使用MIDP(Mobile Information Device Profile)的开发人员经常会抱怨用些什么办法才可以在一个MIDlet上显示动画。MIDP 1.0 没有直接提供对动画的支持(MIDP 2.0支持)，但真要是自己去实现，其实也并非是一件很难的事。
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：
private Image[] loadFrames( String name, int frames )
throws IOException {
Image[] images = new Image[frames];
for( int i = 0; i < frames; ++i ){
images = Image.createImage( name + i +".png" );
}
return images;
}
你也可以传递一个Canvas对象（可选），和一个剪辑列表（clip list）。如果你指定了一个canvas和使用一个timer来自动更换到动画的下一帧，就如下面的例子代码中一样，canvas在动画向前滚动以后自动被重画(repaint)。不过这样的实现办法是可选的，你可以这样做，也可以让程序选择合适的时候重画canvas。
因为MIDP 1.0不支持透明的图片，AnimatedImage 类使用一个剪辑列表来模拟透明的效果，剪辑列表是图片被剪成的方块区域的系列。图片被画出来的时候分开几次，每次画一个剪辑列表里面的剪辑区域。剪辑列表在帧的基础上被定义好，所以你需要为图片的每一帧创建一个数组。数组的大小应该是4的倍数，因为每一个剪辑面积保持了四个数值：左坐标，顶坐标，宽度以及高度。坐标的原点是整个图片的左上角。需要注意的是使用了剪辑列表会使动画慢下来。如果图片更加复杂的话，你应该使用矢量图片。
AnimatedImage类扩展了java.util.TimerTask，允许你设定一个timer。这里有个例子说明如何使用timer做动画：
Timer timer = new Timer(); AnimatedImage ai = ..... // get the image timer.schedule( ai, 200, 200 );
每隔大约200毫秒，timer调用AnimatedImage.run()方法一次，这个方法使得动画翻滚到下一个帧。现在我们需要的是让MIDlet来试试显示动画！我们定义一个简单的Canvas类的子类，好让我们把动画“粘贴上去”。
import java.util.*;
import javax.microedition.lcdui.*;
//A canvas to which you can attach one or more
//animated images. When the canvas is painted,
//it cycles through the animated images and asks
//them to paint their current image.
public class AnimatedCanvas extends Canvas {
private Display display;
private Image offscreen;
private Vector images = new Vector();
public AnimatedCanvas(Display display) {
this.display = display;
// If the canvas is not double buffered by the
// system, do it ourselves...
if (!isDoubleBuffered()) {
offscreen = Image.createImage(getWidth(), getHeight());
}
}
// Add an animated image to the list.
public void add(AnimatedImage image) {
images.addElement(image);
}
// Paint the canvas by erasing the screen and then
// painting each animated image in turn. Double
// buffering is used to reduce flicker.
protected void paint(Graphics g) {
Graphics saved = g;
if (offscreen != null) {
g = offscreen.getGraphics();
}
g.setColor(255, 255, 255);
g.fillRect(0, 0, getWidth(), getHeight());
int n = images.size();
for (int i = 0; i < n; ++i) {
AnimatedImage img = (AnimatedImage) images.elementAt(i);
img.draw(g);
}
if (g != saved) {
saved.drawImage(offscreen, 0, 0, Graphics.LEFT | Graphics.TOP);
}
}
}
AnimatedCanvas 类的代码相当简单，由一个动画导入方法和一个paint方法。canvas画布每次被画，背景都会被擦除然后循环每个导入的AnimatedImage对象，直接画到自己身上来（自己扩展了canvas类）。
import java.io.*;
import java.util.*;
import javax.microedition.lcdui.*;
import javax.microedition.midlet.*;
//MIDlet that displays some simple animations.
//Displays a series of birds on the screen and
//animates them at different (random) rates.
public class AnimationTest extends MIDlet implements CommandListener {
private static final int BIRD_FRAMES = 7;
private static final int NUM_BIRDS = 5;
private Display display;
private Timer timer = new Timer();
private AnimatedImage[] birds;
private Random random = new Random();
public static final Command exitCommand = new Command("Exit", Command.EXIT,
1);
public AnimationTest() {
}
public void commandAction(Command c, Displayable d) {
if (c == exitCommand) {
exitMIDlet();
}
}
protected void destroyApp(boolean unconditional)
throws MIDletStateChangeException {
exitMIDlet();
}
public void exitMIDlet() {
timer.cancel(); // turn it off...
notifyDestroyed();
}
// Generate a non-negative random number...
private int genRandom(int upper) {
return (Math.abs(random.nextInt()) % upper);
}
public Display getDisplay() {
return display;
}
// Initialize things by creating the canvas and then
// creating a series of birds that are moved to
// random locations on the canvas and attached to
// a timer for scheduling.
protected void initMIDlet() {
try {
AnimatedCanvas c = new AnimatedCanvas(getDisplay());
Image[] images = loadFrames("/images/bird", BIRD_FRAMES);
int w = c.getWidth();
int h = c.getHeight();
birds = new AnimatedImage[NUM_BIRDS];
for (int i = 0; i < NUM_BIRDS; ++i) {
AnimatedImage b = new AnimatedImage(c, images);
birds = b;
b.move(genRandom(w), genRandom(h));
c.add(b);
timer.schedule(b, genRandom(1000), genRandom(400));
}
c.addCommand(exitCommand);
c.setCommandListener(this);
getDisplay().setCurrent(c);
} catch (IOException e) {
System.out.println("Could not load images");
exitMIDlet();
}
}
// Load the bird animation, which is stored as a
// series of PNG files in the MIDlet suite.
private Image[] loadFrames(String name, int frames) throws IOException {
Image[] images = new Image[frames];
for (int i = 0; i < frames; ++i) {
images = Image.createImage(name + i + ".png");
}
return images;
}
protected void pauseApp() {
}
protected void startApp() throws MIDletStateChangeException {
if (display == null) {
display = Display.getDisplay(this);
initMIDlet();
}
}
}
七帧图片的动画，你可以看到一个拍着翅膀的小鸟。MIDlet显示了5只小鸟，小鸟的位置和刷新速度是随机的。你可以用一些其他的办法来改进这个程序，但这个程序也应该足够能让你上手了。