/*******************************************************************************
 * 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.jface.viewers.ISelection;
import org.eclipse.jface.viewers.Viewer;
import org.eclipse.swt.dnd.DragSourceEvent;
import org.eclipse.swt.dnd.DragSourceListener;

/**
 * This is an implemention of {@link DragSourceListener}. It allows the selection in effect at the
 * start of the drag and drop interaction to be recorded, which is especially important for a drag
 * and drop interaction within a single view. This is how one of these adapters is typically hooked
 * up:
 * 
 * <pre>
 *   viewer.addDragSupport
 *     (DND.DROP_COPY | DND.DROP_MOVE | DND.DROP_LINK, 
 *      new Transfer [] { LocalTransfer.getInstance() },
 *      ViewerDragAdapter(viewer));
 * </pre>
 * 
 * Doing so simply allows a drag operation to be initiated from the viewer such that the viewer's
 * selection is transferred to the drop target. See {@link EditingDomainViewerDropAdapter} and
 * {@link LocalTransfer} for more details.
 */
public class ViewerDragAdapter implements DragSourceListener {
  /**
   * This keeps track of the viewer to which we are listening.
   */
  protected Viewer viewer;

  /**
   * This keeps track of the selection that is in effect at the start of the drag operation
   */
  protected ISelection selection;

  /**
   * This creates an instance for the given viewer.
   */
  public ViewerDragAdapter(Viewer viewer) {
    super();

    // Remember the viewer and listen to SWT.DragDetect to alert the start of the drag operation.
    //
    this.viewer = viewer;
  }

  /**
   * This is called when dragging is initiated; it records the {@link #selection} of {@link #viewer}
   * .
   */
  public void dragStart(DragSourceEvent event) {
    selection = viewer.getSelection();
  }

  /**
   * This is called when dragging is completed; it forgets the {@link #selection}.
   */
  public void dragFinished(DragSourceEvent event) {
    selection = null;
  }

  /**
   * This is called to transfer the data.
   */
  public void dragSetData(DragSourceEvent event) {
    if (ObjectTransfer.getInstance().isSupportedType(event.dataType)) {
      event.data = selection;
    }
  }
}