本例介绍了一个常用的编程技巧，就是如何基于MIDP1.0实现屏幕的滚动效果。在MIDP2.0可以借助LayerManager的的“可视窗口”来实现。MIDP1.0通过变化坐标系来实现屏幕的滚动效果，只要借助Graphics.translate()方法。
package com.j2medev.translate;
import javax.microedition.midlet.*;
import javax.microedition.lcdui.*;
public class Translate extends MIDlet
{
PRivate Display display; // The display
private TranslateCanvas canvas; // Canvas
public Translate()
{
display = Display.getDisplay(this);
canvas = new TranslateCanvas(this);
}
protected void startApp()
{
display.setCurrent( canvas );
}
protected void pauseApp()
{ }
protected void destroyApp( boolean unconditional )
{ }
public void exitMIDlet()
{
destroyApp(true);
notifyDestroyed();
}
}
/*
* TranslateCanvas.java
*
* Created on 2005年12月1日, 下午10:15
*
* To change this template, choose Tools Options and locate the template under
* the Source Creation and Management node. Right-click the template and choose
* Open. You can then make changes to the template in the Source Editor.
*/
package com.j2medev.translate;
import javax.microedition.lcdui.Canvas;
import javax.microedition.lcdui.Command;
import javax.microedition.lcdui.CommandListener;
import javax.microedition.lcdui.Displayable;
import javax.microedition.lcdui.Graphics;
import javax.microedition.lcdui.Image;
class TranslateCanvas extends Canvas implements CommandListener {
private Command cmExit;
private Translate midlet;
private Image im = null;
private int translatex = 0, translatey = 0;
public TranslateCanvas(Translate midlet) {
this.midlet = midlet;
cmExit = new Command("Exit", Command.EXIT, 1);
addCommand(cmExit);
setCommandListener(this);
try {
//创建图片
im = Image.createImage("/img.png");
} catch (Java.io.IOException e) {
System.err.println("Unable to locate or read .png file");
}
}
protected void paint(Graphics g) {
if (im != null) {
//清屏
g.setColor(255, 255, 255);
g.fillRect(0, 0, getWidth(), getHeight());
//变换坐标，请参考Graphics.translate(x,y)方法说明
g.translate(translatex, translatey);
//总是在原点处画图片，由于前面变换了坐标 看起来同样图片会动起来
g.drawImage(im, 0, 0, Graphics.LEFT Graphics.TOP);
}
}
public void commandAction(Command c, Displayable d) {
if (c == cmExit)
midlet.exitMIDlet();
}
protected void keyPressed(int keyCode) {
switch (getGameAction(keyCode)) {
case UP:
break;
case DOWN:
break;
case LEFT:
// 图片右边缘已经到达屏幕最右边
System.out.println(translatex);
System.out.println(im.getWidth());
if (translatex + im.getWidth() < getWidth())
return;//直接返回
else
translatex -= 2;
break;
case RIGHT:
// 滚动到图片的左边缘
if (translatex>getWidth()-2)
return;
else
translatex += 2;
break;
}
repaint();
}
}
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