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Java Simplified. Extensions and Useful Constructs for the Java Platform. Codeprimate Elements (a.k.a. cp-elements) is a Java library and micro-framework used to simplify the development of software applications written in Java. Elements packages several APIs into one library in order to address various application concerns and aspects of software design and development collectively and conveniently. Elements is a highly simple, yet robust and proven library built on solid OO principles, software design patterns and best practices to effectively solve common and reoccurring problems in software development.

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/*
 * Copyright 2016 Author or Authors.
 *
 * Licensed 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 "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.cp.elements.beans.event;

import java.beans.PropertyChangeEvent;
import java.beans.PropertyChangeListener;
import java.util.Collections;
import java.util.Iterator;
import java.util.Map;
import java.util.TreeMap;

import org.cp.elements.lang.ObjectUtils;

/**
 * The ChangeTracker class is a ChangeListener class used by org.cp.elements.beans.Bean implementations
 * for recording and tracking changes in state to the properties of the "observed" Bean.
 *
 * @author John J. Blum
 * @see java.beans.PropertyChangeEvent
 * @see java.beans.PropertyChangeListener
 * @see java.lang.Iterable
 * @see java.util.Iterator
 * @since 1.0.0
 */
public class ChangeTracker implements Iterable, PropertyChangeListener {

  // The object state Map is a mapping between the property name and property value's hash code.
  private final Map objectStateMap = new TreeMap();

  /**
   * Determines whether the Bean tracked by this ChangeTracker has any modified properties.
   *
   * @return a boolean value indicating whether the Bean tracked by this ChangeTracker has any modified properties.
   * @see #isModified(String)
   */
  public boolean isModified() {
    return !objectStateMap.isEmpty();
  }

  /**
   * Determines whether the specified property on the Bean tracked by this ChangeTracker has been modified.
   *
   * @param propertyName a String specifying the name of the property on the Bean.
   * @return a boolean value indicating whether the specified property on the Bean tracked by this ChangeTracker
   * has been modified.
   * @see #isModified()
   */
  public boolean isModified(final String propertyName) {
    return objectStateMap.containsKey(propertyName);
  }

  /**
   * The event handler method that gets fired when a property of the Bean tracked by this ChangeTracker has been
   * modified.  The PropertyChangeEvent encapsulates all the information pertaining to the property change
   * including the name of the property, it's old and new value and the source Bean of the targeted change.
   *
   * @param event the PropertyChangeEvent encapsulating information about the property change.
   * @see java.beans.PropertyChangeEvent
   */
  public void propertyChange(final PropertyChangeEvent event) {
    if (objectStateMap.containsKey(event.getPropertyName())) {
      if (objectStateMap.get(event.getPropertyName()) == ObjectUtils.hashCode(event.getNewValue())) {
        // NOTE the state of the property identified by the event has been reverted to it's original, persisted value.
        objectStateMap.remove(event.getPropertyName());
      }
    }
    else {
      if (!ObjectUtils.equalsIgnoreNull(event.getOldValue(), event.getNewValue())) {
        objectStateMap.put(event.getPropertyName(), ObjectUtils.hashCode(event.getOldValue()));
      }
    }
  }

  /**
   * Iterates over the set of properties on the Bean tracked by this ChangeTracker that have been modified.
   *
   * @return a Iterator over the modified properties on the Bean tracked by this ChangeTracker.
   * @see java.util.Iterator
   */
  public Iterator iterator() {
    return Collections.unmodifiableSet(objectStateMap.keySet()).iterator();
  }

}