状态模式

1 概念

1 定义

当一个对象内在状态改变时允许改变其行为。比如：停止状态下，不允许快进视频这个行为

类型：行为型

核心角色

1. 环境类（Context）：环境类是拥有状态的对象。它维护一个指向当前状态对象的引用，并将与状态相关的请求委托给状态对象进行处理。
2. 抽象状态类（State）：抽象状态类定义了一个接口，用于封装与环境对象相关的行为。它可以根据具体的状态进行具体的行为实现。
3. 具体状态类（Concrete State）：具体状态类实现了抽象状态类中定义的接口，提供了与具体状态相关的行为实现。

2 应用场景

一个对象存在多个状态（不同状态下行为不同），且状态可以转换

3 优缺点

优点：

• 将不同状态隔离
• 将各种状态的转换逻辑，分布到state的子类中，减少互相依赖
• 增加新的状态简单

缺点：

• 状态多的场景下，导致类数目增加，系统变复杂

2 代码实现

场景：网站的课程视频有暂停，快进，停止，播放状态

// 课程状态抽象类
public abstract class CourseVideoState {
    protected CourseVideoContext courseVideoContext;

    public void setCourseVideoContext(CourseVideoContext courseVideoContext) {
        this.courseVideoContext = courseVideoContext;
    }

    public abstract void play();
    public abstract void speed();
    public abstract void pause();
    public abstract void stop();
}

// 播放状态
public class PlayState extends CourseVideoState {

    @Override
    public void play() {
        System.out.println("正常播放课程视频状态");
    }

    @Override
    public void speed() {
        super.courseVideoContext.setCourseVideoState(CourseVideoContext.SPEED_STATE);
    }

    @Override
    public void pause() {
        super.courseVideoContext.setCourseVideoState(CourseVideoContext.PAUSE_STATE);
    }

    @Override
    public void stop() {
        super.courseVideoContext.setCourseVideoState(CourseVideoContext.STOP_STATE);
    }
}

// 快进状态
public class SpeedState extends CourseVideoState {
    @Override
    public void play() {
        super.courseVideoContext.setCourseVideoState(CourseVideoContext.PLAY_STATE);
    }

    @Override
    public void speed() {
        System.out.println("快进播放课程视频状态");
    }

    @Override
    public void pause() {
        super.courseVideoContext.setCourseVideoState(CourseVideoContext.PAUSE_STATE);
    }

    @Override
    public void stop() {
        super.courseVideoContext.setCourseVideoState(CourseVideoContext.STOP_STATE);
    }
}

// 暂停状态
public class PauseState extends CourseVideoState {

    @Override
    public void play() {
        super.courseVideoContext.setCourseVideoState(CourseVideoContext.PLAY_STATE);
    }

    @Override
    public void speed() {
        super.courseVideoContext.setCourseVideoState(CourseVideoContext.SPEED_STATE);
    }

    @Override
    public void pause() {
        System.out.println("暂停播放课程视频状态");
    }

    @Override
    public void stop() {
        super.courseVideoContext.setCourseVideoState(CourseVideoContext.STOP_STATE);
    }
}

// 停止状态
public class StopState extends CourseVideoState {
    @Override
    public void play() {
        super.courseVideoContext.setCourseVideoState(CourseVideoContext.PLAY_STATE);
    }

    @Override
    public void speed() {
        System.out.println("ERROR 停止状态不能快进!!");
    }

    @Override
    public void pause() {
        System.out.println("ERROR 停止状态不能暂停!!");
    }

    @Override
    public void stop() {
        System.out.println("停止播放课程视频状态");
    }
}

/**
 * 课程状态上下文
 */
public class CourseVideoContext {

    private CourseVideoState courseVideoState;
    public final static PlayState PLAY_STATE = new PlayState();
    public final static StopState STOP_STATE = new StopState();
    public final static PauseState PAUSE_STATE = new PauseState();
    public final static SpeedState SPEED_STATE = new SpeedState();

    public CourseVideoState getCourseVideoState() {
        return courseVideoState;
    }

    public void setCourseVideoState(CourseVideoState courseVideoState) {
        this.courseVideoState = courseVideoState;
        this.courseVideoState.setCourseVideoContext(this);
    }
    public void play(){
        this.courseVideoState.play();
    }

    public void speed(){
        this.courseVideoState.speed();
    }

    public void stop(){
        this.courseVideoState.stop();
    }

    public void pause(){
        this.courseVideoState.pause();
    }
}

public class Test {
    public static void main(String[] args) {
        CourseVideoContext courseVideoContext = new CourseVideoContext();
        courseVideoContext.setCourseVideoState(new PlayState());

        System.out.println("当前状态:"+courseVideoContext.getCourseVideoState().getClass().getSimpleName());
        courseVideoContext.pause();

        System.out.println("当前状态:"+courseVideoContext.getCourseVideoState().getClass().getSimpleName());

        courseVideoContext.speed();

        System.out.println("当前状态:"+courseVideoContext.getCourseVideoState().getClass().getSimpleName());

        courseVideoContext.stop();

        System.out.println("当前状态:"+courseVideoContext.getCourseVideoState().getClass().getSimpleName());
        
        courseVideoContext.speed();
    }
}

输出：

当前状态:PlayState
当前状态:PauseState
当前状态:SpeedState
当前状态:StopState
ERROR 停止状态不能快进!!