23种设计模式-观察者模式《关二爷成名后的生活》

对于许久不用的东西，容易忘记。百度许久，也未能找到自己所要。 从今日起，有些东西就记载下来，不仅方便自己，希望能帮到他人吧！

观察者模式（有时又被称为模型-视图（View）模式、源-收听者(Listener)模式或从属者模式）是软件设计模式的一种。在此种模式中，一个目标物件管理所有相依于它的观察者物件，并且在它本身的状态改变时主动发出通知。这通常透过呼叫各观察者所提供的方法来实现。此种模式通常被用来实现事件处理系统。

定义
观察者模式（Observer）完美的将观察者和被观察的对象分离开。面向对象设计的一个原则是：系统中的每个类将重点放在某一个功能上，而不是其他方面。一个对象只做一件事情，并且将他做好。观察者模式在模块之间划定了清晰的界限，提高了应用程序的可维护性和重用性。
观察者设计模式定义了对象间的一种一对多的组合关系，以便一个对象的状态发生变化时，所有依赖于它的对象都得到通知并自动刷新。

实现方式
观察者模式有很多实现方式，从根本上说，该模式必须包含两个角色：观察者和被观察对象。观察者和被观察者之间存在“观察”的逻辑关联，当被观察者发生改变的时候，观察者就会观察到这样的变化，并且做出相应的响应。

观察者： （Observer）将自己注册到被观察对象（Subject）中，被观察对象将观察者存放在一个容器（Container）里。
被观察： 被观察对象发生了某种变化，从容器中得到所有注册过的观察者，将变化通知观察者。
撤销观察： 观察者告诉被观察者要撤销观察，被观察者从容器中将观察者去除。
观察者将自己注册到被观察者的容器中时，被观察者不应该过问观察者的具体类型，而是应该使用观察者的接口。这样的优点是：假定程序中还有别的观察者，那么只要这个观察者也是相同的接口实现即可。一个被观察者可以对应多个观察者，当被观察者发生变化的时候，他可以将消息一一通知给所有的观察者。基于接口，而不是具体的实现——这一点为程序提供了更大的灵活性。

观察
实现观察者模式的时候要注意，观察者和被观察对象之间的互动关系不能体现成类之间的直接调用，否则就将使观察者和被观察对象之间紧密的耦合起来，从根本上违反面向对象的设计的原则。无论是观察者“观察”观察对象，还是被观察者将自己的改变“通知”观察者，都不应该直接调用。

例子
关二爷出名了，三国演义使得关二爷成为了武圣，各行各业皆跪拜关二哥，可是关二爷依旧是凡人一个，始终谨记–英雄？有人需要你，你就是英雄，没人需要你，你就是毒瘤。

关二爷十分低调，却引发了粉丝们的兴趣，于是各种媒体为了获得关二爷的动态实时监控关二爷。

代码如下:
JDK 中提供了 :java.util.Observable 实现类和 java.util.Observer 接口，我们不需重写，只需要实现即可。

/*
 * Copyright (c) 1994, 1998, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */
package java.util;

/**
 * A class can implement the Observer interface when it
 * wants to be informed of changes in observable objects.
 *
 * @author  Chris Warth
 * @see     java.util.Observable
 * @since   JDK1.0
 */
public interface Observer {
    /**
     * This method is called whenever the observed object is changed. An
     * application calls an Observable object's
     * notifyObservers method to have all the object's
     * observers notified of the change.
     *
     * @param   o     the observable object.
     * @param   arg   an argument passed to the notifyObservers
     *                 method.
     */
    void update(Observable o, Object arg);
}

/*
 * Copyright (c) 1994, 2012, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

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.
 * 

 * An observable object can have one or more observers. An observer
 * may be any object that implements interface Observer. After an
 * observable instance changes, an application calling the
 * Observable's notifyObservers method
 * causes all of its observers to be notified of the change by a call
 * to their update method.
 * 

 * 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.
 * 

 * Note that this notification mechanism has nothing to do with threads
 * and is completely separate from the wait and notify
 * mechanism of class Object.
 * 

 * When an observable object is newly created, its set of observers is
 * empty. Two observers are considered the same if and only if the
 * equals method returns true for them.
 *
 * @author  Chris Warth
 * @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 null 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
     * hasChanged method, then notify all of its observers
     * and then call the clearChanged method to
     * indicate that this object has no longer changed.
     * 

     * Each observer has its update method called with two
     * arguments: this observable object and null. In other
     * words, this method is equivalent to:
     * 

     * notifyObservers(null)
     *
     * @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
     * hasChanged method, then notify all of its observers
     * and then call the clearChanged method to indicate
     * that this object has no longer changed.
     * 

     * Each observer has its update method called with two
     * arguments: this observable object and the arg 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
             */
            // Android-changed: Call out to hasChanged() to figure out if something changes.
            // Upstream code avoids calling the nonfinal hasChanged() from the synchronized block,
            // but that would break compatibility for apps that override that method.
            // if (!changed)
            if (!hasChanged())
                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 Observable object as having been changed; the
     * hasChanged method will now return true.
     */
    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 hasChanged method will now return false.
     * This method is called automatically by the
     * notifyObservers 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  true if and only if the setChanged
     *          method has been called more recently than the
     *          clearChanged method on this object;
     *          false otherwise.
     * @see     java.util.Observable#clearChanged()
     * @see     java.util.Observable#setChanged()
     */
    public synchronized boolean hasChanged() {
        return changed;
    }

    /**
     * Returns the number of observers of this Observable object.
     *
     * @return  the number of observers of this object.
     */
    public synchronized int countObservers() {
        return obs.size();
    }
}

有兴趣的朋友，可以深入研究…

关二爷的类

不管关二爷是否被关注，关二爷始终要生活，所以关二爷有两个方法：1.读春秋；2.练习春秋刀法。

package model.com.hyco.ObserverPattern;

import java.util.ArrayList;
import java.util.List;
import java.util.Observable;
import java.util.Observer;

/**
 * 关二爷，超凡脱俗的人物 
 * 继承Observable，意味着被狗仔队监控了
 */
public class GuanEr extends Observable {

    public void read() {
        System.out.println("关二爷：开始读春秋...");
        //通知所有的观察者
        super.setChanged();
        this.notifyObservers("关二爷读春秋...");
    }

    public void exercise() {
        System.out.println("关二爷：开始练习春秋刀法...");
        //通知所有的观察者
        super.setChanged();
        this.notifyObservers("关二爷开始联系春秋刀法");
    }
}

某媒体 TX、YK，获取到关二爷的消息，立马发布，获取流量。

package model.com.hyco.ObserverPattern;

import java.util.Observable;
import java.util.Observer;

/**
 * 某媒体 TX   当Observable 通知所有观察者后，会调用Observer 中的update（）方法。研读源码可知。
 */
public class TX implements Observer {
    @Override
    public void update(Observable o, Object arg) {
        System.out.println("TX 狗仔队：观察到关二爷的消息...");
        exaggerate((String) arg);
    }

    public void exaggerate(String msg) {
        System.out.println("TX 媒体 发布关二爷的消息 -->" + msg);
    }
}

package model.com.hyco.ObserverPattern;

import java.util.Observable;
import java.util.Observer;

/**
 * 某媒体 YK
 */
public class YK implements Observer {
    @Override
    public void update(Observable o, Object arg) {
        System.out.println("YK 狗仔队：观察到关二爷的消息...");
        exaggerate((String) arg);
    }

    public void exaggerate(String msg) {
        System.out.println("YK 媒体 发布关二爷的消息 -->" + msg);
    }
}

测试代码：

 @Test
    public void testObserver() {
        //1.定义观察者
        Observer tx = new TX();
        Observer yk = new YK();

        //2.定义关二爷 被观察者
        GuanEr guanEr = new GuanEr();

        //2.1注册
        System.out.println("没有注册，获取不到媒体的消息");
        guanEr.read();

        System.out.println("注册");
        guanEr.addObserver(tx);
        guanEr.addObserver(yk);

        //2.2 发布消息
        guanEr.read();
        System.out.println();
        guanEr.exercise();
        System.out.println();
        //2.3注销
        System.out.println("注销");
        guanEr.deleteObserver(tx);
        guanEr.deleteObserver(yk);

        System.out.println("注销之后，获取不到媒体消息");
        guanEr.read();
    }

打印结果:

没有注册，获取不到媒体的消息
关二爷：开始读春秋...
注册
关二爷：开始读春秋...
YK 狗仔队：观察到关二爷的消息...
YK 媒体 发布关二爷的消息 -->关二爷读春秋...
TX 狗仔队：观察到关二爷的消息...
TX 媒体 发布关二爷的消息 -->关二爷读春秋...

关二爷：开始练习春秋刀法...
YK 狗仔队：观察到关二爷的消息...
YK 媒体 发布关二爷的消息 -->关二爷开始联系春秋刀法
TX 狗仔队：观察到关二爷的消息...
TX 媒体 发布关二爷的消息 -->关二爷开始联系春秋刀法

注销
注销之后，获取不到媒体消息
关二爷：开始读春秋...

总结:观察者模式有些类似无序广播，生活中有很多观察者模式。比如我的这篇博文，不管你是否关注，我还是会发布，只是你没有关注，便收不到消息，如果关注了，你会收到通知，取消关注，你又收不到消息。只需要把关注改成观察即可。或者某个应用的通知，也是观察者模式。例如某个粉丝群体，每个粉丝便是单独的观察者，而被关注的人，便是被观察者。