使用GameCanvas制作星空效果

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    GameCanvas是Canvas的子类，因此他同样继承了Canvas类的一些特性，比如showNotify()方法会在Canvas被显示在屏幕 的时候调用，而hideNotify()会在Canvas离开屏幕的时候被调用。我们可以把他们当作{8}来使用，用于初始化和销毁资源。比如
    // When the canvas is shown, start a thread to
    // run the game loop.

    protected void showNotify()
    {
        random = new Random();
        thread = new Thread(this);
        thread.start();
    }
    // When the game canvas is hidden, stop the thread.

    protected void hideNotify()
    {
        thread = null;
    }

在游戏开发中最重要的就是接受用户触发的事件然后重新绘制屏幕，通常我们使用getKeyStates()方法判断哪个键被按下了，然后绘制屏幕， 调用flushGraphics()。在GameCanvas中，系统事实上已经为我们实现了双缓冲技术，因此每次我们绘制的时候就是在off- screen上绘制的。结束后通过flushGraphics把它复制到屏幕上去。下面是典型的接受事件、处理逻辑、绘制屏幕的代码。
 // Get the Graphics object for the off-screen buffer
Graphics g = getGraphics();

while (true) {
      // Check user input and update positions if necessary
      int keyState = getKeyStates();
      if ((keyState & LEFT_PRESSED) != 0) {
          sprite.move(-1, 0);
      }
      else if ((keyState & RIGHT_PRESSED) != 0) {
          sprite.move(1, 0);
      }

// Clear the background to white
g.setColor(0xFFFFFF);
g.fillRect(0,0,getWidth(), getHeight());

      // Draw the Sprite
      sprite.paint(g);

      // Flush the off-screen buffer
      flushGraphics();
}

    下面开始实现我们滚动星空的效果，其实设计的思想非常简单。我们启动一个线程，使用copyArea()方法把屏幕的内容往下复制一个像素的距离。然后绘画第一个空白的直线，随机的在直线上绘画点儿，这样看起来就像星空一样了。逻辑代码如下：
    // The game loop.

    public void run()
    {
        int w = getWidth();
        int h = getHeight() - 1;
        while (thread == Thread.currentThread())
        {
            // Increment or decrement the scrolling interval
            // based on key presses
            int state = getKeyStates();

            if ((state & DOWN_PRESSED) != 0)
            {
                sleepTime += SLEEP_INCREMENT;
                if (sleepTime > SLEEP_MAX)
                    sleepTime = SLEEP_MAX;
            } else if ((state & UP_PRESSED) != 0)
            {
                sleepTime -= SLEEP_INCREMENT;
                if (sleepTime < 0)
                    sleepTime = 0;
            }

            // Repaint the screen by first scrolling the
            // existing starfield down one and painting in
            // new stars...

            graphics.copyArea(0, 0, w, h, 0, 1, Graphics.TOP | Graphics.LEFT);
            graphics.setColor(0, 0, 0);
            graphics.drawLine(0, 0, w, 0);
            graphics.setColor(255, 255, 255);
            for (int i = 0; i < w; ++i)
            {
                int test = Math.abs(random.nextInt()) % 100;
                if (test < 5)
                {
                    graphics.drawLine(i, 0, i, 0);
                }
            }
            flushGraphics();

            // Now wait...

            try
            {
                Thread.currentThread().sleep(sleepTime);
            } catch (InterruptedException e)
            {
            }
        }
    }

下面给出源代码
/*
 * License
 *
 * Copyright 1994-2004 Sun Microsystems, Inc. All Rights Reserved.
 *
 */

import javax.microedition.lcdui.*;
import javax.microedition.lcdui.game.*;
import javax.microedition.midlet.*;

public class GameCanvasTest extends MIDlet implements CommandListener
{

    private Display display;

    public static final Command exitCommand = new Command("Exit", Command.EXIT,
            1);

    public GameCanvasTest()
    {
    }

    public void commandAction(Command c, Displayable d)
    {
        if (c == exitCommand)
        {
            exitMIDlet();
        }
    }

    protected void destroyApp(boolean unconditional)
            throws MIDletStateChangeException
    {
        exitMIDlet();
    }

    public void exitMIDlet()
    {
        notifyDestroyed();
    }

    public Display getDisplay()
    {
        return display;
    }

    protected void initMIDlet()
    {
        GameCanvas c = new StarField();
        c.addCommand(exitCommand);
        c.setCommandListener(this);

        getDisplay().setCurrent(c);
    }

    protected void pauseApp()
    {
    }

    protected void startApp() throws MIDletStateChangeException
    {
        if (display == null)
        {
            display = Display.getDisplay(this);
            initMIDlet();
        }
    }
}
/*
 * License
 *
 * Copyright 1994-2004 Sun Microsystems, Inc. All Rights Reserved.
 */

import java.util.Random;
import javax.microedition.lcdui.*;
import javax.microedition.lcdui.game.GameCanvas;

// A simple example of a game canvas that displays
// a scrolling star field. Use the UP and DOWN keys
// to speed up or slow down the rate of scrolling.

public class StarField extends GameCanvas implements Runnable
{

    private static final int SLEEP_INCREMENT = 10;

    private static final int SLEEP_INITIAL = 150;

    private static final int SLEEP_MAX = 300;

    private Graphics graphics;

    private Random random;

    private int sleepTime = SLEEP_INITIAL;

    private volatile Thread thread;

    public StarField()
    {
        super(true);

        graphics = getGraphics();
        graphics.setColor(0, 0, 0);
        graphics.fillRect(0, 0, getWidth(), getHeight());
    }

    // The game loop.

    public void run()
    {
        int w = getWidth();
        int h = getHeight() - 1;
        while (thread == Thread.currentThread())
        {
            // Increment or decrement the scrolling interval
            // based on key presses
            int state = getKeyStates();

            if ((state & DOWN_PRESSED) != 0)
            {
                sleepTime += SLEEP_INCREMENT;
                if (sleepTime > SLEEP_MAX)
                    sleepTime = SLEEP_MAX;
            } else if ((state & UP_PRESSED) != 0)
            {
                sleepTime -= SLEEP_INCREMENT;
                if (sleepTime < 0)
                    sleepTime = 0;
            }

            // Repaint the screen by first scrolling the
            // existing starfield down one and painting in
            // new stars...

            graphics.copyArea(0, 0, w, h, 0, 1, Graphics.TOP | Graphics.LEFT);
            graphics.setColor(0, 0, 0);
            graphics.drawLine(0, 0, w, 0);
            graphics.setColor(255, 255, 255);
            for (int i = 0; i < w; ++i)
            {
                int test = Math.abs(random.nextInt()) % 100;
                if (test < 5)
                {
                    graphics.drawLine(i, 0, i, 0);
                }
            }
            flushGraphics();

            // Now wait...

            try
            {
                Thread.sleep(sleepTime);
            } catch (InterruptedException e)
            {
            }
        }
    }

    // When the canvas is shown, start a thread to
    // run the game loop.

    protected void showNotify()
    {
        random = new Random();
        thread = new Thread(this);
        thread.start();
    }
    // When the game canvas is hidden, stop the thread.

    protected void hideNotify()
    {
        thread = null;
    }
}