首页 学习 设计模式 HeadFirst 设计模式

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

  1. 报社出版报纸；
  2. 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；
  3. 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；
  4. 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);
}

java.util.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.
 * <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();
    }
}

可以看到 jdk 自带的观察者模式用起来很方便，当我们需要一个新的主题时，只需要继承 Observable 类做一些自己需要的扩展即可（如信息的传递是 pull 还是 push），不必重复造轮子。

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。