J2ME在移动设备上实现动画程序方法

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();

};

};

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

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// 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(); http://www.mscto.com

};

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; 软件开发网

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 );

};