14 行为模式 － 观察者（Observer）模式

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

 

 

 

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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