framework/base/src/main/java/org/apache/ofbiz/base/util/Observable.java

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 * or more contributor license agreements.  See the NOTICE file
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 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
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package org.apache.ofbiz.base.util;

import java.util.concurrent.CopyOnWriteArrayList;
import java.util.concurrent.atomic.AtomicBoolean;

/**
 * An observable object.
 * <p>This class is similar to <code>java.util.Observable</code> but it has some differences:</p>
 * <ul>
 *   <li>It has improved concurrency</li>
 *   <li>It cannot be subclassed</li>
 *   <li>The <code>notifyObservers</code> method does not clear the changed flag</li>
 *   <li>Protected methods have been made public</li>
 * </ul>
 *
 */
public final class Observable {

    private final AtomicBoolean changed = new AtomicBoolean();
    private final CopyOnWriteArrayList<Observer> observers = new CopyOnWriteArrayList<Observer>();

    public Observable() {}

    public Observable(Observable observable) {
        Assert.notNull("observable", observable);
        changed.set(observable.changed.get());
        observers.addAll(observable.observers);
    }

    /**
     * Adds an observer to the set of observers for this object. 
     *
     * @param observer the observer to be added.
     */
    public void addObserver(Observer observer) {
        Assert.notNull("observer", observer);
        observers.addIfAbsent(observer);
    }

    /**
     * Clears the changed flag. 
     */
    public void clearChanged() {
        changed.set(false);
    }

    /**
     * Deletes an observer from the set of observers of this object. 
     * Passing <code>null</code> to this method will have no effect.
     * 
     * @param observer the observer to be deleted.
     */
    public void deleteObserver(Observer observer) {
        observers.remove(observer);
    }

    /**
     * Clears the observer list so that this object no longer has any observers.
     */
    public void deleteObservers() {
        observers.clear();
    }

    /**
     * Returns <code>true</code> if this object has changed. 
     *
     */
    public boolean hasChanged() {
        return changed.get();
    }

    /**
     * Notify all of the observers. 
     * <p>Each <code>Observer</code> 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:
     * </p>
     * <blockquote>
     *   <tt>notifyObservers(null)</tt>
     * </blockquote>
     *
     */
    public void notifyObservers() {
        notifyObservers(null);
    }

    /**
     * Notify all of the observers. 
     * <p>Each observer has its <code>update</code> method called with two
     * arguments: this observable object and the <code>arg</code> argument.</p>
     *
     */
    public void notifyObservers(Object arg) {
        for (Observer observer : observers) {
            observer.update(this, arg);
        }
    }

    /**
     * Sets the changed flag to <code>true</code>. 
     */
    public void setChanged() {
        changed.set(true);
    }
}