用J2ME在移动设备上实现动画

　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(正在开发中的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：

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。