Swing Hacks

288

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Chapter 8, Rendering

#56 Create a Magnifying Glass Component

HACK

Build the Magnifying Glass

The DetachedMagnifyingGlass will need to keep track of the Component it’s

viewing, the current mouse location in that component, a zoom factor, and

its own size. It will also need an instance of the AWT

Robot

for taking screen

grabs. The other thing it needs to do is to have a

MouseMotionListener

, so

that it will get updates on the cursor’s position and, when it changes, do a

new grab and

repaint( )

.

The

DetachedMagnifyingGlass code is shown in Example 8-1.

Example 8-1. JComponent to provide a magnified view of another JComponent

public class DetachedMagnifyingGlass extends JComponent

implements MouseMotionListener {

double zoom;

JComponent comp;

Point point;

Dimension mySize;

Robot robot;

public DetachedMagnifyingGlass (JComponent comp,

Dimension size,

double zoom) {

this.comp = comp;

// flag to say don't draw until we get a MouseMotionEvent

point = new Point (-1, -1);

comp.addMouseMotionListener(this);

this.mySize = size;

this.zoom = zoom;

// if we can't get a robot, then we just never

// paint anything

try {

robot = new Robot( );

} catch (AWTException awte) {

System.err.println ("Can't get a Robot");

awte.printStackTrace( );

}

}

public void paint (Graphics g) {

if ((robot == null) || (point.x == -1))

{

g.setColor (Color.blue);

g.fillRect (0, 0, mySize.width, mySize.height);

return;

}

Rectangle grabRect = computeGrabRect( );

BufferedImage grabImg = robot.createScreenCapture (grabRect);

Image scaleImg =

grabImg.getScaledInstance (mySize.width, mySize.height,

Image.SCALE_FAST);

Create a Magnifying Glass Component #56

Chapter 8, Rendering

|

289

HACK

Looking at the code, the constructor takes the source component, a size for

the magnified component, and a zoom level. It assigns these to its instance

variables, and sets up a

MouseMotionListener to get updates regarding the

cursor position, which is initialized to a “don’t paint me” dummy value of

(-1, -1). It also builds the AWT Robot. Since this can fail on some OSes or

under certain security situations,

the robot == null case will have to be han-

dled gracefully in the painting routine.

The

paint( ) method begins by checking for the two conditions that indi-

cate the mangnified component can’t be painted: if there’s no

Robot or if the

cursor has never entered the source component. In either of these cases, the

magnified component simply fills itself with blue and returns.

On the other hand, if valid data is available, the magnifier needs to figure out

what portion of the source component to grab. For simplicity, you can com-

pute a rectangle whose upper left is the current cursor location. The width

and height of the rectangle are computed by multiplying the zoom factor by

the magnifying component’s height and width. So, for a zoom of

1.0, the

grab is exactly the size of the magnifier. For a zoom of

0.5, it grabs half the

width and half the height, and for

2.0, it grabs twice the width and height.

g.drawImage (scaleImg, 0, 0, null);

}

private Rectangle computeGrabRect( ) {

// width, height are size of this comp / zoom

int grabWidth = (int) ((double) mySize.width / zoom);

int grabHeight = (int) ((double) mySize.height / zoom);

// upper-left corner is current point

return new Rectangle (point.x, point.y, grabWidth, grabHeight);

}

public Dimension getPreferredSize( ) { return mySize; }

public Dimension getMinimumSize( ) { return mySize; }

public Dimension getMaximumSize( ) { return mySize; }

// MouseMotionListener implementations

public void mouseMoved (MouseEvent e) {

Point offsetPoint = comp.getLocationOnScreen( );

e.translatePoint (offsetPoint.x, offsetPoint.y);

point = e.getPoint( );

repaint( );

}

public void mouseDragged (MouseEvent e) {

mouseMoved (e);

}

}

Example 8-1. JComponent to provide a magnified view of another JComponent