ViewerDragAdapter.java

/**
 * Copyright (c) 2002-2006 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
 */
package org.eclipse.emf.edit.ui.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 implementation 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;
    LocalTransfer.getInstance().javaToNative(null, null);
  }
  
  /**
   * This is called to transfer the data.
   */
  public void dragSetData(DragSourceEvent event)
  {
    if (LocalTransfer.getInstance().isSupportedType(event.dataType))
    {
      event.data = selection;
    }
  }
}