状态模式

• 定义：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
• 适用场景：一个对象存在多个状态（不同状态下行为不同），且状态可以相互转换
• 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
• 缺点：状态多的业务场景导致类数目增加，系统变复杂
• 相关设计模式：
  • 状态模式和享元模式：有时候可以配合一起使用，可以使用享元模式在多个上下文角色之间共享这些状态实例

Coding

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();
    }
}
/**
 * 课程视频状态抽象类
 */
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();

}
/**
 * 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);
    }
}
/**
 * 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);
    }
}
/**
 * 快放状态
 */
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);
    }
}
/**
 * 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");
    }
}

UML

测试

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!!

源码中的应用

在一些业务场景中可以用状态模式，比如不同电商网站关于订单扭转的状态，可以自己设置一个和业务场景非常契合的状态机，还有一些比如我们的例子视频状态扭转、办公系统流程上的扭转等等都是和具体的业务场景相关的，可以使用上状态模式