Swing Hacks

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Chapter 1, Basic JComponents

#6 Watermark Your Scroll Panes

HACK

H A C K

#6

Watermark Your Scroll Panes

Hack #6

This hack creates a text area with a tiled background image that is fixed,

even when the text area scrolls, and also a fixed foreground image that

appears above the text, much like the station badges now affixed to the

lower-righthand corner of most TV broadcasts.

The Swing framework was designed to let developers override portions of

every component, both the visual appearance (the view) and the behavior

(the model and controller). This design gives developers great flexibility.

One of my favorites is the

JScrollPane. Its nested composite design allows

developers to create some stunning effects.

Once again, the idea is to override the drawing code of a standard compo-

nent to create the visual effects

[Hack #5]. The difference here is that you must

deal with a composite object, the

JScrollPane. A JScrollPane is not a single

Swing component—it’s actually a wrapper around two scrollbars and the

component that does the real scrolling is a

JViewport. This viewport is the

actual target component; you will subclass it to draw both above and below

the

View component (as seen in Example 1-12). The View is the Swing wid-

get being scrolled; in this case, it is a

JTextArea.

The

ScrollPaneWatermark class inherits from JViewport, adding two meth-

ods:

setBackgroundTexture( ) and setForegroundBadge( ). Each takes a URL

instead of a File to allow for images loaded from places other than the local

disk, such as a web server or JAR file.

setBackgroundTexture( ) does the same thing that the WatermarkTextField

did in the previous hack. It loads the image, creates a same-size rectangle,

then initializes a

TexturePaint for later use. setForegroundBadge( ) is even

simpler, only loading the image and storing it in the

fgimage variable.

Example 1-12. Modifying the viewport for watermarking

public class ScrollPaneWatermark extends JViewport {

BufferedImage fgimage, bgimage;

TexturePaint texture;

public void setBackgroundTexture(URL url) throws IOException {

bgimage = ImageIO.read(url);

Rectangle rect = new Rectangle(0,0,

bgimage.getWidth(null),bgimage.getHeight(null));

texture = new TexturePaint(bgimage, rect);

}

public void setForegroundBadge(URL url) throws IOException {

fgimage = ImageIO.read(url);

}

Watermark Your Scroll Panes #6

Chapter 1, Basic JComponents

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27

HACK

With the class set up, it’s time to draw. The code below calls super.

paintComponent( )

first, and then draws the texture on top of the component.

This is because the existing background might need to show through in case

the texture has translucent sections. This would be especially important if the

standard view background isn’t just a solid color. Under Mac OS X, for

example, the background is often a striped, light blue pattern. Here’s the

code to handle texturing:

public void paintComponent(Graphics g) {

// do the superclass behavior first

super.paintComponent(g);

// paint the texture

if(texture != null) {

Graphics2D g2 = (Graphics2D)g;

g2.setPaint(texture);

g.fillRect(0,0,getWidth(),getHeight( ));

}

}

ScrollPaneWatermark draws the foreground image badge by overriding the

paintChildren( ) method, calling the superclass, and then drawing the

image. This ensures that the badge is always on top of the children or view:

public void paintChildren(Graphics g) {

super.paintChildren(g);

if(fgimage != null) {

g.drawImage(fgimage,

getWidth( )-fgimage.getWidth(null), 0,

null);

}

}

The view (a text area in this example) will usually draw its own back-

ground. Because, by definition, the view is as big as the viewport (if not big-

ger), its background will cover up the viewport’s nice texture completely. To

stop that, you need to call

setOpaque( ) on the view:

public void setView(JComponent view) {

view.setOpaque(false);

super.setView(view);

}

The setView( ) method overrides the existing version (from JViewport) to

call

setOpaque(false) on the view before calling the super( ) method. By

putting this call here, instead of calling

setOpaque( ) from the normal setup

routines, it frees the developer using the

ScrollPaneWatermark class from

having to call

setOpaque( ) manually, making the class more reusable.