什么是状态设计模式？

状态设计模式—> 行为型模式

基本介绍

状态模式（State Pattern）它主要用来解决对象在多种状态转换时，需要对外输出不同的行为的问题。状态和行为是一一对应的，状态之间可以相互转换。
当一个对象的内在状态改变时，允许改变其行为，这个对象看起来像是改变了其类。

Context 类为环境角色，用于维护 State 实例，这个实例定义当前状态；
State 是抽象状态角色，定义一个接口封装与 Context 的一个特点接口相关行为；
ConcreteState 具体的状态角色，每个子类实现一个与Context的一个状态相关行为；

状态设计模式应用实例

State

public abstract class State {

    public abstract void deductMoney();

    public abstract boolean lottery();

    public abstract void dispensePrize();
}

NoLotteryState

//不可抽奖状态
public class NoLotteryState extends State {

    LotteryActivity activity;

    public NoLotteryState(LotteryActivity activity) {
        this.activity = activity;
    }

    @Override
    public void deductMoney() {
        System.out.println("扣取成功，可以抽奖！");
        //TODO 将状态改为 可以抽奖状态
        activity.setState(activity.getCanLotteryState());
    }

    @Override
    public boolean lottery() {
        System.out.println("买过票，才可以抽奖！");
        return false;
    }

    @Override
    public void dispensePrize() {
        System.out.println("不能发放奖品！");
    }
}

CanLotteryState

//可以抽奖状态
public class CanLotteryState extends State {

    LotteryActivity activity;

    public CanLotteryState(LotteryActivity activity) {
        this.activity = activity;
    }

    @Override
    public void deductMoney() {
        System.out.println("扣取成功，可以抽奖！");
    }

    @Override
    public boolean lottery() {
        System.out.println("正在抽奖，请稍等...");
        Random r = new Random();
        int num = r.nextInt(10);//TODO 10%中奖机会
        if (num == 0) {
            //TODO 改变活动状态为 发放奖品
            activity.setState(activity.getDispenseState());
            return true;
        } else {
            System.out.println("很遗憾没有中奖！");
            //TODO 改变活动状态为 不能抽奖
            activity.setState(activity.getNoLotteryState());
            return false;
        }
    }

    @Override
    public void dispensePrize() {
        System.out.println("没有中奖，不能发放奖品！");
    }
}

DispenseState

//发放奖品状态
public class DispenseState extends State {

    LotteryActivity activity;

    public DispenseState(LotteryActivity activity) {
        this.activity = activity;
    }

    @Override
    public void deductMoney() {
        System.out.println("不能扣钱！");
    }

    @Override
    public boolean lottery() {
        System.out.println("不能抽奖！");
        return false;
    }

    @Override
    public void dispensePrize() {
        if (activity.getCount() > 0) {
            System.out.println("===================> 恭喜中奖，发放奖品！");
            //TODO 改变活动状态为 不能抽奖
            activity.setState(activity.getNoLotteryState());
        } else {
            System.out.println("===================> 已中奖，但很遗憾，奖品发放完毕！");
             activity.setState(activity.getDispensOutState());
//            System.out.println("抽奖活动结束");//TODO 可以注释
//            System.exit(0);//TODO 可以注释
        }
    }
}

DispenseOutState

//奖品发放完毕状态
public class DispenseOutState extends State {

    LotteryActivity activity;

    public DispenseOutState(LotteryActivity activity) {
        this.activity = activity;
    }

    @Override
    public void deductMoney() {
        //System.out.println("奖品发放完毕，请下次参加！");
    }

    @Override
    public boolean lottery() {
        System.out.println("奖品发放完毕，请下次参加！");
        return false;
    }

    @Override
    public void dispensePrize() {
        System.out.println("奖品发放完毕，请下次参加！");
    }
}

LotteryActivity

public class LotteryActivity {

    State state = null;//当前活动状态
    int count = 0;//奖品数量

    State canLotteryState = new CanLotteryState(this);
    State noLotteryState = new NoLotteryState(this);

    State dispenseState = new DispenseState(this);
    State dispensOutState = new DispenseOutState(this);

    public LotteryActivity(int count) {
        this.state = getNoLotteryState();
        this.count = count;
    }

    public void debuctMoney() {
        state.deductMoney();
    }

    public void lottery() {
        if (state.lottery()) {
            state.dispensePrize();
        }
    }

    public State getState() {
        return state;
    }

    public void setState(State state) {
        this.state = state;
    }

    public int getCount() {
        int curCount = count;
        count--;
        return curCount;
    }

    public void setCount(int count) {
        this.count = count;
    }


    public State getNoLotteryState() {
        return noLotteryState;
    }

    public void setNoLotteryState(State noLotteryState) {
        this.noLotteryState = noLotteryState;
    }

    public State getCanLotteryState() {
        return canLotteryState;
    }

    public void setCanLotteryState(State canLotteryState) {
        this.canLotteryState = canLotteryState;
    }

    public State getDispenseState() {
        return dispenseState;
    }

    public void setDispenseState(State dispenseState) {
        this.dispenseState = dispenseState;
    }

    public State getDispensOutState() {
        return dispensOutState;
    }

    public void setDispensOutState(State dispensOutState) {
        this.dispensOutState = dispensOutState;
    }
}

Client

public class Client {
    public static void main(String[] args) {
        LotteryActivity activity = new LotteryActivity(1);

        for (int i = 1; i <= 20; i++) {
            System.out.println("--------第" + i + "抽奖----------");
            activity.debuctMoney();
            activity.lottery();
        }
    }
}

状态模式的注意事项和细节

代码有很强的可读性。状态模式将每个状态的行为封装到对应的一个类中方便维护；
将容易产生问题的 if/else 语句删除了，如果把每个状态的行为都放到一个类中，每次调用方法时都要判断当前是什么状态，不但会产出很多 if/else 语句，而且容易出错；
符合 “开闭原则”，容易增删状态；
会产生很多类。每个状态都要一个对应的类，当状态过多时会产生很多类，加大维护难度；
应用场景：当一个事件或者对象有很多种状态，状态之间会相互转换，对不同的状态要求有不同的行为的时候，可以考虑使用状态模式。

感谢阅读

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状态设计模式（State）

什么是状态设计模式？

状态设计模式应用实例

State

NoLotteryState

CanLotteryState

DispenseState

DispenseOutState

LotteryActivity

Client

状态模式的注意事项和细节

感谢阅读

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