设计模式—状态机模式

1.概要

定义

状态机由状态寄存器和组合逻辑电路构成，能够根据控制信号按照预先设定的状态进行状态转移，是协调相关信号动作、完成特定操作的控制中心。（状态机是有限状态自动机的简称，是现实事物运行规则抽象而成的一个数学模型。）

四要素

• State：状态。一个状态机至少要包含两个状态

• Event：事件。事件就是执行某个操作的触发条件或者口令

• Action：动作。事件发生以后要执行动作

• Transition：变换。也就是从一个状态变化为另一个状态。指的是两个状态之间的一种关系，表明对象在第一个状态中执行一定的动作，并将在某个事件发生同时某个特定条件满足时进行第二个状态。

2. 状态机模式实现

以音乐播放器为示例，状态转换图如下:

类图如下：

源码如下：

PlayerState表示状态抽象类

public abstract class PlayerState {

    protected IPlayer mPlayer;

    public PlayerState(IPlayer player) {
        this.mPlayer=player;
    }

    public abstract void handle(int action) throws Exception;
}

PausedState

public class PausedState extends PlayerState {


    public PausedState(IPlayer player) {
        super(player);
    }

    @Override
    public void handle(int action) throws Exception {
        if(!Objects.equals(action, StateEnum.STOPPED.getValue())
                && !Objects.equals(action, StateEnum.PLAYING.getValue())) {
            System.out.println("state is error, it should be stopped/playing status");
            throw new Exception("error");
        }

        // 播放
        if(Objects.equals(action, StateEnum.PLAYING.getValue())) {
            System.out.println("开始播放......");
            mPlayer.playVedio();
            mPlayer.setState(new PlayingState(mPlayer));
        }

        // 停止
        if(Objects.equals(action, StateEnum.STOPPED.getValue())) {
            System.out.println("开始停止......");
            mPlayer.stop();
            mPlayer.setState(new StoppedState(mPlayer));
        }
    }
}

PlayingState

public class PlayingState extends PlayerState {


    public PlayingState(IPlayer player) {
        super(player);
    }

    @Override
    public void handle(int action) throws Exception {
        if(!Objects.equals(action, StateEnum.PAUSED.getValue())
                && !Objects.equals(action, StateEnum.STOPPED.getValue())) {
            System.out.println("state is error, it should be paused/stopped status");
            throw new Exception("error");
        }

        // 暂停
        if(Objects.equals(action, StateEnum.PAUSED.getValue())) {
            System.out.println("开始暂停......");
            mPlayer.pause();
            mPlayer.setState(new PausedState(mPlayer));
        }

        // 停止
        if(Objects.equals(action, StateEnum.STOPPED.getValue())) {
            System.out.println("开始停止......");
            mPlayer.stop();
            mPlayer.setState(new StoppedState(mPlayer));
        }
    }
}

StoppedState

public class StoppedState extends PlayerState {


    public StoppedState(IPlayer player) {
        super(player);
    }

    @Override
    public void handle(int action) throws Exception {
        if(!Objects.equals(action, StateEnum.PLAYING.getValue())) {
            System.out.println("state is error, it should be playing status");
            throw new Exception("error");
        }
        System.out.println("开始播放音乐......");
        mPlayer.playVedio();
        mPlayer.setState(new PlayingState(mPlayer));
    }
}

音乐器

public abstract class IPlayer {

    public abstract void request(int flag);

    public abstract void setState(PlayerState state);

    public abstract void playVedio();

    public abstract void pause();

    public abstract void stop();

}

public class VedioPlayer extends IPlayer {

    private PlayerState initState = new StoppedState(this);

    @Override
    public void request(int flag) {
        try {
            initState.handle(flag);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    @Override
    public void setState(PlayerState state) {
        this.initState = state;
    }

    @Override
    public void playVedio() {
        System.out.println("play vedio!");
    }

    @Override
    public void pause() {
        System.out.println("pause vedio!");
    }

    @Override
    public void stop() {
        System.out.println("stop vedio!");
    }
}

测试函数

public class Main {

    public static void main(String[] args) {
        Scanner sc=new Scanner(System.in);
        IPlayer player=new VedioPlayer();
        int i=-1;
        while((i=sc.nextInt())!=-1){
            player.request(i);
        }
    }
}


--------------以下输出
1
state is error, it should be playing status
java.lang.Exception: error
	at state.StoppedState.handle(StoppedState.java:20)
	at state.VedioPlayer.request(VedioPlayer.java:14)
	at state.Main.main(Main.java:16)
2
开始播放音乐......
play vedio!
3
开始暂停......
pause vedio!
1
开始停止......
stop vedio!
2
开始播放音乐......
play vedio!