/*******************************************************************************
 * Copyright (c) 2004, 2005 IBM Corporation and others.
 * All rights reserved. This program and the accompanying materials
 * are made available under the terms of the Eclipse Public License v1.0
 * which accompanies this distribution, and is available at
 * http://www.eclipse.org/legal/epl-v10.html
 * 
 * Contributors:
 *     IBM Corporation - Initial API and implementation
 *     Jens Lukowski/Innoopract - initial renaming/restructuring
 *******************************************************************************/
package org.eclipse.wst.common.ui.internal.dnd;


import org.eclipse.swt.dnd.ByteArrayTransfer;
import org.eclipse.swt.dnd.TransferData;


/**
 * This derived implementation of a byte array transfer short circuits the transfer process
 * so that a local transfer does not serialize the object
 * and hence can and will return the original object, not just a clone.
 * You only really need ever know about {@link #getInstance ObjectTransfer.getInstance()},
 * so that you can include it in when adding drag support to a viewer.
 * See {@link EditingDomainViewerDropAdapter} and {@link ViewerDragAdapter} for more details.
 * <p>
 * As an addded guard, the time is recorded and serialized in javaToNative
 * to that navive to java can ensure that it's returns the value that was really to have been transferred.
 */
public class ObjectTransfer extends ByteArrayTransfer
{
  /**
   * This is the register transfer type name.
   */
  protected static final String TYPE_NAME = "local-transfer-format";

  /**
   * This is the ID that is registered to the name.
   */
  protected static final int TYPE_ID = registerType(TYPE_NAME);

  /**
   * This is initialized and returned by {@link #getInstance}.
   */
  protected static ObjectTransfer instance;

  /**
   * This returns the one instance of this transfer agent.
   */
  public static ObjectTransfer getInstance()
  {
    if (instance == null)
    {
      instance = new ObjectTransfer();
    }

    return instance;
  }

  /**
   * This records the time at which the transfer data was recorded.
   */
  protected long startTime;

  /**
   * This records the data being transferred.
   */
  protected Object object;

  /**
   * This creates an instance; typically you get one from {@link #getInstance}.
   */
  protected ObjectTransfer()
  {
  }

  /**
   * This returns the transfer ids that this agent supports.
   */
  protected int[] getTypeIds() 
  {
    return new int[] { TYPE_ID };
  }

  /**
   * This returns the transfer names that this agent supports.
   */
  public String[] getTypeNames() 
  {
    return new String[] { TYPE_NAME };
  }

  /**
   * This records the object and current time and encodes only the current time into the transfer data.
   */
  public void javaToNative(Object object, TransferData transferData) 
  {
    startTime = System.currentTimeMillis();
    this.object = object;
    if (transferData != null)
    {
      super.javaToNative(String.valueOf(startTime).getBytes(), transferData);
    }
  }

  /**
   * This decodes the time of the transfer and returns the recorded the object if the recorded time and the decoded time match.
   */
  public Object nativeToJava(TransferData transferData) 
  {
    long startTime  = Long.valueOf(new String((byte[])super.nativeToJava(transferData))).longValue();
    return
      this.startTime == startTime ?
        object :
        null;
  }
}