设计模式——观察者模式

观察者模式，用于一对多关系对象之间消息传递。

观察者模式的实现需要以下几种角色：

1、观察者的抽象角色，一般就是一个接口，里面只有一个update方法，由观察者自己去实现当收到消息后的处理

2、观察者角色，实现观察者的抽象

3、主题的抽象角色，一般是一个抽象类

　　用于将观察者聚拢在一个集合里面，并提供观察者的增删方法，同时，提供通知所有观察者的方法。

4、主题角色，继承于抽象主题角色，一般是在发生变化时，调用父类（抽象主题角色）的通知方法。

关于推模型和拉模型：二者的区别在于通知观察者时传递参数不同，推模型传递的是观察者所需要的信息，拉模型传递的是被观察者本身，由观察者自动提取有效信息。

Java的util包中提供了关于观察者模型的一些类，使用Java实现观察者模型很方便！

1、观察者的抽象接口（Java提供）

/*
 * Copyright (c) 1994, 1998, 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
 * @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);
}

值得我们借鉴的就是在写接口的时候，方法前面尽可能不加修饰符！

2、被观察者的抽象（Java提供）

/*
 * Copyright (c) 1994, 2004, 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
 * @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();
    }
}

值得学习或模仿的地方：

　　* 变量没有直接初始化，而是放在构造函数里面初始化的

　　* 对于观察者集合，使用了线程安全的Vector类

　　* 没必要暴露给外面的变量，采用了private保护，并且没有提供get方法

　　* 在通知之前，修改标识变量的时候（clearChanged()方法调用时）增加了锁，考虑了线程安全，避免重复通知。

3、主题/被观察者的实现

/*
 * Copyright (c) 2017. Xiaomi.Co.Ltd All rights reserved
 */

package com.pt.observer;

import java.util.Observable;
import java.util.Vector;

/**
 * @description 主题，被观察者
 * @author panteng
 * @date 17-2-24.
 */
public class Subject extends Observable {
    Vector<String> messages;
    public Subject(){
        super();
        //根据源码来看，初始化都是放在构造函数中的
        messages = new Vector<String>();
    }
    public void addMsg(String msg){
        messages.add(msg);
        setChanged();           //设置状态，标识已改变，需要通知观察者
        notifyObservers(msg);  //通知观察者
    }

    public Vector<String> getMessages(){
        return messages;
    }
    public void setMessages(Vector<String> messages){
        this.messages = messages;
    }
}

4、观察者的实现

/*
 * Copyright (c) 2017. Xiaomi.Co.Ltd All rights reserved
 */

package com.pt.observer;

import java.util.Observable;

/**
 * @description
 * @author panteng
 * @date 17-2-24.
 */
public class Watcher implements java.util.Observer {
    String name;
    public Watcher(){
    }
    public Watcher(String name){
        this.name = name;
    }
    /**
     * 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>
     */
    public void update(Observable o, Object arg){
        Subject mySubject = (Subject) o;
        System.out.println(this.name + ":共有 " + mySubject.getMessages().size() + " 条消息，最新消息：" + arg);
    }

    public String getName(){
        return name;
    }
    public void setName(String name){
        this.name = name;
    }
}

5、测试

/*
 * Copyright (c) 2017. Xiaomi.Co.Ltd All rights reserved
 */

package com.pt.observer;

import org.junit.Test;

/**
 * @description
 * @author panteng
 * @date 17-2-24.
 */
public class ObserverModelTest {
    @Test
    public void observerModelTest(){
        Subject subject = new Subject();
        Watcher watcher1 = new Watcher("watcher1");
        Watcher watcher2 = new Watcher("watcher2");
        Watcher watcher3 = new Watcher("watcher3");
        subject.addObserver(watcher1);
        subject.addObserver(watcher2);
        subject.addObserver(watcher3);
        subject.addMsg("第一条消息");
        subject.addMsg("第二条消息");

        subject.deleteObserver(watcher3);
        subject.addMsg("第三条消息");
    }
}