状态模式

一、状态模式介绍

1、定义与类型

允许一个对象在其内部状态改变时，改变它的行为
类型：行为型

2、适用场景

一个对象存在多个状态（不同状态下行为不同），且状态可相互转换

3、优点

将不同的状态隔离
把各种状态的转换逻辑，分布到State的子类中，减少相互间依赖
增加新的状态非常简单

4、缺点

状态多的业务场景导致类数目增加，系统变复杂

5、相关设计模式

状态模式和享元模式 有时候可以配合进行使用

二、代码示例

模拟场景：看视频时，可以切换为播放状态、暂停状态、快进状态、停止状态

视频状态抽象类：

public abstract class VideoVideoState {
    // 与上下文 互相嵌套
    protected VideoStateContext videoStateContext;

    public void setVideoStateContext(VideoStateContext videoStateContext) {
        this.videoStateContext = videoStateContext;
    }

    public abstract void paly();

    public abstract void speed();

    public abstract void pause();

    public abstract void stop();
}

播放状态类：

public class PlayState extends VideoVideoState {
    @Override
    public void paly() {
        System.out.println("正常播放课程视频的状态");
    }

    @Override
    public void speed() {
        System.out.println("快进了视频");
        super.videoStateContext.setVideoVideoState(VideoStateContext.SPEED_STATE);
    }

    @Override
    public void pause() {
        System.out.println("暂停了视频");
        super.videoStateContext.setVideoVideoState(VideoStateContext.PAUSE_STATE);
    }

    @Override
    public void stop() {
        System.out.println("停止了视频");
        super.videoStateContext.setVideoVideoState(VideoStateContext.STOP_STATE);
    }
}

暂停状态类：

public class PauseState extends VideoVideoState {
    @Override
    public void paly() {
        System.out.println("播放了视频");
        super.videoStateContext.setVideoVideoState(VideoStateContext.PLAY_STATE);
    }

    @Override
    public void speed() {
        System.out.println("快进了视频");
        super.videoStateContext.setVideoVideoState(VideoStateContext.SPEED_STATE);
    }

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

    @Override
    public void stop() {
        System.out.println("停止了视频");
        super.videoStateContext.setVideoVideoState(VideoStateContext.STOP_STATE);
    }
}

快进状态类：

public class SpeedState extends VideoVideoState {
    @Override
    public void paly() {
        System.out.println("播放了视频");
        super.videoStateContext.setVideoVideoState(VideoStateContext.PLAY_STATE);
    }

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

    @Override
    public void pause() {
        System.out.println("暂停了视频");
        super.videoStateContext.setVideoVideoState(VideoStateContext.PAUSE_STATE);
    }

    @Override
    public void stop() {
        System.out.println("停止了视频");
        super.videoStateContext.setVideoVideoState(VideoStateContext.STOP_STATE);
    }
}

停止状态类：

public class StopState extends VideoVideoState {
    @Override
    public void paly() {
        System.out.println("播放了视频");
        super.videoStateContext.setVideoVideoState(VideoStateContext.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("停止播放课程视频的状态");
    }
}

视频状态上下文（用于管理状态）：

public class VideoStateContext {
    // 与视频状态类 互相嵌套
    private VideoVideoState videoVideoState;
    public final static PlayState PLAY_STATE = new PlayState();
    public final static SpeedState SPEED_STATE = new SpeedState();
    public final static PauseState PAUSE_STATE = new PauseState();
    public final static StopState STOP_STATE = new StopState();

    public VideoStateContext() {
        // 设置默认停止状态
        this.videoVideoState = STOP_STATE;
        this.videoVideoState.setVideoStateContext(this);
    }

    public VideoVideoState getVideoVideoState() {
        return videoVideoState;
    }

    public void setVideoVideoState(VideoVideoState videoVideoState) {
        // 切换状态
        this.videoVideoState = videoVideoState;
        this.videoVideoState.setVideoStateContext(this);
    }

    public void paly() {
        this.videoVideoState.paly();
    }

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

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

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

测试类：

public class Test {
    public static void main(String[] args) {
        VideoStateContext videoStateContext = new VideoStateContext();

        videoStateContext.paly();

        System.out.println("当前状态：" + videoStateContext.getVideoVideoState().getClass().getSimpleName());

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

        videoStateContext.speed();
        System.out.println("当前状态：" + videoStateContext.getVideoVideoState().getClass().getSimpleName());

        videoStateContext.stop();
        System.out.println("当前状态：" + videoStateContext.getVideoVideoState().getClass().getSimpleName());

        videoStateContext.speed();
    }
}

输出：
播放了视频
当前状态：PlayState
暂停了视频
当前状态：PauseState
快进了视频
当前状态：SpeedState
停止了视频
当前状态：StopState
ERROR 停止状态不能快进！

类图：

三、 源码示例

1、jsf中的LifeCycle