Head First 设计模式——观察者模式(Observer Pattern) 之二

上一篇博文中介绍了观察者模式，并且实现了“推”的方式的代码，但是关于“拉”模式的介绍很少，网上很多材料介绍的拉模式其实根本不是观察者模式，下面通过一些分析来看看JDK里面是怎么实现的

本书介绍了Java 中内置的观察者模式：

import java.util.Observable;import java.util.Observer;public class WeatherData extends Observable{private float temperature;private float humidity;private float pressure;//在这里我们不需要追踪观察者了，也不需要管理注册与删除(让超类代劳即可).//把注册、添加、通知的相关代码删除public WeatherData(){//我们的构造器不再需要为了记住观察者们而建立数据结构了}public void measurementsChanged(){setChanged();notifyObservers();}public void setMeasurements(float temperature,float humidity,float pressure){this.temperature = temperature;this.humidity = humidity;this.pressure = pressure;measurementsChanged();}public float getTemperature(){return temperature;}public float getHumidity(){return humidity;}public float getPressure(){return pressure;}}

import java.util.Observable;import java.util.Observer;public class CurrentConditionsDisplay implements Observer{Observable observable;private float temperature;private float humidity;public CurrentConditionsDisplay(Observable observable){this.observable = observable;observable.addObserver(this);}@Overridepublic void update(Observable obs, Object arg) {// TODO Auto-generated method stubif(obs instanceof WeatherData){WeatherData weatherData = (WeatherData) obs;this.temperature = weatherData.getTemperature();this.humidity = weatherData.getHumidity();display();}}public void display(){System.out.println("Current conditions:" + temperature + "F degrees and " + humidity + "% humidity");}}

public class Test {public static void main(String[] arg){WeatherData weatherData = new WeatherData();CurrentConditionsDisplay currentConditionsDisplay = new CurrentConditionsDisplay(weatherData);weatherData.setMeasurements(80, 65, 30.4f);weatherData.setMeasurements(82,70,29.2f);}}

结果是和之前相同的。

开始的时候我看到这里就没有完全搞懂：

假如Subject 的内容发生了变化，那么它就改变标志位。观察者采用拉的方式从Subject 取得数据。想不明白的一件事情是：观察者怎么立即知道Subject 有变化？我自己YY了一种实现。就是当Subject 发生变化时，不需要通知观察者，但是要让Observer 进行Update。这就像订报纸，报纸每天更新之后放到信箱里面去，订报人想起来的时候，去信箱看一下，假若有新的报纸，那么消息就传出来了。但是，这种想法有些局限1）时效性，不能在发生变化的同时更新 2）每一个观察者都需要一个线程，来定时的获取信息。

这是与观察者模式出发点是不同的，有些博客上说是另外一种模式，可能有它的应用场景。

后来问了一下同学，查看JDK中的代码，发现其内部的实现和推的模式是类似的，也是有一个结构来维护Subject要通知的对象。只是现在当Subject 里面调用notifyObserver 的时候会带着Subject 的引用，Observer 需要根据这个引用再多走一步去Subject里面取得自己所需要的数据

/* * 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();    }}

/* * 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);}

“推”的方式是指，Subject维护一份观察者的列表，每当有更新发生，Subject会把更新消息主动推送到各个Observer去。“拉”的方式是指，Subject 变化时给Observer 一个通知，然后由Observer自行决定用合适的方式去Subject获取相应的更新数据。 

“推”的好处包括：

1、高效。如果没有更新发生，不会有任何更新消息推送的动作，即每次消息推送都发生在确确实实的更新事件之后，都是有意义的。

2、实时。事件发生后的第一时间即可触发通知操作。

3、可以由Subject确立通知的时间，可以避开一些繁忙时间。

4、可以表达出不同事件发生的先后顺序。

 “拉”的好处包括：

1、Observer可以不理会它不关心的变更事件，只需要去获取自己感兴趣的事件即可。

2、Observer可以自行决定获取更新事件的时间和方式。

3、拉的形式可以让Subject更好地控制各个Observer每次查询更新的访问权限，因为Observer 是利用Subject 的getter方法来获取数据。