观察者模式的定义：当一个主题对象（或是可观察者）的状态发生改变时，其它依赖者（观察者）都会收到通知。
下面的Demo是基于Java jar包中的观察者来写的：
首先看主题对象的代码部分：
import java.util.Observable;
public class ObserverSubject extends Observable {
public ObserverSubject() {
super();
// TODO Auto-generated constructor stub
}
public void sendNotify(String content){
setChanged();
notifyObservers(content);
}
}
两个依赖者的代码部分：
依赖者一的代码：
import java.util.Observable;
import java.util.Observer;
public class ReceverOne implements Observer {
Observable observable;
public ReceverOne(Observable observable) {
super();
this.observable = observable;
this.observable.addObserver(this);
}
@Override
public void update(Observable o, Object arg) {
// TODO Auto-generated method stub
if(o instanceof ObserverSubject){
System.out.println("I am ReceverOne,I have receiver:"+arg.toString());
}
}
}依赖者二的代码
import java.util.Observable;
import java.util.Observer;
public class ReceverTwo implements Observer {
Observable observable;
public ReceverTwo(Observable observable) {
super();
this.observable = observable;
this.observable.addObserver(this);
}
@Override
public void update(Observable o, Object arg) {
// TODO Auto-generated method stub
if(o instanceof ObserverSubject){
System.out.println("I am ReceverTwo,I have receiver:"+arg.toString());
}
}
}发送一个消息测试这个观察者模式： public static void main(String[] args) {
// TODO Auto-generated method stub
ObserverSubject observer=new ObserverSubject();
new ReceverOne(observer);
new ReceverTwo(observer);
observer.sendNotify("Hello ,this is the first notifition");
}
以上代码运行的结果为：
I am ReceverTwo,I have receiver:Hello ,this is the first notifition
I am ReceverOne,I have receiver:Hello ,this is the first notifition
看结果可知：编程时不要依赖观察者被通知的顺序！
接下来所介绍的是自定义观察者模式：
先定义要成为观察者的接口，这是主题接口，对象使用此接口注册为观察者，或者从观察者中移除（这个是成为观察者的条件）
public interface DefineObservAble {
void registerObserver(DefineObserver observer);
void removeObserver(DefineObserver observer);
void notifyObserver(Object o);
}再定义观察者的接口，所有潜在的观察者，都必须实现这个接口----观察者接口，有通知时调用uPdate方法（这个是观察者的义务）
public interface DefineObserver {
void update(Object o);
}
public class MyObservAble implements DefineObservAble {
private ArrayList observers=null;
public MyObservAble(){
observers=new ArrayList<>();
}
@Override
public void registerObserver(DefineObserver observer) {
// TODO Auto-generated method stub
if(observers==null)return;
synchronized(observers){
int i=observers.indexOf(observer);
if(i==-1){
observers.add(observer);
}
}
}
@Override
public void removeObserver(DefineObserver observer) {
// TODO Auto-generated method stub
if(observers==null)return;
synchronized (observer) {
int i=observers.indexOf(observer);
if(i!=-1){
observers.remove(i);
}
}
}
@Override
public void notifyObserver(Object o) {
// TODO Auto-generated method stub
if(observers==null)return;
int size=observers.size();
for(int i=0;i<size;i++){
DefineObserver observer=(DefineObserver) observers.get(i);
observer.update(o);
}
}
public void sendNotify(String content){
notifyObserver(content);
}
}创建两个可以用于观察的类：public class ObserverOne implements DefineObserver {
public ObserverOne(DefineObservAble able) {
able.registerObserver(this);
}
@Override
public void update(Object o) {
// TODO Auto-generated method stub
System.out.println("This is the DefineObserver,I am ObserverOne, I have receive:"+o);
}
}
public class ObserverTwo implements DefineObserver {
public ObserverTwo(DefineObservAble able) {
able.registerObserver(this);
}
@Override
public void update(Object o) {
// TODO Auto-generated method stub
System.out.println("This is the DefineObserver,I am ObserverTwo, I have receive:"+o);
}
}
/**
* @param args
*/
public static void main(String[] args) {
// TODO Auto-generated method stub
MyObservAble able=new MyObservAble();
new ObserverOne(able);
new ObserverTwo(able);
able.sendNotify(" hello world!");
}
设计模式---观察者模式（学习笔记）,布布扣,bubuko.com
原文：http://blog.csdn.net/zouchengxufei/article/details/24055993