LocalTransfer.java

/**
 * <copyright> 
 *
 * Copyright (c) 2002, 2009 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 - Initial API and implementation
 *
 * </copyright>
 *
 * $Id: LocalTransfer.java,v 1.6 2008/12/22 14:26:18 emerks Exp $
 */
package net.enilink.komma.edit.ui.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 LocalTransfer.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 added guard, the time is recorded and serialized in javaToNative to
 * that native to java can ensure that it's returns the value that was really to
 * have been transferred.
 */
public class LocalTransfer 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 LocalTransfer instance;

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

		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 LocalTransfer() {
		super();
	}

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

	/**
	 * This returns the transfer names that this agent supports.
	 */
	@Override
	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.
	 */
	@Override
	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.
	 */
	@Override
	public Object nativeToJava(TransferData transferData) {
		byte[] bytes = (byte[]) super.nativeToJava(transferData);
		if (bytes == null)
			return null;

		try {
			long startTime = Long.parseLong(new String(bytes));
			return this.startTime == startTime ? object : null;
		} catch (NumberFormatException exception) {
			return null;
		}
	}
}