基于MIDP1.0实现屏幕滚动

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。

基于MIDP1.0实现屏幕滚动

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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