利用J2ME在移动设备上实现动画的解决方案

7/25/2004来源:移动短信技术人气:22738

  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。