设计模式-Observer(观察者模式)

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设计模式-Observer(观察者模式)

Observer模式应该可以说是应用最多、影响最广的模式之一,因为Observer的一个实例Model/View/Control(MVC)结构在系统开发架构设计中有着很重要的地位和意义,MVC实现了业务逻辑和表示层的解耦。个人也认为Observer模式是软件开发过程中必须要掌握和使用的模式之一。在MFC中,Doc/View(文档视图结构)提供了实现MVC的框架结构(有一个从设计模式(Observer模式)的角度分析分析Doc/View的文章正在进一步的撰写当中,遗憾的是时间:))。在Java阵容中,Struts则提供和MFC中Doc/View结构类似的实现MVC的框架。另外Java语言本身就提供了Observer模式的实现接口,这将在讨论中给出。
当然,MVC只是Observer模式的一个实例。Observer模式要解决的问题为:建立一个一(Subject)对多(Observer)的依赖关系,并且做到当“一”变化的时候,依赖这个“一”的多也能够同步改变。最常见的一个例子就是:对同一组数据进行统计分析时候,我们希望能够提供多种形式的表示(例如以表格进行统计显示、柱状图统计显示、百分比统计显示等)。这些表示都依赖于同一组数据,我们当然需要当数据改变的时候,所有的统计的显示都能够同时改变。Observer模式就是解决了这一个问题。


模式结构图:



实现:

Observer: JDK中提供的

/* * %W% %E% * * Copyright (c) 2006, 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 * @version %I%, %G% * @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);}

FatherObserver:

package com.designpatten.observer;import java.util.Observable;import java.util.Observer;public class FatherObserver implements Observer {@Overridepublic void update(Observable o, Object arg) {ChildObservable to = (ChildObservable)o ;to.sick() ;System.out.println("FatherObserver.update()=====" + arg);}}

MotherObserver:

package com.designpatten.observer;import java.util.Observable;import java.util.Observer;public class MotherObserver implements Observer {@Overridepublic void update(Observable o, Object arg) {ChildObservable to = (ChildObservable)o ;to.sick() ;System.out.println("MotherObserver.update()=====" + arg);}}

Observable:   JDK中提供的

/* * %W% %E% * * Copyright (c) 2006, 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 * @version %I%, %G% * @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();    }}


ChildObservable:

package com.designpatten.observer;import java.util.Observable;public class ChildObservable extends Observable {public void notifyObservers(String arg){super.notifyObservers(arg) ;}public void sick(){System.out.println(this.getClass().getName() + "------> sick()");}public void changeT(){super.setChanged() ;}}


ObserverMain:

package com.designpatten.observer;public class ObserverMain {public static void main(String[] args) {ChildObservable observable = new ChildObservable() ;FatherObserver father = new FatherObserver() ;MotherObserver mother = new MotherObserver() ;observable.addObserver(father) ;observable.addObserver(mother) ;observable.changeT() ;if(observable.hasChanged()){observable.notifyObservers("sick...") ;}}}



讨论:

Observer是影响极为深远的模式之一,也是在大型系统开发过程中要用到的模式之一。除了MFC、Struts提供了MVC的实现框架,在Java语言中还提供了专门的接口实现Observer模式:通过专门的类Observable及Observer接口来实现MVC编程模式,其UML图可以表示为:


这里的Observer就是观察者,Observable则充当目标Subject的角色。
Observer模式也称为发布-订阅(publish-subscribe),目标就是通知的发布者,观察者则是通知的订阅者(接受通知)。



参考:C++设计模式 一书