观察者模式，用于一对多关系对象之间消息传递。
观察者模式的实现需要以下几种角色：
1、观察者的抽象角色，一般就是一个接口，里面只有一个update方法，由观察者自己去实现当收到消息后的处理
2、观察者角色，实现观察者的抽象
3、主题的抽象角色，一般是一个抽象类
用于将观察者聚拢在一个集合里面，并提供观察者的增删方法，同时，提供通知所有观察者的方法。
4、主题角色，继承于抽象主题角色，一般是在发生变化时，调用父类（抽象主题角色）的通知方法。
关于推模型和拉模型：二者的区别在于通知观察者时传递参数不同，推模型传递的是观察者所需要的信息，拉模型传递的是被观察者本身，由观察者自动提取有效信息。
Java的util包中提供了关于观察者模型的一些类，使用Java实现观察者模型很方便！
1、观察者的抽象接口（Java提供）
/* * Copyright (c) 1994, 1998, Oracle and/or its affiliates. All rights reserved. * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. */ package java.util; /** * A class can implement the <code>Observer</code> interface when it * wants to be informed of changes in observable objects. * * @author Chris Warth * @see java.util.Observable * @since JDK1.0 */ public interface Observer { /** * This method is called whenever the observed object is changed. An * application calls an <tt>Observable</tt> object's * <code>notifyObservers</code> method to have all the object's * observers notified of the change. * * @param o the observable object. * @param arg an argument passed to the <code>notifyObservers</code> * method. */ void update(Observable o, Object arg); }
值得我们借鉴的就是在写接口的时候，方法前面尽可能不加修饰符！
2、被观察者的抽象（Java提供）
/* * Copyright (c) 1994, 2004, Oracle and/or its affiliates. All rights reserved. * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. */ package java.util; /** * This class represents an observable object, or "data" * in the model-view paradigm. It can be subclassed to represent an * object that the application wants to have observed. * <p> * An observable object can have one or more observers. An observer * may be any object that implements interface <tt>Observer</tt>. After an * observable instance changes, an application calling the * <code>Observable</code>'s <code>notifyObservers</code> method * causes all of its observers to be notified of the change by a call * to their <code>update</code> method. * <p> * The order in which notifications will be delivered is unspecified. * The default implementation provided in the Observable class will * notify Observers in the order in which they registered interest, but * subclasses may change this order, use no guaranteed order, deliver * notifications on separate threads, or may guarantee that their * subclass follows this order, as they choose. * <p> * Note that this notification mechanism is has nothing to do with threads * and is completely separate from the <tt>wait</tt> and <tt>notify</tt> * mechanism of class <tt>Object</tt>. * <p> * When an observable object is newly created, its set of observers is * empty. Two observers are considered the same if and only if the * <tt>equals</tt> method returns true for them. * * @author Chris Warth * @see java.util.Observable#notifyObservers() * @see java.util.Observable#notifyObservers(java.lang.Object) * @see java.util.Observer * @see java.util.Observer#update(java.util.Observable, java.lang.Object) * @since JDK1.0 */ public class Observable { private boolean changed = false; private Vector obs; /** Construct an Observable with zero Observers. */ public Observable() { obs = new Vector(); } /** * Adds an observer to the set of observers for this object, provided * that it is not the same as some observer already in the set. * The order in which notifications will be delivered to multiple * observers is not specified. See the class comment. * * @param o an observer to be added. * @throws NullPointerException if the parameter o is null. */ public synchronized void addObserver(Observer o) { if (o == null) throw new NullPointerException(); if (!obs.contains(o)) { obs.addElement(o); } } /** * Deletes an observer from the set of observers of this object. * Passing <CODE>null</CODE> to this method will have no effect. * @param o the observer to be deleted. */ public synchronized void deleteObserver(Observer o) { obs.removeElement(o); } /** * If this object has changed, as indicated by the * <code>hasChanged</code> method, then notify all of its observers * and then call the <code>clearChanged</code> method to * indicate that this object has no longer changed. * <p> * Each observer has its <code>update</code> method called with two * arguments: this observable object and <code>null</code>. In other * words, this method is equivalent to: * <blockquote><tt> * notifyObservers(null)</tt></blockquote> * * @see java.util.Observable#clearChanged() * @see java.util.Observable#hasChanged() * @see java.util.Observer#update(java.util.Observable, java.lang.Object) */ public void notifyObservers() { notifyObservers(null); } /** * If this object has changed, as indicated by the * <code>hasChanged</code> method, then notify all of its observers * and then call the <code>clearChanged</code> method to indicate * that this object has no longer changed. * <p> * Each observer has its <code>update</code> method called with two * arguments: this observable object and the <code>arg</code> argument. * * @param arg any object. * @see java.util.Observable#clearChanged() * @see java.util.Observable#hasChanged() * @see java.util.Observer#update(java.util.Observable, java.lang.Object) */ public void notifyObservers(Object arg) { /* * a temporary array buffer, used as a snapshot of the state of * current Observers. */ Object[] arrLocal; synchronized (this) { /* We don't want the Observer doing callbacks into * arbitrary code while holding its own Monitor. * The code where we extract each Observable from * the Vector and store the state of the Observer * needs synchronization, but notifying observers * does not (should not). The worst result of any * potential race-condition here is that: * 1) a newly-added Observer will miss a * notification in progress * 2) a recently unregistered Observer will be * wrongly notified when it doesn't care */ if (!changed) return; arrLocal = obs.toArray(); clearChanged(); } for (int i = arrLocal.length-1; i>=0; i--) ((Observer)arrLocal[i]).update(this, arg); } /** * Clears the observer list so that this object no longer has any observers. */ public synchronized void deleteObservers() { obs.removeAllElements(); } /** * Marks this <tt>Observable</tt> object as having been changed; the * <tt>hasChanged</tt> method will now return <tt>true</tt>. */ protected synchronized void setChanged() { changed = true; } /** * Indicates that this object has no longer changed, or that it has * already notified all of its observers of its most recent change, * so that the <tt>hasChanged</tt> method will now return <tt>false</tt>. * This method is called automatically by the * <code>notifyObservers</code> methods. * * @see java.util.Observable#notifyObservers() * @see java.util.Observable#notifyObservers(java.lang.Object) */ protected synchronized void clearChanged() { changed = false; } /** * Tests if this object has changed. * * @return <code>true</code> if and only if the <code>setChanged</code> * method has been called more recently than the * <code>clearChanged</code> method on this object; * <code>false</code> otherwise. * @see java.util.Observable#clearChanged() * @see java.util.Observable#setChanged() */ public synchronized boolean hasChanged() { return changed; } /** * Returns the number of observers of this <tt>Observable</tt> object. * * @return the number of observers of this object. */ public synchronized int countObservers() { return obs.size(); } }
值得学习或模仿的地方：
* 变量没有直接初始化，而是放在构造函数里面初始化的
* 对于观察者集合，使用了线程安全的Vector类
* 没必要暴露给外面的变量，采用了private保护，并且没有提供get方法
* 在通知之前，修改标识变量的时候（clearChanged()方法调用时）增加了锁，考虑了线程安全，避免重复通知。
3、主题/被观察者的实现
/* * Copyright (c) 2017. Xiaomi.Co.Ltd All rights reserved */ package com.pt.observer; import java.util.Observable; import java.util.Vector; /** * @description 主题，被观察者 * @author panteng * @date 17-2-24. */ public class Subject extends Observable { Vector<String> messages; public Subject(){ super(); //根据源码来看，初始化都是放在构造函数中的 messages = new Vector<String>(); } public void addMsg(String msg){ messages.add(msg); setChanged(); //设置状态，标识已改变，需要通知观察者 notifyObservers(msg); //通知观察者 } public Vector<String> getMessages(){ return messages; } public void setMessages(Vector<String> messages){ this.messages = messages; } }
4、观察者的实现
/* * Copyright (c) 2017. Xiaomi.Co.Ltd All rights reserved */ package com.pt.observer; import java.util.Observable; /** * @description * @author panteng * @date 17-2-24. */ public class Watcher implements java.util.Observer { String name; public Watcher(){ } public Watcher(String name){ this.name = name; } /** * This method is called whenever the observed object is changed. An * application calls an <tt>Observable</tt> object's * <code>notifyObservers</code> method to have all the object's * observers notified of the change. * @param o the observable object. * @param arg an argument passed to the <code>notifyObservers</code> */ public void update(Observable o, Object arg){ Subject mySubject = (Subject) o; System.out.println(this.name + ":共有 " + mySubject.getMessages().size() + " 条消息，最新消息：" + arg); } public String getName(){ return name; } public void setName(String name){ this.name = name; } }
5、测试
/* * Copyright (c) 2017. Xiaomi.Co.Ltd All rights reserved */ package com.pt.observer; import org.junit.Test; /** * @description * @author panteng * @date 17-2-24. */ public class ObserverModelTest { @Test public void observerModelTest(){ Subject subject = new Subject(); Watcher watcher1 = new Watcher("watcher1"); Watcher watcher2 = new Watcher("watcher2"); Watcher watcher3 = new Watcher("watcher3"); subject.addObserver(watcher1); subject.addObserver(watcher2); subject.addObserver(watcher3); subject.addMsg("第一条消息"); subject.addMsg("第二条消息"); subject.deleteObserver(watcher3); subject.addMsg("第三条消息"); } }
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