【图解设计模式】State模式

Posted on | In

用类来表示状态。

示例

一个警戒状态每小时会改变一次的报警系统。

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类图

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时序图

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State接口

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public interface State {
    public abstract void doClock(Context context, int hour);
    public abstract void doUse(Context context);
    public abstract void doAlarm(Context context);
    public abstract void doPhone(Context context);
}

DayState类

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public class DayState implements State {
    private static DayState singleton = new DayState();

    private DayState() {}

    public static DayState getInstance() {
        return singleton;
    }

    @Override
    public void doClock(Context context, int hour) {
        if (hour < 9 || hour >= 17)
            context.changeState(NightState.getInstance());
    }

    @Override
    public void doUse(Context context) {
        context.recordLog("使用金库（白天）");
    }

    @Override
    public void doAlarm(Context context) {
        context.callSecurityCenter("按下警铃（白天）");
    }

    @Override
    public void doPhone(Context context) {
        context.callSecurityCenter("正常通话（白天）");
    }

    @Override
    public String toString() {
        return "[白天]";
    }
}

NightState类

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public class NightState implements State {
    private static NightState singleton = new NightState();

    private NightState() {}

    public static NightState getInstance() {
        return singleton;
    }

    @Override
    public void doClock(Context context, int hour) {
        if (hour >= 9 && hour < 17)
            context.changeState(DayState.getInstance());
    }

    @Override
    public void doUse(Context context) {
        context.callSecurityCenter("紧急：晚上使用金库！");
    }

    @Override
    public void doAlarm(Context context) {
        context.callSecurityCenter("按下警铃（晚上）");
    }

    @Override
    public void doPhone(Context context) {
        context.recordLog("晚上的通话录音");
    }

    @Override
    public String toString() {
        return "[晚上]";
    }
}

Context接口

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public interface Context {
    public abstract void setClock(int hour);
    public abstract void changeState(State state);
    public abstract void callSecurityCenter(String msg);
    public abstract void recordLog(String msg);
}

SafeFrame类

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import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

public class SafeFrame extends Frame implements ActionListener, Context {
    private TextField textClock = new TextField(60);
    private TextArea textScreen = new TextArea(10, 60);
    private Button buttonUse = new Button("使用金库");
    private Button buttonAlarm = new Button("按下警铃");
    private Button buttonPhone = new Button("正常通话");
    private Button buttonExit = new Button("结束");

    private State state = DayState.getInstance();

    public SafeFrame(String title) {
        super(title);
        setBackground(Color.lightGray);
        setLayout(new BorderLayout());
        add(textClock, BorderLayout.NORTH);
        textClock.setEditable(false);
        add(textScreen, BorderLayout.CENTER);
        textScreen.setEditable(false);
        Panel panel = new Panel();
        panel.add(buttonUse);
        panel.add(buttonAlarm);
        panel.add(buttonPhone);
        panel.add(buttonExit);
        add(panel, BorderLayout.SOUTH);

        pack();
        show();

        buttonUse.addActionListener(this);
        buttonAlarm.addActionListener(this);
        buttonPhone.addActionListener(this);
        buttonExit.addActionListener(this);
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        System.out.println(e.toString());

        if (e.getSource() == buttonUse)
            state.doUse(this);
        else if (e.getSource() == buttonAlarm)
            state.doAlarm(this);
        else if (e.getSource() == buttonPhone)
            state.doPhone(this);
        else if (e.getSource() == buttonExit)
            System.exit(0);
        else
            System.out.println("?");
    }

    @Override
    public void setClock(int hour) {
        String clockstring = "现在时间是";

        if (hour < 10)
            clockstring += "0" + hour + ":00";
        else
            clockstring += hour + ":00";

        System.out.println(clockstring);
        textClock.setText(clockstring);
        state.doClock(this, hour);
    }

    @Override
    public void changeState(State state) {
        System.out.println("从" + this.state + "状态变为了" + state + "状态。");
        this.state = state;
    }

    @Override
    public void callSecurityCenter(String msg) {
        textScreen.append("call! " + msg + "\n");
    }

    @Override
    public void recordLog(String msg) {
        textScreen.append("record ... " + msg + "\n");
    }
}

Main类

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public class Main {

    public static void main(String[] args) {
        SafeFrame frame = new SafeFrame("State Sample");

        while (true) {
            for (int hour = 0; hour < 24; hour++) {
                frame.setClock(hour);

                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                }
            }
        }
    }
}

运行结果

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登场角色

State（状态）

State角色表示状态，定义了根据不同状态进行不同处理的接口（API）。该接口（API）是那些处理内容依赖于状态的方法的集合。在示例程序中，由State接口扮演此角色。

ConcreteState（具体状态）

ConcreteState角色表示各个具体的状态，它实现了State接口。在示例程序中，由DayState类和NightState类扮演此角色。

Context（状况、前后关系、上下文）

Context角色持有表示当前状态的ConcreteState角色。此外，它还定义了供外部调用者使用State模式的接口（API）。在示例程序中，由Context接口和SafeFrame类扮演此角色。

类图

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