertQuarter();
        gumballMachine.turnCrank();
        gumballMachine.insertQuarter();
        gumballMachine.turnCrank();
        gumballMachine.ejectQuarter();
        
        System.out.println(gumballMachine);
        
        gumballMachine.insertQuarter();
        gumballMachine.insertQuarter();
        gumballMachine.turnCrank();
        gumballMachine.insertQuarter();
        gumballMachine.turnCrank();
        gumballMachine.insertQuarter();
        gumballMachine.turnCrank();
        
        System.out.println(gumballMachine);
        
    }
}

将每一个状态局部化到它自己的类中。将容易产生问题的if语句删除，方便日后的维护。

这样，每一个状态“对修改关闭”，糖果机“对扩展开放”

状态模式：允许对象在内部状态改变时改变他的行为，对象看起来好像修改了它的类。

类图：

现在我们改变需求，希望有10%的概率中奖，也就是投一个币，获得两个糖果。

我们只需要添加一个新的状态类。

public class WinnerState implements State {
    
    GumballMachine gumballMachine;
    
    public WinnerState(GumballMachine gumballMachine) {
        this.gumballMachine = gumballMachine;
    }

    @Override
    public void insertQuarter() {
        System.out.println("Please wait, we're already giving you a gumball");
    }

    @Override
    public void ejectQuarter() {
        System.out.println("Sorry, you already turned the crank");
    }

    @Override
    public void turnCrank() {
        System.out.println("Turning twice doesn't get you another gumball!");
    }

    @Override
    public void dispense() {
        System.out.println("YOU'RE A WINNER! YOU GET TWO GUMBALLS FOR YOUR QUARTERS");
        gumballMachine.releaseBall();
        if (gumballMachine.getCount() == 0) {
            gumballMachine.setState(gumballMachine.getSoldOutState());
        } else {
            gumballMachine.releaseBall();
            if (gumballMachine.getCount() > 0) {
                gumballMachine.setState(gumballMachine.getNoQuarterState());
            } else {
                System.out.println("Oops, out of gumballs!");
                gumballMachine.setState(gumballMachine.getSoldOutState());
            }
        }
    }

}

在HasQuarterState中有几率跳到WinnerState状态：

public class HasQuarterState implements State {
    
    GumballMachine gumballMachine;
    Random randomWinner = new Random(System.currentTimeMillis());

    // ....

    @Override
    public void turnCrank() {
        System.out.println("You turned...");
        int winner = randomWinner.nextInt(10);
        if (winner == 0 && (gumballMachine.getCount() > 1)) {
            gumballMachine.setState(gumballMachine.getWinnerState());
        } else {
            gumballMachine.setState(gumballMachine.getSoldState());
        }
    }

    // ....

}

记得在糖果机类中添加一个WinnerState状态类，这个新的需求就实现了。

• 状态模式允许Context随着状态的改变而改变行为。
• 状态装换可以由State类或者Context类控制。
• 使用状态模式通常会导致设计中的类数目大量增加。
• 状态类可以被多个Context实例共享。