1.用意
观察者模式定义了一种一对多的依赖关系,让多个观察者对象同时监听某一个主题对象,这个主题对象在状态上发生变化时,会通知所有观察者对象,使它们能够自动更新自己。
2.类图
第一种简单方式
第二种方式
第三种方式 Java的内在支持
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3.代码
第一种方式代码:
/* * 抽象观察者角色 */ public interface Observer { /* * 调用这个方法会更新自己 */ void update(); } /* * 具体观察者角色 */ public class ConcreteObserver implements Observer{ /* * 调用这个方法会更新自己 * @see com.javapatterns.observer.Observer#update() */ public void update() { System.out.println("I am notified."); } } /* * 抽象主题角色 */ public interface Subject { /* * 调用这个方法登记一个新的观察者对象 */ public void attach(Observer observer); /* * 调用这个方法删除一个已经登记过的观察者对象 */ public void detach(Observer observer); /* * 调用这个方法通知所有登记过的观察者对象 */ void notifyObserver(); } /* * 具体主题角色 */ public class ConcreteSubject implements Subject { private Vector observersVector = new Vector(); public void attach(Observer observer) { observersVector.addElement(observer); } public void detach(Observer observer) { observersVector.removeElement(observer); } public Enumeration observers() { return ((Vector) observersVector.clone()).elements(); } public void notifyObserver() { Enumeration enumeration = observers(); while (enumeration.hasMoreElements()) { ((Observer) enumeration.nextElement()).update(); } } } /* * 测试类 */ public class Test { public static void main(String[] args) { Subject subject = new ConcreteSubject(); subject.attach(new ConcreteObserver()); subject.attach(new ConcreteObserver()); subject.attach(new ConcreteObserver()); subject.notifyObserver(); } }
第二种方式代码:
public abstract class Subject { private Vector observersVector = new Vector(); public void attach(Observer observer) { observersVector.addElement(observer); S.s("Attached an observer."); } public void detach(Observer observer) { observersVector.removeElement(observer); S.s("Detached an observer."); } public Enumeration observers() { return ((Vector) observersVector.clone()).elements(); } public void notifyObserver() { Enumeration enumeration = observers(); while (enumeration.hasMoreElements()) { ((Observer) enumeration.nextElement()).update(); } } } /* * 具体主题角色,现在只有一个改变内部状态的商业方法 */ public class ConcreteSubject extends Subject{ private String state; public void change(String newState){ state = newState; this.notifyObserver(); } } /* * 测试类 */ public class Test { private static ConcreteSubject subject; private static Observer observer; public static void main(String[] args) { subject = new ConcreteSubject(); observer = new ConcreteObserver(); subject.attach(observer); subject.change("new state"); } }
第3种方式,Java的内在支持:
/* * 被观察者对象 */ public class Watched extends Observable { private String data = ""; public String retrieveData() { return data; } public void changeDate(String data) { if (!this.data.equals(data)) { this.data = data; //只有setChange过观察者才会执行update方法,详见Observable源代码 setChanged(); } //通知观察者执行update方法 notifyObservers(); } } /* * 观察者 */ public class Watcher implements Observer { public Watcher(Watched w) { w.addObserver(this); } public void update(Observable o, Object arg) { S.s("Data has been changed to: " + ((Watched) o).retrieveData() + ""); } } /* * 测试类 */ public class Test { static private Watched watched; static private Observer watcher; public static void main(String[] args) { watched = new Watched(); watcher = new Watcher(watched); watched.changeDate("In C, we create bugs."); watched.changeDate("In Java, we inherit bugs."); watched.changeDate("In Java, we inherit bugs."); watched.changeDate("In visual Basic, we visualize bugs."); } }
输出:
Observer源代码:
/* * %W% %E% * * Copyright (c) 2006, Oracle and/or its affiliates. All rights reserved. * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. */ package sam.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); }
Observable源代码:
/* * %W% %E% * * Copyright (c) 2006, Oracle and/or its affiliates. All rights reserved. * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. */ package sam.java.util; import java.util.Vector; /** * 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(); } }
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