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用J2ME在移动设备上实现动画
摘要: 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(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。