Swing Hacks

Create Menus with Drop Shadows #11

Chapter 1, Basic JComponents

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HACK

One nice thing about assembling the drop-down from standard compo-

nents is that it will still look good when used with a different Look and Feel.

Everything the user sees on the screen is some subclass of the standard

JButton, but it is just presented in a non-traditional manner. If you switch to

another theme where standard buttons are shaped differently, the custom

component adapts automatically.

H A C K

#11

Create Menus with Drop Shadows Hack #11

This hack explores a simple way to create drop shadows on menus

throughout an entire application with minimal code changes.

Many modern operating systems provide menus with interesting effects to

make them jump off the screen. One of the most common is the drop

shadow. Some programs even provide shadows themselves when the host

operating system does not. For years, a lack of low-level graphics support

has denied Swing programs access to these kinds of cool effects. But not any

more! Most of the effects can be duplicated with Swing’s robust theming

ability.

Most custom effects require either subclassing a component or messing with

graphics overlays. I tried a variety of techniques to create this hack, but I

kept coming across the same problem over and over. If I wanted to draw a

shadow, I had to change the sizing of each menu item, plus its background,

plus the pop-up frame itself. That is a lot of components to manage. It

would be a lot simpler if I could tell the components to make themselves a

little bit bigger and give me the extra slice of screen real estate to draw in.

The solution was right under my nose: the border. Every Swing component

can use a custom border, without subclassing, and the border will automati-

cally resize the component to fit. If the border is lopsided, then it will create

a kind of shadow effect. Perfect!

Figure 1-28. A drop-down color chooser in a test JFrame

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Chapter 1, Basic JComponents

#11 Create Menus with Drop Shadows

HACK

Every standard Swing component is actually drawn by a UI helper class, and

pop-up menus are no exception. I took the

BasicPopupMenuUI in the javax.

swing.plaf.basic

package and created a subclass called

CustomPopupMenuUI

(shown in Example 1-25). It only does two things special: adds a custom

border to the pop up’s parent panel and sets the panel to be transparent.

The

getPopup( ) method generates the actual menu object in the superclass.

This version grabs the pop up’s parent—a

JPanel—and adds the custom

ShadowBorder with offsets of 3 pixels, both horizontally and vertically. It also

calls

setOpaque(false). This tells the component that it does not have to

completely fill its boundaries and that the parent component may show

through. For most components, this turns off its background, which is the

desired effect here.

With the custom pop-up class done, it’s time to create the border.

ShadowBorder, shown in Example 1-26, is a subclass of AbstractBorder,

which handles all of the messy details of border management and lets the

developer get down to the actual drawing code pretty quickly.

Example 1-25. Extending the pop-up menu’s UI

public class CustomPopupMenuUI extends BasicPopupMenuUI {

public static ComponentUI createUI(JComponent c) {

return new CustomPopupMenuUI( );

}

public Popup getPopup(JPopupMenu popup, int x, int y) {

Popup pp = super.getPopup(popup,x,y);

JPanel panel = (JPanel)popup.getParent( );

panel.setBorder(new ShadowBorder(3,3));

panel.setOpaque(false);

return pp;

}

}

Example 1-26. Adding a shadowed border

class ShadowBorder extends AbstractBorder {

int xoff, yoff;

Insets insets;

public ShadowBorder(int x, int y) {

this.xoff = x;

this.yoff = y;

insets = new Insets(0,0,xoff,yoff);

}

public Insets getBorderInsets( Component c ) {

return insets;

}