Swing Hacks

Animating a Sheet Dialog #45

Chapter 6, Transparent and Animated Windows

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237

HACK

startAnimation( ) begins by refreshing the glass pane with a repaint( ). It

then informs the

AnimatingSheet of its new source (i.e., the sheet) and adds

the

AnimatingSheet

to the layout. It then sets up a

javax.swing.Timer

to get

callbacks to an

actionPerformed( )

method that performs each step of the

animation. The

ANIMATION_SLEEP

parameter affects how smooth the anima-

tion will look—shorter sleeps will result in a higher frame rate, but higher

CPU use.

I’ve set this value as low as 2 ms and haven’t had a problem,

but I’m on a pretty fast box (a dual 1.8 GHz G5 Power Mac).

You might want to play with this value to get an ideal

performance-to-smoothness ratio.

stopAnimation( ) just stops the Timer, as you might expect.

The

actionPerformed( ) method is used by the Timer callbacks. It has two

tasks: calculating the height of the sheet to show, and wrapping up the ani-

mation if it has run its course. Notice that the progress calculation doesn’t

assume that it has been called back in accordance with the

ANIMATION_SLEEP.

It calculates the current time offset from when the animation began, and cal-

culates a progress from that, which in turn allows it to figure out how much

of the sheet to show. This is a really good practice because if your computer

can’t keep up with your desired rate, the

Timer will combine multiple call-

backs into one. By just using the current time (instead of a possibly incor-

rect assumption about frame rate), you’ll keep up with the specified rate and

duration. On screen, combined callbacks will look like dropped frames, but

a choppier animation is preferable to one that takes longer than it should.

If

actionPerformed( ) decides that the animation has finished, it calls

stopAnimation( ) and then cleans up the glass pane by either removing its

contents (if the direction is

OUTGOING) or by calling finishShowingSheet( ) (if

direction is

INCOMING). The finishShowingSheet( ) method clears the glass

pane and puts the real sheet in place, just like

showJDialogAsSheet( ) did in

the last lab.

Self-Painting

So, the frame is responsible for adding, removing, and animating the

AnimatedSheet, and for adding the real sheet when the incoming animation

completes. What that leaves for the

AnimatedSheet is the ability to paint itself

(from a region of the original dialog) and report an accurate size for the ben-

efit of the glass pane’s

LayoutManager.

238

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Chapter 6, Transparent and Animated Windows

#45 Animating a Sheet Dialog

HACK

Example 6-10 shows the code for the AnimatingSheet inner class.

Example 6-10. Inner class to paint sheet during animation

class AnimatingSheet extends JPanel {

Dimension animatingSize = new Dimension (0, 1);

JComponent source;

BufferedImage offscreenImage;

public AnimatingSheet ( ) {

super( );

setOpaque(true);

}

public void setSource (JComponent source) {

this.source = source;

animatingSize.width = source.getWidth( );

makeOffscreenImage(source);

}

public void setAnimatingHeight (int height) {

animatingSize.height = height;

setSize (animatingSize);

}

private void makeOffscreenImage(JComponent source) {

GraphicsConfiguration gfxConfig =

GraphicsEnvironment.getLocalGraphicsEnvironment( )

.getDefaultScreenDevice( )

.getDefaultConfiguration( );

offscreenImage =

gfxConfig.createCompatibleImage(source.getWidth( ),

source.getHeight( ));

Graphics2D offscreenGraphics =

(Graphics2D) offscreenImage.getGraphics( );

source.paint (offscreenGraphics);

}

public Dimension getPreferredSize( ) { return animatingSize; }

public Dimension getMinimumSize( ) { return animatingSize; }

public Dimension getMaximumSize( ) { return animatingSize; }

public void paint (Graphics g) {

// get the bottom-most n pixels of source and

// paint them into g, where n is height

BufferedImage fragment =

offscreenImage.getSubimage (0,

offscreenImage.getHeight( ) -

animatingSize.height,

source.getWidth( ),

animatingSize.height);

// g.drawImage (fragment, 0, 0, this);

g.drawImage (fragment, 0, 0, this);

}

}