觀察者（Observer）模式，又叫 發(fā)布-訂閱（Publish/Subscribe）模式，模型-視圖（Model-View）模式
定義了一種一對(duì)多的依賴關(guān)系，當(dāng)主題對(duì)象發(fā)生改變的時(shí)候會(huì)通知所有的觀察者對(duì)象。

一個(gè)變化其他的相應(yīng)變化
設(shè)計(jì)目標(biāo)：低耦合 + 行動(dòng)的協(xié)調(diào)一致

上類圖：

觀察者模式.png

java類庫(kù)對(duì)該模式的支持：
java JDK中定義了Observer接口，以及 Observable的主題角色

AWT1.1+，Servlet的委派事件模型（DEM Delegation Event Model）

優(yōu)缺點(diǎn)：
優(yōu)點(diǎn):觀察者和被觀察者建立了一個(gè)抽象的耦合
缺點(diǎn)：

1. 如果觀察者比較多，全部通知一下比較耗時(shí)
2. 如果被觀察者之間有循環(huán)依賴會(huì)導(dǎo)致系統(tǒng)崩潰

• 代碼示例：

1. 定義個(gè)抽象觀察者對(duì)象，可以直接用JDK自帶的Observer接口

package com.byedbl.observer;

/**
 *  Observer interface
 */
public interface Observer {
    public void update(Subject s);
}

2. 實(shí)現(xiàn)兩個(gè)觀察者

package com.byedbl.observer;
/**
 *  A concrete observer
 *  This concrete observer can change subject through call
 *  a concrete subject setState function
 */
import java.util.Vector;

public class ObserverA implements Observer {
    private Vector strVector;
    private Subject sub;
    public ObserverA(Subject s) {
        sub = s;
        //strVector = new Vector();
    }
    public void update(Subject subject) {
        strVector = subject.getState();
        System.out.println("----- ObserverA will be updated -----");
        for(int i = 0; i < strVector.size(); i++) {
            System.out.println("Num " + i + " is :" + (String)strVector.get(i));
        }
    }
    public void change(String action, String str) {
        sub.setState(action, str);
        //we can auto update
        //sub.sendNotify();
    }
    public void notifySub() {
        sub.sendNotify();
    }
}

package com.byedbl.observer; /**
 *  A concrete observer
 *  This observer, can not change subject, 
 *  but it can print the content in vector sorted by alphameric
 */
import java.util.Vector;

public class ObserverB implements Observer {
    private Vector strVector;
    public ObserverB() {
        strVector =  new Vector();
    }
    public void update(Subject subject) {
        strVector = (Vector)(subject.getState()).clone();
        //-----  Sorted vector  ---------------------------
        for (int i = strVector.size(); --i >= 0; ) {
            for (int j = 0; j < i; j++) {
                String str1 = (String)strVector.get(j);
                String str2 = (String)strVector.get(j+1);
                if((str1.compareTo(str2)) > 0) {
                    strVector.setElementAt(str2, j);
                    strVector.setElementAt(str1, j+1);
                }
            }
        }
        System.out.println("----- ObserverB will be updated -----");
        for(int i = 0; i < strVector.size(); i++) {
            System.out.println("Num " + i + " is :" + (String)strVector.get(i));
        }
    }
    
}

3. 定義抽象主題角色

package com.byedbl.observer; /**
 *  Subject interface
 *  In this interface , we can only declare top 3 function, 
 *  other function we can define in an abstract class which implements
 *  this interface
 */
import java.util.*;

public interface Subject  {
    public abstract void attach(Observer o);
    public abstract void detach(Observer o);
    public abstract void sendNotify();

    public abstract Vector getState();
    public abstract void setState(String act, String str);
}

4. 實(shí)現(xiàn)抽象主題角色

package com.byedbl.observer;
/**
 * A concrete subject
 */

import java.util.LinkedList;
import java.util.Vector;

public class ConcreteSubject implements Subject {
    private LinkedList observerList;
    private Vector strVector;

    public ConcreteSubject() {
        observerList = new LinkedList();
        strVector = new Vector();
    }

    public void attach(Observer o) {
        observerList.add(o);
    }

    public void detach(Observer o) {
        observerList.remove(o);
    }

    public void sendNotify() {
        for (int i = 0; i < observerList.size(); i++) {
            ((Observer) observerList.get(i)).update(this);
        }
    }

    public void setState(String act, String str) {
        if (act.equals("ADD")) {
            strVector.add(str);
        } else if (act.equals("DEL")) {
            strVector.remove(str);
        }
    }

    public Vector getState() {
        return strVector;
    }
}

5. 客戶端代碼

package com.byedbl.observer;
/**
 * A concrete subject
 */

import java.util.LinkedList;
import java.util.Vector;

public class ConcreteSubject implements Subject {
    private LinkedList observerList;
    private Vector strVector;

    public ConcreteSubject() {
        observerList = new LinkedList();
        strVector = new Vector();
    }

    public void attach(Observer o) {
        observerList.add(o);
    }

    public void detach(Observer o) {
        observerList.remove(o);
    }

    public void sendNotify() {
        for (int i = 0; i < observerList.size(); i++) {
            ((Observer) observerList.get(i)).update(this);
        }
    }

    public void setState(String act, String str) {
        if (act.equals("ADD")) {
            strVector.add(str);
        } else if (act.equals("DEL")) {
            strVector.remove(str);
        }
    }

    public Vector getState() {
        return strVector;
    }
}