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使用GameCanvas制作星空效果

編輯：J2ME

MIDP2.0中提供了游戲開發專用的API,比如GameCanvas等類。他們位於Javax.microedition.lcdui.game包內。本文介紹GameCanvas的基本使用方法並實現一種滾動星空的效果。您可以參考Game Canvas Basic獲得更詳細的信息。

    GameCanvas是Canvas的子類，因此他同樣繼承了Canvas類的一些特性，比如showNotify()方法會在Canvas被顯示在屏幕的時候調用，而hideNotify()會在Canvas離開屏幕的時候被調用。我們可以把他們當作監聽器來使用，用於初始化和銷毀資源。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;
    }

在游戲開發中最重要的就是接受用戶觸發的事件然後重新繪制屏幕，通常我們使用getKeyStates()方法判斷哪個鍵被按下了，然後繪制屏幕，調用flushGraphics()。在GameCanvas中，系統事實上已經為我們實現了雙緩沖技術，因此每次我們繪制的時候就是在off-screen上繪制的。結束後通過flushGraphics把它復制到屏幕上去。下面是典型的接受事件、處理邏輯、繪制屏幕的代碼。
// 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();
}

下面開始實現我們滾動星空的效果，其實設計的思想非常簡單。我們啟動一個線程，使用copyArea()方法把屏幕的內容往下復制一個像素的距離。然後繪畫第一個空白的直線，隨機的在直線上繪畫點兒，這樣看起來就像星空一樣了。邏輯代碼如下：
// 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)
            {
            }
        }
    }

下面給出源代碼
/*
* 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.

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

// 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;
    }
}