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 ...