/** Construct an Observable with zero Observers. */
publicObservable() { obs = newVector<>(); }
/** * 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. */ publicsynchronizedvoidaddObserver(Observer o) { if (o == null) thrownewNullPointerException(); 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. */ publicsynchronizedvoiddeleteObserver(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) */ publicvoidnotifyObservers() { 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) */ publicvoidnotifyObservers(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 (inti= 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. */ publicsynchronizedvoiddeleteObservers() { obs.removeAllElements(); }
/** * Marks this <tt>Observable</tt> object as having been changed; the * <tt>hasChanged</tt> method will now return <tt>true</tt>. */ protectedsynchronizedvoidsetChanged() { 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) */ protectedsynchronizedvoidclearChanged() { 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() */ publicsynchronizedbooleanhasChanged() { return changed; }
/** * Returns the number of observers of this <tt>Observable</tt> object. * * @return the number of observers of this object. */ publicsynchronizedintcountObservers() { return obs.size(); } }
3、Observer接口:
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publicinterfaceObserver
一个可在观察者要得到 observable 对象更改通知时可实现 Observer 接口的类。
3.1、相关方法:
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publicinterfaceObserver { /** * 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. */ voidupdate(Observable o, Object arg); }