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solution is simply to display a translucent copy of the component itself,

referred to as a ghost in the code. Figure 9-10 shows a ghost in mid-drag.

This time, you will not use the package

java.awt.dnd; you’ll need your own

framework for this level of sophistication. By breaking out of the pre-built

box, you gain full control of drag-and-drop.

Drawing a Ghost

The very first thing you need is a glass pane able to draw a translucent pic-

ture over the UI. A glass pane is nothing more than a transparent

JPanel.

The code of

GhostGlassPane is very simple, as shown in Example 9-7.

Figure 9-10. A ghost is a translucent copy of a Swing component

Example 9-7. Creating a glass pane for ghosting

import java.awt.*;

import java.awt.image.*;

import javax.swing.*;

public class GhostGlassPane extends JPanel

{

private AlphaComposite composite;

private BufferedImage dragged = null;

private Point location = new Point(0, 0);

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This glass pane has two properties: the picture to be displayed and its loca-

tion. To make the picture translucent, use an

AlphaComposite instance.

Besides colors, strokes, and painters, Java2D drawings can be affected by

composites that define how the newly drawn pixels are mixed with the

underlying pixels. In this case, you want the new pixels—i.e., the picture—

to be drawn at 50% of their initial opacity. You can change the second

parameter of

getInstance( ) to choose the opacity. The valid value range is

from a totally opaque

0.0 to a totally transparent 1.0. An AlphaComposite can

be used to define how the alpha channels of the source (the pixels being

drawn), and the target (the existing pixels) are mixed together. You can

select the mixing mode with the first parameter of

getInstance( ) and refer

to the documentation of

AlphaComposite to get a comprehensive list of possi-

ble modes. It turns out that the

AlphaComposite.SRC_OVER composite gives

the best result for our job.

public GhostGlassPane( )

{

setOpaque(false);

composite = AlphaComposite.getInstance(AlphaComposite.SRC_OVER, 0.5f);

}

public void setImage(BufferedImage dragged)

{

this.dragged = dragged;

}

public void setPoint(Point location)

{

this.location = location;

}

public void paintComponent(Graphics g)

{

if (dragged == null)

return;

Graphics2D g2 = (Graphics2D) g;

g2.setComposite(composite);

g2.drawImage(dragged,

(int) (location.getX( ) - (dragged.getWidth(this) / 2)),

(int) (location.getY( ) - (dragged.getHeight(this) / 2)),

null);

}

}

Example 9-7. Creating a glass pane for ghosting (continued)

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The only job of the paintComponent( ) will be to draw the dragged picture.

Therefore, there is no need to call

super.paintComponent( ), as it will per-

form unnecessary operations.

Do not forget to set the composite in this method, or you

might draw the picture fully opaque.

The center of the picture is painted at the location specified in setLocation( ).

This means assuming the location given is the location of the mouse cursor.

Now, you’ve got a fully functional glass pane, and you just need to add it to

a Swing frame:

glassPane = new GhostGlassPane( );

setGlassPane(glassPane);

Also, do not forget that a glass pane is not visible by default: you will have to

call

setVisible(true) when a drag-and-drop operation is initiated. The final

step is the activation of drag-and-drop on components requiring it. You can

use a picture as drag-and-drop feedback:

JLabel label = new JLabel("New Sale");

GhostDropAdapter pictureAdapter;

pictureAdapter = new GhostPictureAdapter(glassPane,

"new_sale",

"images/new_sale.png")

label.addMouseListener(pictureAdapter);

label.addMouseMotionListener(new GhostMotionAdapter(glassPane));

You can also use a ghost feedback:

JButton button = new JButton("Ghost Feedback"));

GhostDropAdapter componentAdapter;

componentAdapter = new GhostComponentAdapter(glassPane, "button_pushed");

button.addMouseListener(componentAdapter);

button.addMouseMotionListener(new GhostMotionAdapter(glassPane));

Whatever choice you make, you need two adapters to perform drag-and-

drop. The first one is an adapter for the

MouseListener interface. It can be

either a

GhostPictureAdapter, to handle pictures, or a GhostComponentAdapter,

to handle a component ghost. The role of this adapter is to handle the begin-

ning and the end of a drag-and-drop gesture. When a drop action is per-

formed, an event is fired to every

GhostDropListener registered by the

adapter. The code in Example 9-8 shows how

GhostComponentAdapter works.

The code of

GhostPictureAdapter is almost the same, and it’s left to you to

check out (all the code for this book is online; visit http://www.oreilly.com/

catalog/swinghks).

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Example 9-8. Handling component ghosts

import java.awt.*;

import java.awt.event.*;

import java.awt.image.*;

import javax.swing.*;

public class GhostComponentAdapter extends GhostDropAdapter

{

public GhostComponentAdapter(GhostGlassPane glassPane, String action) {

super(glassPane, action);

}

public void mousePressed(MouseEvent e)

{

Component c = e.getComponent( );

BufferedImage image = new BufferedImage(c.getWidth( ),

c.getHeight( ),

BufferedImage.TYPE_INT_ARGB);

Graphics g = image.getGraphics( );

c.paint(g);

glassPane.setVisible(true);

Point p = (Point) e.getPoint().clone( );

SwingUtilities.convertPointToScreen(p, c);

SwingUtilities.convertPointFromScreen(p, glassPane);

glassPane.setPoint(p);

glassPane.setImage(image);

glassPane.repaint( );

}

public void mouseReleased(MouseEvent e)

{

Component c = e.getComponent( );

Point p = (Point) e.getPoint().clone( );

SwingUtilities.convertPointToScreen(p, c);

Point eventPoint = (Point) p.clone( );

SwingUtilities.convertPointFromScreen(p, glassPane);

glassPane.setPoint(p);

glassPane.setVisible(false);

glassPane.setImage(null);

fireGhostDropEvent(new GhostDropEvent(action, eventPoint));

}

}