The Game API - Beginning J2ME: From Novice to Professional - page 252

CHAPTER 14
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The Game API
M IDP is an attractive platform for games, the leading edge of consumer J2ME software. In
the next chapter, you'll read about the exciting new 3D graphics capabilities available, and in
Chapter 16 you will read about MIDP's support for multimedia. This chapter describes the
Game API, which simplifies writing 2D games.
Overview
The Game API builds on the Canvas and Graphics classes you read about in Chapter 13. The
entire API is composed of five classes in the javax.microedition.lcdui.game package. One
class, GameCanvas , provides methods for animation and key polling. The other four classes deal
with layers, which can be used to compose scenes from several different elements.
GameCanvas offers two main advantages over Canvas . First, your application has control
over exactly when the display is updated, instead of having to wait for the system software to
call paint() . Second, you can control what region of the screen is updated. GameCanvas gives
your application very specific control of display updates.
Driving Animation with GameCanvas
GameCanvas extends javax.microedition.lcdui.Canvas with methods for animation and key
state polling. GameCanvas is used differently from Canvas :
•To use Canvas , you subclass it and define the paint() method. Inside paint() , you use
Graphics to render graphics on the screen. When you change something and want to
update the screen, you call repaint() , and the system calls paint() again for you.
•To use GameCanvas , you subclass it. To draw on the screen, you use the Graphics returned
from getGraphics() . When you want updates to appear on the screen, call flushGraphics() ,
which does not return until the screen is updated. For more specific updates, use the
method flushGraphics(int x, int y, int width, int height) , which only updates a
region of the screen.
public void flushGraphics(int x, int y, int width, int height)
GameCanvas 's model of use makes it easy to use inside a game loop like this:
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