old/org.eclipse.emf.ecp.common/src/org/eclipse/emf/ecp/common/dnd/DragSourcePlaceHolder.java

/*******************************************************************************
 * Copyright (c) 2008-2011 Chair for Applied Software Engineering,
 * Technische Universitaet Muenchen.
 * 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:
 ******************************************************************************/
package org.eclipse.emf.ecp.common.dnd;

import java.util.List;

/**
 * This class is just a temporary location to hold dragSource in a DnD operation. Typically we should not have used such
 * a class to keep track of dragSource. Instead, one should use a transfer class. This is not the case because of the following:
 * 
 * <ol>
 * 	<li>Using a transfer is not guaranteed to work on all platforms and always.</li>
 *  <li>On platforms other than Windows, dragSource is not set until drop event fires and we need dragSource during events like dragOver</li>
 *  <li>We are not planning to drop something out of ECP environment, i.e. form ECP navigator on file system.</li>
 * </ol> 
 *  
 *  This class is intended to be used as a singleton.
 * 
 * @author Hodaie
 */
public final class DragSourcePlaceHolder {

	private static Object dragSource;
	
	/**
	 * Initializes the singleton instance statically.
	 */
	private static class SingletonHolder { 
		public static final DragSourcePlaceHolder INSTANCE = new DragSourcePlaceHolder();
	}

	/**
	 * Returns the singleton instance.
	 * 
	 * @return The DragSourcePlaceHolder instance
	 */
	public static DragSourcePlaceHolder getInstance() {
		return SingletonHolder.INSTANCE;
	}

	/**
	 * Private constructor.
	 */
	private DragSourcePlaceHolder() {

	}

	/**
	 * This method will be called from drag adaptors, in dragStart() event. This sets the drag source on which initially
	 * the drag started. This can be a single object of a collection.
	 * 
	 * @param dragSource the drag source
	 */
	@SuppressWarnings("rawtypes")
	public static void setDragSource(Object dragSource) {
		if ((dragSource instanceof List) && ((List) dragSource).size() == 0) {
			DragSourcePlaceHolder.dragSource = null;
		}
		DragSourcePlaceHolder.dragSource = dragSource;
	}

	/**
	 * This will be called from drop adaptors to get the drag source.
	 * 
	 * @return drag source; object(s) on which drag started
	 */
	public static Object getDragSource() {
		return dragSource;
	}

}