使用MIDP(Mobile Information Device PRofile)的開發人員經常會抱怨用些什么辦法才可以在一個MIDlet上顯示動畫。 MIDP 1.0 沒有直接提供對動畫的支持(正在開發中的MIDP 2.0支持)，但真要是自己去實現，其實也并非是一件很難的事。 任何動畫的最基本的前提，是要在足夠快的時間內顯示和更換一張張的圖片，讓人的眼睛看到動的畫面效果。圖片必須按照順序畫出來。從一張圖片到下一張圖片之間的變化越小，效果會越好。 首先要做的，是使用你的圖片處理軟件（比如ps或者firework）創建一系列相同大小的圖片來組成動畫。每張圖片代表動畫一幀。需要制作一定數量的禎－－越多的幀會讓你的動畫看上去越平滑。制作好的圖片一定要保存成PNG(Portable Network Graphics)格式，MIDP唯一支持的圖片格式；有兩個辦法讓你剛做好的圖片在MIDlet上變成動畫。第一，把圖片都放到一個web服務器上，讓MIDlet下載他們，MIDP內置的HTTP支持。第二個辦法更簡單，把圖片用MIDlet打包成jar文件。假如你使用的是J2ME開發工具，把PNG文件放入你的項目文件里面就可以了。 動畫的過程其實更像帳本記錄：顯示當前幀，然后適當地更換到下一幀。那么使用一個類來完成這個工作應該是很恰當的，那好，我們就先定義一個AnimatedImage類： import java.util.*; import javax.microedition.lcdui.*; // 定義了一個動畫，該動畫其實只是一系列相同大小的圖片 // 輪流顯示，然后模擬出的動畫 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 ); }; }; 你實例化一個AnimatedImage對象的時候你必須給AnimatedImage類的構造方法傳一個Image對象數組，該數組代表動畫的每一幀。 使用的所有圖片必須具有相同的高度和寬度。用Image.createImage()方法從jar文件里面加載圖片： 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; }; 你也可以傳遞一個Canvas對象（可選），和一個剪輯列表（clip list）。假如你指定了一個canvas和使用一個timer來自動更換到動畫的下一幀，就如下面的例子代碼中一樣，canvas在動畫向前滾動以后自動被重畫(repaint)。不過這樣的實現辦法是可選的，你可以這樣做，也可以讓程序選擇合適的時候重畫canvas。 因為MIDP 1.0不支持透明的圖片，AnimatedImage 類使用一個剪輯列表來模擬透明的效果，剪輯列表是圖片被剪成的方塊區域的系列。圖片被畫出來的時候分開幾次，每次畫一個剪輯列表里面的剪輯區域。剪輯列表在幀的基礎上被定義好，所以你需要為圖片的每一幀創建一個數組。數組的大小應該是4的倍數，因為每一個剪輯面積保持了四個數值：左坐標，頂坐標，寬度以及高度。坐標的原點是整個圖片的左上角。需要注重的是使用了剪輯列表會使動畫慢下來。假如圖片更加復雜的話，你應該使用矢量圖片。 AnimatedImage類擴展了java.util.TimerTask，答應你設定一個timer。這里有個例子說明如何使用timer做動畫： Timer timer = new Timer(); AnimatedImage ai = ..... // get the image timer.schedule( ai, 200, 200 ); 每隔大約200毫秒，timer調用AnimatedImage.run()方法一次，這個方法使得動畫翻滾到下一個幀。 現在我們需要的是讓MIDlet來試試顯示動畫！我們定義一個簡單的Canvas類的子類，好讓我們把動畫“粘貼上去”。 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 類的代碼相當簡單，由一個動畫導入方法和一個paint方法。canvas畫布每次被畫，背景都會被擦除，然后循環每個導入的AnimatedImage對象，直接畫到自己身上來（自己擴展了canvas類）。 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(); }; }; }; 七幀圖片的動畫，你可以看到一個拍著翅膀的小鳥。MIDlet顯示了5只小鳥，小鳥的位置和刷新速度是隨機的。你可以用一些其他的辦法來改進這個程序，但這個程序也應該足夠能讓你上手了。
