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 ...