学习笔记：设计模式之状态模式

请编写程序完成APP抽奖活动 具体要求如下:
① 假如每参加一次这个活动要扣除用户50积分，中奖概率是10%
② 奖品数量固定，抽完就不能抽奖
③ 活动有四个状态: 可以抽奖、不能抽奖、发放奖品和奖品领完
④ 活动的四个状态转换关系图

状态模式

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。

状态模式原理类图

对原理类图的说明

1. Context 类为环境角色, 用于维护State实例,这个实例定义当前状态
2. 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
3. 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

代码实现：

public interface State {
    public void deducePoint(int point);
    public boolean drawPrice();
    public void receivePrice(int priceCount);
}

public class Activity {
    private int point = 0;
    private int priceCount = 0;
    private State state = null;
    private boolean isOver = false;
    private boolean isWin = false;
    private boolean isNext = true;
    private State prepareState = new PrepareState(this);
    private State receivePriceState = new ReceivePriceState(this);
    private State drawPriceState = new DrawPriceState(this);
    public Activity(int point, int priceCount) {
        state = getPrepareState();
        this.point = point;
        this.priceCount = priceCount;
    }
    public void deducePoint() {
        state.deducePoint(point);
    }
    public void drawPrice() {
        if(isNext) {
            isWin = state.drawPrice();
        }
    }
    public void receivePrice() {
        if(isWin) {
            state.receivePrice(priceCount);
        }
    }
    public State getPrepareState() {
        return prepareState;
    }
    public State getReceivePriceState() {
        return receivePriceState;
    }
    public State getDrawPriceState() {
        return drawPriceState;
    }
    public void setState(State state) {
        this.state = state;
        isOver = false;
        isWin = false;
        isNext = true;
    }
    public int getPoint() {
        return point;
    }
    public void setPoint(int point) {
        this.point = point;
    }
    public int getPriceCount() {
        return priceCount;
    }
    public void setPriceCount(int priceCount) {
        this.priceCount = priceCount;
    }
    public boolean isOver() {
        return isOver;
    }
    public void setOver(boolean isOver) {
        this.isOver = isOver;
    }
    public boolean isNext() {
        return isNext;
    }
    public void setNext(boolean isNext) {
        this.isNext = isNext;
    }
}

public class PrepareState implements State{
    private Activity activity;
    public PrepareState(Activity activity) {
        this.activity = activity;
    }
    @Override
    public void deducePoint(int point) {
        if(point >= 50 && activity.getPriceCount() > 0) {
            point -= 50;
            activity.setPoint(point);
            activity.setState(activity.getDrawPriceState());
            System.out.println("已经扣除50积分！");
        }else {
            activity.setNext(false);
            activity.setOver(true);
            if(point < 50) {
                System.out.println("积分不足，不能参加抽奖！");
            }else {
                System.out.println("真遗憾，已经没有奖品了！");
            }
        }
    }
    @Override
    public boolean drawPrice() {
        System.out.println("请先扣除积分!");
        return false;
    }
    @Override
    public void receivePrice(int priceCount) {
        System.out.println("请先扣除积分!");
    }
}

public class DrawPriceState implements State{
    private Activity activity;
    public DrawPriceState(Activity activity) {
        this.activity = activity;
    }
    @Override
    public void deducePoint(int point) {
        System.out.println("已经扣除了积分，不用再次扣除！");
    }
    @Override
    public boolean drawPrice() {
        System.out.println("正在抽奖，请耐心等待！");
        Random random = new Random();
        int i = random.nextInt(10);
        if(i == 0) {
            activity.setState(activity.getReceivePriceState());
            System.out.println("恭喜您中奖了！");
            return true;
        }else {
            activity.setState(activity.getPrepareState());
            System.out.println("很遗憾，没中奖，欢迎下次再来！");
            return false;
        }
    }
    @Override
    public void receivePrice(int priceCount) {
        System.out.println("请先参与抽奖！ ");
    }

}

public class ReceivePriceState implements State{
    private Activity activity;
    public ReceivePriceState(Activity activity) {
        this.activity = activity;
    }
    @Override
    public void deducePoint(int point) {
        System.out.println("您已经被扣除了积分，不用再次扣除！");
    }
    @Override
    public boolean drawPrice() {
        System.out.println("您已经参与抽奖了！");
        return false;
    }
    @Override
    public void receivePrice(int priceCount) {
        if(priceCount > 0) {
            priceCount --;
            activity.setPriceCount(priceCount);
            activity.setState(activity.getPrepareState());
            System.out.println("奖品已经发放成功，欢迎下次再来！");
        }else {
            activity.setOver(true);
            System.out.println("真遗憾，没有奖品了！");
        }
    }

}

public class Test {
    public static void main(String[] args) {
        Activity activity = new Activity(500, 1);
        int i = 1;
        while(!activity.isOver()) {
            System.out.println("==============第"+ i +"次抽奖==================");
            activity.deducePoint();
            activity.drawPrice();
            activity.receivePrice();
            i++;
        }
    }
}

状态模式UML类图

评价：

优势：

① 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
② 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
③ 符合“开闭原则”，容易增删状态

不足：

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