behavior_observer

观察者模式

本章任务

1.观察者模式的定义

 

定义：观察者模式定义了一种一队多的依赖关系，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，使他们能够自动更新自己。

2.网上商店中商品在名称 价格等方面有变化,系统自动通知会员。

 

/*
 * @(#)Observer.java	1.20 05/11/17
 *
 * Copyright 2006 Sun Microsystems, Inc. All rights reserved.
 * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */
package com.aptech.behavior.observer;

/**
 * A class can implement the <code>Observer</code> interface when it
 * wants to be informed of changes in observable objects.
 *
 * @author  Chris Warth
 * @version 1.20, 11/17/05
 * @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.java 1.39 05/11/17
 * 
 * Copyright 2006 Sun Microsystems, Inc. All rights reserved.
 * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */

package com.aptech.behavior.observer;

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 1.39, 11/17/05
 * @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 final 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();
	}
}

 

package com.aptech.behavior.observer;

/**
 * 
 * 文件名: Product.java
 * 版权： 方勇
 * 描述:产品类 ,主要执行产品数据库插入 更新
 * Email:[e-mail]
 * 修改时间： 2010-7-9下午09:31:48
 */
public class Product extends Observable {

	// 产品名
	private String	name;
	// 产品价格
	private float		price;

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
		// 设置变化点
		setChanged();
		notifyObservers(name);
	}

	public float getPrice() {
		return price;
	}
	public void setPrice(float price) {
		this.price = price;
		// 设置变化点
		setChanged();
		notifyObservers(new Float(price));
	}

}

 

package com.aptech.behavior.observer;



/**
 * 
 * 文件名: NameObserver.java
 * 版权： 方勇
 * 描述: 观察者NameObserver主要用来对产品名称(name)进行观察的
 * Email:[e-mail]
 * 修改时间： 2010-7-9下午09:40:49
 */
public class NameObserver implements Observer {

	private String	name	= null;

	public void update(Observable arg0, Object arg1) {
		if (arg1 instanceof String){
			name=(String)arg1;
			// 产品名称改变值在name中
			System.out.println("NameObserver :name changet to "+name);
		}
	}
}

 

package com.aptech.behavior.observer;



/**
 * 
 * 文件名: NameObserver.java
 * 版权： 方勇
 * 描述: 观察者PriceObserver主要用来对产品价格(price)进行观察的
 * Email:[e-mail]
 * 修改时间： 2010-7-9下午09:40:49
 */
public class PriceObserver implements Observer {

	private float	price	= 0;

	public void update(Observable arg0, Object arg1) {
		if (arg1 instanceof Float) {
			price = ((Float) arg1).floatValue();
			System.out.println("PriceObserver :price changet to " + price);
		}
	}
}

 

package com.aptech.behavior.observer;

import junit.framework.TestCase;

public class ObserverTest extends TestCase {

	@Override
	protected void setUp() throws Exception {
		super.setUp();
	}

	public void testObserver() {
		NameObserver nameobs = new NameObserver();
		PriceObserver priceobs = new PriceObserver();
		// 加入观察者
		Product product = new Product();

		product.addObserver(nameobs);
		product.addObserver(priceobs);

		product.setName("基围虾");
		product.setPrice(15.82f);
	}
}

   

本章目标

1. 理解观察者模式