设计模式之观察者模式

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实现代码（推的方式）

目标对象

public class Subject {    protected List<Observer> list = new ArrayList<Observer>();    public void registerObserver(Observer obs){        list.add(obs);    }    public void removeObserver(Observer obs){        list.add(obs);    }    //通知所有的观察者更新状态    public void notifyAllObservers(){        for (Observer obs : list) {            obs.update(this);        }    }}public class ConcreteSubject extends Subject {    private int state;    public int getState() {        return state;    }    public void setState(int state) {        this.state = state;        //主题对象(目标对象)值发生了变化，请通知所有的观察者        this.notifyAllObservers();    }}

观察者

public interface Observer {    void  update(Subject subject);}public class ObserverA implements Observer {    private int myState;   //myState需要跟目标对象的state值保持一致！    @Override    public void update(Subject subject) {        myState = ((ConcreteSubject)subject).getState();    }    public int getMyState() {        return myState;    }    public void setMyState(int myState) {        this.myState = myState;    }}

客户端调用

public class Client {    public static void main(String[] args) {        //目标对象        ConcreteSubject subject = new ConcreteSubject();        //创建多个观察者        ObserverA  obs1 = new ObserverA();        ObserverA  obs2 = new ObserverA();        ObserverA  obs3 = new ObserverA();        //将这三个观察者添加到subject对象的观察者队伍中        subject.registerObserver(obs1);        subject.registerObserver(obs2);        subject.registerObserver(obs3);        //改变subject的状态        subject.setState(3000);        System.out.println("########################");        //我们看看，观察者的状态是不是也发生了变化        System.out.println(obs1.getMyState());        System.out.println(obs2.getMyState());        System.out.println(obs3.getMyState());        //改变subject的状态        subject.setState(30);        System.out.println("########################");        //我们看看，观察者的状态是不是也发生了变化        System.out.println(obs1.getMyState());        System.out.println(obs2.getMyState());        System.out.println(obs3.getMyState());    }}

类图

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Javase提供的类来实现观察者模式

java.util.observable和java.util.observer

//目标对象public class ConcreteSubject extends Observable {    private int state;     public void set(int s){        state = s;  //目标对象的状态发生了改变        setChanged();  //表示目标对象已经做了更改        notifyObservers(state);  //通知所有的观察者    }    public int getState() {        return state;    }    public void setState(int state) {        this.state = state;    }}

public class ObserverA implements Observer {    private int myState;    @Override    public void update(Observable o, Object arg) {        myState = ((ConcreteSubject)o).getState();    }    public int getMyState() {        return myState;    }    public void setMyState(int myState) {        this.myState = myState;    }}

public class Client {    public static void main(String[] args) {        //创建目标对象Obserable        ConcreteSubject subject = new ConcreteSubject();        //创建观察者        ObserverA obs1 = new ObserverA();        ObserverA obs2 = new ObserverA();        ObserverA obs3 = new ObserverA();        //将上面三个观察者对象添加到目标对象subject的观察者容器中        subject.addObserver(obs1);        subject.addObserver(obs2);        subject.addObserver(obs3);        //改变subject对象的状态        subject.set(3000);        System.out.println("===============状态修改了！");        //观察者的状态发生了变化        System.out.println(obs1.getMyState());        System.out.println(obs2.getMyState());        System.out.println(obs3.getMyState());        subject.set(600);        System.out.println("===============状态修改了！");        //观察者的状态发生了变化        System.out.println(obs1.getMyState());        System.out.println(obs2.getMyState());        System.out.println(obs3.getMyState());          }}

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