用J2ME在移动设备上实现动画

　任何动画的最基本的前提，是要在足够快的时间内显示和更换一张张的图片，让人的眼睛看到动的画面效果。图片必须按照顺序画出来。从一张图片到下一张图片之间的变化越小，效果会越好。

　首先要做的，是使用你的图片处理软件（比如ps或者firework）创建一系列相同大小的图片来组成动画。每张图片代表动画一帧。

　你需要制作一定数量的祯－－越多的帧会让你的动画看上去越平滑。制作好的图片一定要保存成PNG(Portable Network Graphics) 格式，MIDP唯一支持的图片格式；（有两个办法让你刚做好的图片在MIDlet上变成动画。第一，把图片都放到一个web服务器上，让MIDlet下载他们，MIDP内置的HTTP支持。第二个办法更简单，把图片用MIDlet打包成jar文件。如果你使用的是J2ME开发工具，把PNG文件放在你的项目文件里面就可以了。

　动画的过程其实更像帐本记录：显示当前帧，然后适当地更换到下一帧。那么使用一个类来完成这个工作应该是很恰当的，那好，我们就先定义一个AnimatedImage类：
import java.util.*; import javax.microedition.lcdui.*; // 定义了一个动画，该动画其实只是一系列相同大小的图片 // 轮流显示，然后模拟出的动画 public class AnimatedImage extends TimerTask {; private Canvas canvas; private Image[] images; private int[][] clipList; private int current; private int x; private int y; private int w; private int h; // Construct an animation with no canvas. public AnimatedImage( Image[] images ){; this( null, images, null ); }; // Construct an animation with a null clip list. public AnimatedImage( Canvas canvas, Image[] images ){; this( canvas, images, null ); }; // Construct an animation. The canvas can be null, // but if not null then a repaint will be triggered // on it each time the image changes due to a timer // event. If a clip list is specified, the image is // drawn multiple times, each time with a different // clip rectangle, to simulate transparent parts. public AnimatedImage( Canvas canvas, Image[] images, int[][] clipList ){; this.canvas = canvas; this.images = images; this.clipList = clipList; if( images != null && clipList != null ){; if( clipList.length < images.length ){; throw new IllegalArgumentException(); }; }; if( images != null && images.length > 0 ){; w = images[0].getWidth(); h = images[0].getHeight(); }; }; // Move to the next frame, wrapping if necessary. public void advance( boolean repaint ){; if( ++current >= images.length ){; current = 0; }; if( repaint && canvas != null && canvas.isShown() ){; canvas.repaint( x, y, w, h ); canvas.serviceRepaints(); }; }; // Draw the current image in the animation. If // no clip list, just a simple copy, otherwise // set the clipping rectangle accordingly and // draw the image multiple times. public void draw( Graphics g ){; if( w == 0 || h == 0 ) return; int which = current; if( clipList == null || clipList[which] == null ){; g.drawImage( images[which], x, y, g.TOP | g.LEFT ); }; else {; int cx = g.getClipX(); int cy = g.getClipY(); int cw = g.getClipWidth(); int ch = g.getClipHeight(); int[] list = clipList[which]; for( int i = 0; i + 3 <= list.length; i += 4 ){; g.setClip( x + list[0], y + list[1], list[2], list[3] ); g.drawImage( images[which], x, y, g.TOP | g.LEFT ); }; g.setClip( cx, cy, cw, ch ); }; }; // Moves the animation's top left corner. public void move( int x, int y ){; this.x = x; this.y = y; }; // Invoked by the timer. Advances to the next frame // and causes a repaint if a canvas is specified. public void run(){; if( w == 0 || h == 0 ) return; advance( true ); }; };
　你实例化一个AnimatedImage对象的时候你必须给AnimatedImage类的构造方法传一个Image对象数组，该数组代表动画的每一帧。使用的所有图片必须具有相同的高度和宽度。用Image.createImage()方法从jar文件里面加载图片：

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