Observable与Observer源码分析及用法

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观察者模式是对象的行为模式，又叫发布-订阅(Publish/Subscribe)模式、模型-视图(Model/View)模式、源-监听器(Source/Listener)模式或从属者(Dependents)模式。
　　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

一、Observable类

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package java.util;
/**
 * Observable is used to notify a group of Observer objects when a change
 * occurs. On creation, the set of observers is empty. After a change occurred,
 * the application can call the {@link #notifyObservers()} method. This will
 * cause the invocation of the {@code update()} method of all registered
 * Observers. The order of invocation is not specified. This implementation will
 * call the Observers in the order they registered. Subclasses are completely
 * free in what order they call the update methods.
 *
 * @see Observer
 */
public class Observable {
    List<Observer> observers = new ArrayList<Observer>();
    boolean changed = false;
    /**
     * Constructs a new {@code Observable} object.
     */
    public Observable() {
    }
    /**
     * Adds the specified observer to the list of observers. If it is already
     * registered, it is not added a second time.
     *
     * @param observer
     *            the Observer to add.
     */
    public void addObserver(Observer observer) {
        if (observer == null) {
            throw new NullPointerException("observer == null");
        }
        synchronized (this) {
            if (!observers.contains(observer))
                observers.add(observer);
        }
    }
    /**
     * Clears the changed flag for this {@code Observable}. After calling
     * {@code clearChanged()}, {@code hasChanged()} will return {@code false}.
     */
    protected void clearChanged() {
        changed = false;
    }
    /**
     * Returns the number of observers registered to this {@code Observable}.
     *
     * @return the number of observers.
     */
    public int countObservers() {
        return observers.size();
    }
    /**
     * Removes the specified observer from the list of observers. Passing null
     * won't do anything.
     *
     * @param observer
     *            the observer to remove.
     */
    public synchronized void deleteObserver(Observer observer) {
        observers.remove(observer);
    }
    /**
     * Removes all observers from the list of observers.
     */
    public synchronized void deleteObservers() {
        observers.clear();
    }
    /**
     * Returns the changed flag for this {@code Observable}.
     *
     * @return {@code true} when the changed flag for this {@code Observable} is
     *         set, {@code false} otherwise.
     */
    public boolean hasChanged() {
        return changed;
    }
    /**
     * If {@code hasChanged()} returns {@code true}, calls the {@code update()}
     * method for every observer in the list of observers using null as the
     * argument. Afterwards, calls {@code clearChanged()}.
     * <p>
     * Equivalent to calling {@code notifyObservers(null)}.
     */
    public void notifyObservers() {
        notifyObservers(null);
    }
    /**
     * If {@code hasChanged()} returns {@code true}, calls the {@code update()}
     * method for every Observer in the list of observers using the specified
     * argument. Afterwards calls {@code clearChanged()}.
     *
     * @param data
     *            the argument passed to {@code update()}.
     */
    @SuppressWarnings("unchecked")
    public void notifyObservers(Object data) {
        int size = 0;
        Observer[] arrays = null;
        synchronized (this) {
            if (hasChanged()) {
                clearChanged();
                size = observers.size();
                arrays = new Observer[size];
                observers.toArray(arrays);
            }
        }
        if (arrays != null) {
            for (Observer observer : arrays) {
                observer.update(this, data);
            }
        }
    }
    /**
     * Sets the changed flag for this {@code Observable}. After calling
     * {@code setChanged()}, {@code hasChanged()} will return {@code true}.
     */
    protected void setChanged() {
        changed = true;
    }
}

从源码可以看出，Observable类中有两个方法对Observer特别重要，一个是setChange()方法用来设置一个内部标志位注明数据发生了变化；一个是notifyObservers()方法会去调用一个列表中所有的Observer的update()方法，通知它们数据发生了变化。
Observer通过Observable的addObserver()方法把自己添加到这个列表中。这个列表虽然由Observable拥有，但Observable并不知道到底有哪些Observer正在观察等待通知。Observable只提供一个方法让Observer能把自己添加进列表，并保证会去通知Observer发生了变化。通过这种机制，可以有任意多个Observer对Observable进行观察，而不影响Observable的实现。
Observable的所有方法都是同步的，保证了在一个线程对其标志位、列表进行操作时，不会有其它线程也在操作它。
Observable的notifyObservers(Object obj)形式可以再调用update时将参数传进去。通知顺序通常时越晚加入列表的越先通知。update会被依次调用，由于一个update返回后下一个update才被调用，这样当update中有大量操作时，最好将其中的工作拿到另一个线程或者Observer本身同时也是一个Thread类，Observer先挂起，在update中被唤醒，这样会有一个隐患，Observer线程还没来得及挂起，update就被调用了，通知消息就这样被错过了，一种解决办法是在Observer中实现一个同步的队列结构，并有一个类来封装参数，update实现一个参数类的对象把接收到的通知消息的参数封装在里面，然后把其加进队列，run方法从队列中移除参数对象，并进行处理，这保证了没有通知信息被丢失。

二、Observer接口

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package java.util;
/**
 * {@code Observer} is the interface to be implemented by objects that
 * receive notification of updates on an {@code Observable} object.
 *
 * @see Observable
 */
public interface Observer {
    /**
     * This method is called if the specified {@code Observable} object's
     * {@code notifyObservers} method is called (because the {@code Observable}
     * object has been updated.
     *
     * @param observable
     *            the {@link Observable} object.
     * @param data
     *            the data passed to {@link Observable#notifyObservers(Object)}.
     */
    void update(Observable observable, Object data);
}

从源码可以看出Observer接口只有update()方法，当Observable类调用setChange()方法注明数据发生了变化；同时调用notifyObservers()方法通知列表中所有的Observer调用update()方法，通知它们数据发生了变化，更新数据。

三、简单用法

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import java.util.Observable;  
   
public class SimpleObservable extends Observable  
{  
   private int data = 0;  
   
   public int getData(){  
       return data;  
   }  
   
   public void setData(int i){  
       if(this.data != i){   
         this.data = i;  
          setChange(); //注明数据发生了变化
       }  
          notifyObservers();  
         //只有在setChange()被调用后，notifyObservers()才会去调用update()，否则什么都不干。  
       }  
   }  
}
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import java.util.Observable;  
import java.util.Observer;  
   
public class SimpleObserver implements Observer  
{  
   public SimpleObserver(SimpleObservable so){  
      so.addObserver(this );  
   }  
   
   public void update(Observable o,Object arg/*任意对象，用于传递参数*/){  
      System.out.println(“Data has changed to” + (SimpleObservable)o.getData());  
   }  
}  
   
public class SimpleTest  
{  
   public static void main(String[] args){  
      SimpleObservable doc = new SimpleObservable ();  
      SimpleObserver view = new SimpleObserver (doc);  
      doc.setData(1);  
      doc.setData(2);  
      doc.setData(2);  
      doc.setData(3);  
   }  
}

打印结果：
Data has changed to 1
Data has changed to 2 //第二次setData(2)时由于没有setChange，所以update没被调用
Data has changed to 3

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