Java 基础 观察者模式复习

观察者模式的概念：

属于对象行为型模型，也被称为发布-订阅模式或者依赖模式，定义对象间一对多的依赖关系，当一个对象发生改变状态时，所有依赖于它的对象都会收到通知并被自动更新。

观察者职责：收到通知后做一些事情

被观察者职责： 管理和通知观察者

优点：

• 观察者和被观察者之间是抽象解耦： 
• 建立了一套触发机制

缺点：

• 因为一个被观察者多个观察者，开发调试会比较复杂，Java是消息通知默认是顺序执行，如果一个观察者卡了，会影响整体的执行效率，这种情况下，一般采用异步方式。
• 多级触发时效率问题，意外的更新：因为一个观察者并不知道其他观察者的存在，它可能对改变目标的最终代价一无所知，在目标的操作可能因为一系列对观察者以及依赖于这些观察者的那些对象的更新，此外，如果依赖准则的定义或维护不当，常常会引起错误的更新，这些错误通常很难捕捉。

使用场景：

• 关联行为场景
• 事件需要多级触发的场景
• 跨系统的消息交互场景，如消息队列的处理机制 
• 当一个抽象模型有两个方面，其中一个依赖于另一个，将这二封装在独立的对象中使他们可以各自独立的改变和复用
• 当一个对象的改变需要同时改变其他对象，而不知道具体有多少对象有待改变
• 对一个对象必须通知其他对象，而它又不能假定其他对象是谁，就是这些对象不能紧密耦合。

MVC

代码实例：

Java已经为我们抽象出了观察者的API

/**
 * 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);
}

 

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();
    }
}