观察者模式（Observer）

场景：

1.进攻队员射门。

2.防守队员（后卫、门将）作出反应（封堵，扑救）。

使用jdk自带的observer和observale。

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);
}

public class Observable {
    private boolean changed = false;
    private Vector obs;
   
    /** Construct an Observable with zero Observers. */

    public Observable() {
	obs = new Vector();
    }

    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) {
	    if (!changed)
                return;
            arrLocal = obs.toArray();
            clearChanged();
        }

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

Observable和Observer 维护了一个一对多关系。

类图：（代码见附件observer.rar）




Defend在这里作为接口，实现为Keeper和MicBack。此步骤与观察者模式无关。

观察者模式(Observer)：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受到通知并自动更新。

设计原则：为了交互对象之间的松耦合设计而努力。

这里，有一点需要指出，由于java单继承的特性，Observable采用抽象类的形式不可避免的会造成不必要的麻烦（比如需要有自己的业务类需要继承，Observable内方法都为protected，除非继承否则不能将Observable的实例组合到自己的对象中来），此处建议自己实现一个观察者模式。