【GOF23设计模式】观察者模式

来源：http://www.bjsxt.com/ 
一、【GOF23设计模式】_观察者模式、广播机制、消息订阅、网络游戏对战原理

package com.test.observer;

import java.util.ArrayList;
import java.util.List;

public class Subject {
    protected List<Observer> list = new ArrayList<Observer>();

    public void registerObserver(Observer obs){
        list.add(obs);
    }
    public void removeObserver(Observer obs){
        list.remove(obs);
    }

    //通知所有的观察者更新状态
    public void notjfyAllObservers(){
        for (Observer obs : list) {
            obs.update(this);
        }
    }
}

package com.test.observer;

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)的值发生了变化，请通知所有的观察者
        this.notjfyAllObservers();
    }
}

package com.test.observer;

public interface Observer {
    void update(Subject subject);
}

package com.test.observer;

public class ObserverA implements Observer{
    private int myState;//myState需要跟目标对象的state值保持一致

    public int getMyState() {
        return myState;
    }

    public void setMyState(int myState) {
        this.myState = myState;
    }

    @Override
    public void update(Subject subject) {
        myState = ((ConcreteSubject)subject).getState(); 
    }
}

package com.test.observer;

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());
    }
}

控制台输出：
#################
3000
3000
3000
#################
30
30
30

二、【GOF23设计模式】_观察者模式、obserable类和observer接口、应用场景总结

package com.test.observer2;

import java.util.Observable;
/**
 * 目标对象
 */
public class ConcreteSubject extends Observable{
    private int state;

    public int getState() {
        return state;
    }

    public void setState(int state) {
        this.state = 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
    }
}

package com.test.observer2;

import java.util.Observable;
import java.util.Observer;

public class ObserverA implements Observer{
    private int myState;

    public int getMyState() {
        return myState;
    }

    public void setMyState(int myState) {
        this.myState = myState;
    }

    @Override
    public void update(Observable o, Object arg) {
        myState = ((ConcreteSubject)o).getState();
    }
}

package com.test.observer2;

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());

        //改变subject对象的状态
        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());
    }
}

控制台输出：
==================状态修改了
3000
3000
3000
==================状态修改了
600
600
600