How to change the mouse icon/feedback when we drag a node to the canvas from the tool palette in a GEF/GMF based editor

In this blog post I suggest a simple way to change the drag icon/feedback to indicate to the user that it is to drop or illegal to drop what they are dragging at the current location in a GEF or GMF based editor.

You can do it simply by implementing custom DragDropEditPolicy and installing to your EditPart.

a snippet of my code follows.

import java.awt.MouseInfo;
import java.awt.Point;

import org.eclipse.draw2d.FigureCanvas;
import org.eclipse.draw2d.IFigure;
import org.eclipse.draw2d.ImageFigure;
import org.eclipse.draw2d.geometry.Rectangle;
import org.eclipse.gef.Request;
import org.eclipse.gef.commands.Command;
import org.eclipse.gef.requests.ChangeBoundsRequest;
import org.eclipse.gmf.runtime.diagram.ui.editparts.ShapeCompartmentEditPart;
import org.eclipse.gmf.runtime.diagram.ui.editpolicies.DragDropEditPolicy;
import org.eclipse.gmf.runtime.diagram.ui.requests.CreateUnspecifiedTypeRequest;
import org.eclipse.swt.graphics.Image;
import org.eclipse.swt.widgets.Control;
import org.eclipse.wb.swt.SWTResourceManager;

/**
 * Installing FeedbackIndicateDragDropEditPolicy will indicate a feedback when drag a node
 * to the canvas from the tool palette
 */
public class FeedbackIndicateDragDropEditPolicy extends DragDropEditPolicy {
 
 private IFigure feedbackFigure;
 private Image feedbackImage;

 private static final String ALLOW_DROP_ICON_PATH = "/icons/custom/check.png";
 private static final String NOT_ALLOW_DROP_ICON_PATH = "/icons/custom/exclamation.png";

 /**
  * {@inheritDoc}
  */
 protected Command getDropCommand(ChangeBoundsRequest request) {
  if ((getHost() instanceof ShapeCompartmentEditPart)) {
   //avoiding drag and drop nodes between compartments.
   return null;
  }
  return super.getDragCommand(request);
 }

 /**
  * {@inheritDoc}
  */
 public void showTargetFeedback(Request request) {
  super.showTargetFeedback(request);
  if (request instanceof CreateUnspecifiedTypeRequest) {
   if (getHost() instanceof ShapeCompartmentEditPart) {
    ShapeCompartmentEditPart host = (ShapeCompartmentEditPart) getHost();
    Command command = host.getCommand(request);
    if (command != null) {
     if (command.canExecute()) {
      showFeedBackFigure(true);
     } else {
      showFeedBackFigure(false);
     }
    } else {
     showFeedBackFigure(false);
    }
   } else {
    showFeedBackFigure(false);
   }
  }
 }

 /**
  * {@inheritDoc}
  */
 public void eraseTargetFeedback(Request request) {
  if (feedbackFigure != null) {
   removeFeedback(feedbackFigure);
   feedbackFigure = null;
  }
  super.eraseTargetFeedback(request);
 }

 /**
  * Get feedback figure
  * @param allowDrop
  * @return
  */
 public IFigure getFeedbackFigure(boolean allowDrop) {
  if (feedbackFigure == null) {
   Image feedbackImage = SWTResourceManager.getImage(this.getClass(),
     (allowDrop) ? ALLOW_DROP_ICON_PATH : NOT_ALLOW_DROP_ICON_PATH);
   IFigure figure = new ImageFigure(feedbackImage);
   addFeedback(figure);
   this.feedbackImage = feedbackImage;
   feedbackFigure = figure;
  }
  return feedbackFigure;
 }

 /**
  * Show feedback figure in canvas
  * @param allowDrop
  */
 private void showFeedBackFigure(boolean allowDrop) {
  Point pointer = MouseInfo.getPointerInfo().getLocation();
  IFigure feedbackFigure = getFeedbackFigure(allowDrop);
  int x = (int) pointer.getX();
  int y = (int) pointer.getY();
  Control ctrl = getHost().getViewer().getControl();
  FigureCanvas canvas = (FigureCanvas) ctrl;
  int horizontal = canvas.getHorizontalBar().getSelection();
  int vertical = canvas.getVerticalBar().getSelection();
  horizontal += 15;
  vertical -= 15;
  org.eclipse.swt.graphics.Point p = canvas.toDisplay(0, 0);
  feedbackFigure.setBounds(new Rectangle((x - p.x) + horizontal, (y - p.y) + vertical,
    feedbackImage.getBounds().width, feedbackImage.getBounds().height));
 }

}