Swing Hacks

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Chapter 8, Rendering

#56 Create a Magnifying Glass Component

HACK

With a grab rectangle calculated, call Robot.createScreenCapture( ) to grab

those pixels and return them as a

BufferedImage. Unless your zoom factor is

1.0

, this image will be larger or smaller than the

DetachedMagnifyingGlass

component, so you need to use

getScaledInstance( )

to scale it to your com-

ponent size. In terms of a scaling behavior, the code uses the

SCALE_FAST

constant because the moving mouse will be calling for many repaints, and

thus many grabs and scales, every second. Finally, with your properly sized

image in memory, you paint to the

Graphics to get the grabbed data into

your component.

One TODO item I haven’t shown here, but that might pro-

vide a performance boost, would be to

flush( ) the various

temporary images after you’ve called

drawImage( ).

To ensure the AWT LayoutManagers respect the size that’s set for the compo-

nent, have

getPreferredSize( ), getMinimumSize( ), and getMaximumSize( ) all

return the size that was originally sent to the constructor, since the size of

the magnifier component is critical in computing what to grab from the

source component.

The last bit of code in this class is a

MouseMotionListner, which is used to

track the mouse’s location. You need to do this so you’ll always have an up-

to-date point when the

paint( ) method is called. You’ll only get events

when the cursor is over the source component, and it’s easy enough to cache

the point in the

MouseEvent, but there’s a catch: that point is relevant to the

coordinate system of the source component, not the screen, so unless the

component is at the upper-left corner of the screen, you will be grabbing the

wrong pixels in

paint( ). The fix is to translate the point from the source

component’s coordinate system to the screen’s coordinate system. Do this

by getting the component’s onscreen location with

getLocationOnScreen( ),

and then translate the

Point with MouseEvent.translate( ).

Testing the Magnifier Out

To take DetachedMagnifyingGlass out for a spin, Example 8-2 shows a

TestDetachedMagnifyingGlass class, which opens an image in a JFrame and

sets a

DetachedMagnifyingGlass next to it.

Example 8-2. Testing the MagnifyingGlassComponent

public class TestDetachedMagnifyingGlass extends Object {

public TestDetachedMagnifyingGlass(File f) {

// image frame

ImageIcon i = new ImageIcon (f.getPath( ));

JLabel l = new JLabel (i);

Create a Magnifying Glass Component #56

Chapter 8, Rendering

|

291

HACK

This class simply brings up a JFileChooser to pick an image file, which it

loads into an

ImageIcon and then into a JLabel. It then creates a

DetachedMagnifyingGlass from the JLabel, with a size of 150 × 150 and a

magnification factor of 2.0, and moves it to the immediate right of the

source component.

Figure 8-1 shows what the test looks like when run.

JFrame imgFrame = new JFrame ("Image");

imgFrame.getContentPane( ).add(l);

imgFrame.pack( );

imgFrame.setVisible(true);

// magnifying glass frame

JFrame magFrame = new JFrame ("Mag");

DetachedMagnifyingGlass mag =

new DetachedMagnifyingGlass (l, new Dimension (150, 150), 2.0);

magFrame.getContentPane( ).add (mag);

magFrame.pack( );

magFrame.setLocation (new Point (

imgFrame.getLocation().x + imgFrame.getWidth( ),

imgFrame.getLocation( ).y));

magFrame.setVisible(true);

}

public static void main (String[] args) {

JFileChooser chooser = new JFileChooser( );

chooser.showOpenDialog(null);

File f = chooser.getSelectedFile( );

new TestDetachedMagnifyingGlass (f);

}

}

Figure 8-1. Use of the DetachedMagnifyingGlass

Example 8-2. Testing the MagnifyingGlassComponent (continued)