观察者模式
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平时写程序，基本是追求功能的实现，并不注重代码的质量及程序的设计，或许既有代码已经设计到了一些模式，但是没有发现，今天有时间，看看奥莱利O'Reilly的Head First系列书《设计模式》
以报纸和杂志的订阅为例子，可以这样想
- 报社出版报纸；
- 小红订了报纸，那么，如果以后报社有新报纸，只要小红还是订户，小红就会持续收到报纸，不间断；
- 小红失恋了，不想看报纸了，可以取消订阅，于是报社就不再给小红送了；
- 但是报社只要在运营，就会有源源不断的类似小红的客户前来订阅（或取消订阅）；
从上面我们可以总结出：
- 观察者模式 = 出版者 + 订阅者
出版者也称主题，即Subject，订阅者也可以称为观察者，即Observer。
这听起来是不是很像关注某个人的微博一样，例如当你成为赵丽颖的粉丝的时候，此时，赵丽颖就在扮演一个主题，你就是观察者。你可以持续地接收到赵丽颖更新的微博消息。
而在这里，赵丽颖也可以被当作是一名观察者去关注他人，这是很正常的，通常情况下，集两种角色于一身是非常合理的。
下面看一下观察者模式的真正定义：
- 观察者模式定义了对象之间的一对多依赖，这样一来，当一个对象改变状态时，它的所有依赖者都会收到通知并自动更新。
在上面，赵丽颖作为一个主题，只需知道有哪些人关注了我，而不需要知道粉丝们具体身份细节。这实现了主题和观察者之间的松耦合。即便有新的观察者出现也不需要做修改，新观察者只需实现此观察者接口，然后注册为观察者即可。这样实现了主题和观察者的复用。
设计原则：为了交互对象之间的松耦合而努力。
书中介绍了以气象站广播天气为例，可以参考博客。
在java api中有内置的观察者模式。如java.util包内的Observer接口与Observable类。
java.util.Observer源码：
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);}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(); }}然而，java.util.Observable类同样存在不好的地方，因为你会发现我们的可观察者Observable竟然tmd是一个类，而不是一个接口，更糟糕的是它竟然没有实现任何一个接口。自己把自己给限制了，毕竟java不支持多重继承，只是单继承的，如果你想拥有另一个超类的行为，显然是不可能的，复用的潜力比较差，当然这并不是说它没有提供有用的功能。
简单说一下jdk Observable设计违反的两个设计原则：
- 第一种就是上面说的扩展复用问题；
- 第二种就是Observable竟然将关键的方法保护起来了（可以看到setChanged()方法修饰符是protected类型的），违反了”多用组合，少用继承“原则；
所以，在使用jdk自带的Observable类时，如果你能扩展，那么就可能符合你的需求，否则只能自己写一套观察者模式了。
类似的在jdk中出现观察者模式的地方还有JavaBeans和Swing等。
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