Change mouseListener while mouse is pressed

Question

I'm working on a game in Java and have the following challenge.

I have 2 JPanels and need to visually drag shapes from one JPanel to another. I've got this working using the GlassPane from the JFrame. When I press the mouse to drag a shape, the GlassPane activates and transfers the shape to the glassPane. Because of this you need to transfer the mousePressed state from the JPanels mouseAdapter to the glassPanes mouseAdapter. I solved this by using the Robot Class which simulates another mousePressed event after the glassPane has been acivated.

Now here comes the problem, this workaround only works on windows and not on mac osx, on osx the mouse keeps talking to the JPanels mouseAdapter as long as the mousebutton is pressed. So does anyone know how to transfer the mousePressed state from one mouseAdapter to another while pressing the mousebutton, in a proper way? (Releasing the button and pressing it again is not an option as this would defeat the purpose of dragging.)

Answer 1

Why not add the MouseListener just to the glasspane, and in the mousePressed method, get the mouse's location (Point) and then get the draggable component by calling getComponentAt(Point p) on the Container that holds your Component? You can then place the component into the glasspane and drag it there. For instance here is one way I've done this using a JLayeredPane (which is similar to using a glasspane): DragLabelOnLayeredPane

Or another thought: why not simply add the MouseAdapter on the dragged component itself and leave it on the component? As long as you take care to get mouse position relative to the screen and move the component relative to its container, you should have no problem whether the component is in the contentPane or the glasspane.

edit: or go with MeBigFatGuy's excellent suggestion.

edit 2: A semi-drunk late at night attempt at a not-brief-as-I'd-like proof of concept program, a program that moves shapes by adding a MouseListener to the glass pane only.

import java.awt.BasicStroke;
import java.awt.Color;
import java.awt.Component;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.GridLayout;
import java.awt.Point;
import java.awt.RenderingHints;
import java.awt.Shape;
import java.awt.Stroke;
import java.awt.event.*;
import java.awt.geom.*;
import java.util.ArrayList;
import java.util.List;
import java.util.Random;

import javax.swing.*;

@SuppressWarnings("serial")
public class DragShapesMainPanel extends JPanel {
   private static final Dimension RIGHT_PANEL_SIZE = new Dimension(300, 450);
   private static final int SHAPE_COUNT = 10;
   private static final int SHAPE_WIDTH = 40;
   private static final int SHAPE_HEIGHT = SHAPE_WIDTH;
   private Shape draggedShape = null;
   private DragShapesPanel leftPanel = new DragShapesPanel(Color.blue, Color.black);
   private DragShapesPanel rightPanel = new DragShapesPanel(Color.blue, Color.black);
   private DragShapesGlassPanel glassPanel = new DragShapesGlassPanel(Color.pink, Color.gray);
   private Random random = new Random();

   public DragShapesMainPanel() {
      setLayout(new GridLayout(1, 0));
      setBackground(Color.black);
      rightPanel.setPreferredSize(RIGHT_PANEL_SIZE);
      leftPanel.setPreferredSize(RIGHT_PANEL_SIZE);
      rightPanel.setBorder(BorderFactory.createLineBorder(Color.black, 1));
      leftPanel.setBorder(BorderFactory.createLineBorder(Color.black, 1));
      add(leftPanel);
      add(rightPanel);

      MouseAdapter myMouseAdapter = new MyMouseAdapter();
      glassPanel.addMouseListener(myMouseAdapter);
      glassPanel.addMouseMotionListener(myMouseAdapter);

      glassPanel.setOpaque(false);
      glassPanel.setVisible(true);

      for (int i = 0; i < SHAPE_COUNT; i++) {
         leftPanel.addShape(createRandomShape(i));
      }
   }

   private Shape createRandomShape(int i) {
      Dimension size = rightPanel.getPreferredSize();
      int x = random.nextInt(size.width - SHAPE_WIDTH);
      int y = random.nextInt(size.height - SHAPE_HEIGHT);
      switch (i % 3) {
      case 0:
         return new Ellipse2D.Double(x, y, SHAPE_WIDTH, SHAPE_HEIGHT);

      case 1:
         return new Rectangle2D.Double(x, y, SHAPE_WIDTH, SHAPE_HEIGHT);

      case 2:
         return new RoundRectangle2D.Double(x, y, SHAPE_WIDTH, SHAPE_HEIGHT, 15, 15);

      default:
         break;
      }
      return null;
   }

   public JPanel getGlassPanel() {
      return glassPanel;
   }

   private class MyMouseAdapter extends MouseAdapter {
      Point initialLocation = null;

      @Override
      public void mousePressed(MouseEvent e) {
         if (e.getButton() != MouseEvent.BUTTON1) {
            return;
         }
         Component componentAt = getComponentAt(e.getPoint());
         if (!(componentAt instanceof DragShapesPanel)) {
            return;
         }

         initialLocation = e.getPoint();
         DragShapesPanel dsPanel = (DragShapesPanel) getComponentAt(initialLocation);

         int x = initialLocation.x - dsPanel.getLocation().x;
         int y = initialLocation.y - dsPanel.getLocation().y;
         Point p = new Point(x, y);

         Shape shape = dsPanel.getShapeAtPoint(p);
         if (shape == null) {
            initialLocation = null;
            return;
         }

         dsPanel.removeShape(shape);
         dsPanel.repaint();

         int tx = dsPanel.getLocation().x;
         int ty = dsPanel.getLocation().y;
         draggedShape = AffineTransform.getTranslateInstance(tx, ty).createTransformedShape(shape);

         glassPanel.setShape(draggedShape);
         glassPanel.repaint();
      }

      @Override
      public void mouseDragged(MouseEvent e) {
         if (initialLocation == null) {
            return;
         }

         Point currentLocation = e.getPoint();
         int x = currentLocation.x - initialLocation.x;
         int y = currentLocation.y - initialLocation.y;

         glassPanel.translate(new Point(x, y));
         glassPanel.repaint();
      }

      @Override
      public void mouseReleased(final MouseEvent e) {
         if (initialLocation == null) {
            return;
         }
         SwingUtilities.invokeLater(new Runnable() {
            public void run() {
               Component componentAt = getComponentAt(e.getPoint());
               if (!(componentAt instanceof DragShapesPanel)) {
                  return;
               }

               DragShapesPanel dsPanel = (DragShapesPanel) getComponentAt(e.getPoint());

               Point currentLocation = e.getPoint();
               int x = currentLocation.x - initialLocation.x;
               int y = currentLocation.y - initialLocation.y;

               Point dspPoint = dsPanel.getLocation();
               int tx = x - dspPoint.x;
               int ty = y - dspPoint.y;
               draggedShape = AffineTransform.getTranslateInstance(tx, ty).createTransformedShape(
                        draggedShape);

               dsPanel.addShape(draggedShape);
               dsPanel.repaint();

               initialLocation = null;
               glassPanel.setShape(null);
               glassPanel.translate(new Point(0, 0));
            }
         });
      }
   }

   private static void createAndShowUI() {
      DragShapesMainPanel dragShapesMain = new DragShapesMainPanel();
      JFrame frame = new JFrame("DragShapes");
      frame.getContentPane().add(dragShapesMain);
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.pack();
      frame.setGlassPane(dragShapesMain.getGlassPanel());
      frame.getGlassPane().setVisible(true);
      frame.setLocationRelativeTo(null);
      frame.setVisible(true);
   }

   public static void main(String[] args) {
      java.awt.EventQueue.invokeLater(new Runnable() {
         public void run() {
            createAndShowUI();
         }
      });
   }
}

@SuppressWarnings("serial")
class DragShapesPanel extends JPanel {
   private static final float STROKE_WIDTH = 3;
   private static final Stroke SHAPE_STROKE = new BasicStroke(STROKE_WIDTH);
   private List<Shape> shapeList = new ArrayList<Shape>();
   private Color shapeFillColor;
   private Color shapeBorderColor;

   public DragShapesPanel(Color fillColor, Color borderColor) {
      this.shapeFillColor = fillColor;
      this.shapeBorderColor = borderColor;
   }

   public void addShape(Shape s) {
      shapeList.add(s);
   }

   public void removeShape(Shape s) {
      shapeList.remove(s);
   }

   public Shape getShapeAtPoint(Point p) {
      Shape shapeAtPoint = null;
      for (int i = shapeList.size() - 1; i >= 0; i--) {
         if (shapeList.get(i).contains(p)) {
            return shapeList.get(i);
         }
      }
      return shapeAtPoint;
   }

   @Override
   protected void paintComponent(Graphics g) {
      super.paintComponent(g);
      Graphics2D g2 = (Graphics2D) g;
      g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
      for (Shape shape : shapeList) {
         g2.setColor(shapeFillColor);
         g2.fill(shape);
         g2.setStroke(SHAPE_STROKE);
         g2.setColor(shapeBorderColor);
         g2.draw(shape);
      }
   }
}

@SuppressWarnings("serial")
class DragShapesGlassPanel extends JPanel {
   private static final float STROKE_WIDTH = 1.5f;
   private static final Stroke SHAPE_STROKE = new BasicStroke(STROKE_WIDTH);
   private Shape shape = null;
   private Color shapeFillColor;
   private Color shapeBorderColor;
   private AffineTransform transform = new AffineTransform();

   public DragShapesGlassPanel(Color fillColor, Color borderColor) {
      this.shapeFillColor = fillColor;
      this.shapeBorderColor = borderColor;
   }

   public void setShape(Shape shape) {
      this.shape = shape;
   }

   public void translate(Point p) {
      transform = AffineTransform.getTranslateInstance(p.x, p.y);
   }

   @Override
   protected void paintComponent(Graphics g) {
      super.paintComponent(g);
      if (shape == null) {
         return;
      }
      Graphics2D g2 = (Graphics2D) g;
      g2.transform(transform);
      g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
      g2.setColor(shapeFillColor);
      g2.fill(shape);
      g2.setStroke(SHAPE_STROKE);
      g2.setColor(shapeBorderColor);
      g2.draw(shape);
   }
}