设计模式-Observer(观察者模式)
2013-04-10 15:43 java20130722 阅读(264) 评论(0) 编辑 收藏 举报设计模式-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 { @Override public 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 { @Override public 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