观察者模式在Android开发场景中运用之通过Java源码分析(一)

对于观察者，很多开发者并不陌生，在日常开发过程中，这也是一个非常常见的设计模式，尤其是Android小伙伴，很多人都知道broadcast就是一个典型的观察者模式，还有最近很火的rxjava，响应式编程中，观察者模式扮演着一个很重要的角色，但观察者模式具体是怎么样运转的，部分小伙伴就有点模糊了。

先从日常生活中一个例子开始说起，在看电视的过程中，我们经常看到一些抗日神剧中有这么一个剧情，鬼子进村，在进村的过程中，总会有一些一些人通风报信，然后通知村里的人能躲的躲，能藏的藏，能跑的跑，或者中路再搞个埋伏，抓到了以后是手撕还是其它方式处理，在此就先不做讨论。。。其实这个过程中就是一个典型的观察者模式，下面，我们先看一下手撕鬼子的UML。

DevilsSubject.java

/**
 * 
 * created by zm on 2016-5-28
 * 继承Observable，此类等同于上述UML的Devil(小鬼子)，其它对号入座
 * 观察鬼子是否来袭击 
 *
 */
public class DevilsSubject extends Observable
{
    private String assault;

    public String isAssault() {
        return assault;
    }

    public void setAssault(String assault) {
        this.assault = assault;
        //可通过this.hasChanged()获取是否发生改变，这里我们统一设置成改变，以便测试
        this.setChanged();
        this.notifyObservers(assault);
    }
}

VillagerObserver.java

/**
 * 
 * created by zm on 2016-5-28
 * 
 * VillagerObserver(放哨的村民)，观察小鬼子行动
 *
 */
public class VillagerObserver implements Observer
{

    public void update(Observable o, Object obj) {
        // TODO Auto-generated method stub
        String assault = (String) obj;
        System.out.println(assault);
    }
}

Client.java

public class Client
{
    public static void main(String[] args) {
        VillagerObserver yes = new VillagerObserver();
        VillagerObserver no = new VillagerObserver();
        DevilsSubject devilsSubject = new DevilsSubject();
        //如果观察者与集合中已有的观察者不同，则向对象的观察者集中添加此观察者。
        devilsSubject.addObserver(yes);
        devilsSubject.addObserver(no);
        devilsSubject.setAssault("前方有一坨鬼子来了");
        devilsSubject.setAssault("鬼子见阎王了，在来村的路上就被村民手撕了");
        //返回 Observable 对象的观察者数目
        System.out.println(devilsSubject.countObservers());
        System.out.println("................");
        devilsSubject.deleteObserver(yes);
        devilsSubject.setAssault("鬼子来了");
        System.out.println(devilsSubject.countObservers());
    }
}

运行的结果：

前方有一坨鬼子来了
前方有一坨鬼子来了
鬼子见阎王了，在来村的路上就被村民手撕了
鬼子见阎王了，在来村的路上就被村民手撕了
Observable对象的观察者数目:2个
................
鬼子来了
Observable对象的观察者数目:1个

下面是observable源码

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
             */
            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 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();
    }
}

再附上Observable的api

根据源码中最上部分的注释，翻译成中文后，大体的意思是此类是一个被观察者。它可以派生子类来表示一个应用程序想要观察的对象。一个可观察到的对象(observable)可以有一个或多个观察者(observer)。一个观察者可以是任何实现接口的观察者的对象。修改后可观察到的实例,应用程序调用notifyObservers方法使所有的观察者调用更新方法。通知的顺序将是未指定的。请注意,这与线程通知机制无关,完全独立于类对象的等待和通知机制。当一个可观察的对象是新创建的,它的观察是空的。当且仅当这个方法返回true，两个观察者是同步的。

源码中，起关键性作用的就是vector和changed，在observable实例化的时候，就初始化了一个空的vector，可以通过vector添加和移除vector操作后，当observable发生改变时，通过changed去判断是否通知，在我们的上述示例代码中使用setChanged()，主要是因为第一次加入的时候，不会去调用observer的update方法，也就是changed为false，当changed为false时，直接从notifyObservers方法中return，只有changed为true的时候才通知刷新，刷新之前，重新把changed赋值为false，提取上述源码中的关键代码如下：

public void notifyObservers(Object arg) {
        Object[] arrLocal;
        synchronized (this) {
            if (!changed)
                return;
            arrLocal = obs.toArray();
            clearChanged();
        }

        for (int i = arrLocal.length-1; i>=0; i--)
            ((Observer)arrLocal[i]).update(this, arg);
    }

observer类

/**
 * 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);
}

observer就是一个接口，里面一个update方法，这个类没太多需要解释的，有点Java基础的都可以明白。

现在一目了然了，Observer模式是一种行为模式，它的作用是当一个对象的状态发生改变的时候，能够自动通知其他关联对象，自动刷新对象状态。Observer模式提供给关联对象一种同步通信的手段，使其某个对象与依赖它的其他对象之间保持状态同步。

抽象主题角色（Subject）内部其实就是一个Vector，在addObserver的时候，就把需要的观察者添加到Vector中。在deleteObserver的时候，就把传进来的观察者从容器中移除掉。主题角色又叫抽象被观察者角色(observable)，一般用一个抽象类或者接口来实现。

observable与observer是一种一对多的依赖关系，可以让多个观察者对象同时监听某一个主题对象。观察者模式有时被称作发布/订阅模式(Publish/Subscribe)，对于这名称很贴切的，就好比我们订阅了报纸，每次报社新报纸出版发售的时候，就会根据订阅的客户一一发报纸，通知客户阅读。

ConcreteSubject：具体主题角色，将相关状态存入具体观察者对象。具体主题角色又叫具体被观察者角色(ConcreteObservable)。

ConcreteObserver：具体观察者角色，实现抽象观察者角色(observer)所需要的更新接口，以便使自己状态和主题状态相协调。

总结：通过依赖抽象而不是依赖具体类，去实现一个类中某个状态的改变，而通知相关的一些类去做出相应的改变，进而保持同步状态。实现这样的方式或许有很多种，但是为了使系统能够易于复用，应该选择第耦合度的方案。减少对象之间的耦合度有利于系统的复用，在保证低耦合度的前提下并且能够维持行动的协调一致，保证高度协作，观察者模式是一种很好的设计方案。