使用MIDP2.0开发游戏 (1) GameCanvas基础

MIDP2.0提供了对游戏的强有力支持，通过javax.microedition.lcdui.game包，原来在MIDP1.0中很多需要自
己写的功能现在都被当作标准API实现了，包括GameCanvas，Sprite，Layer等等。

我们将使用MIDP2.0编写一个坦
克大战的手机游戏，我也是初学J2ME不久，准备边看书边做，争取把这个游戏做出来！J2ME高手请多指点，和我一样学习中的朋友欢迎多多交流！

我
们的开发环境为Windows XP SP1 + J2DK1.4 + J2ME
WTK2.1 + Eclipse 3.0 + EclipseMe

// MyGameCanvas.java
// 编写Canvas类
import javax.microedition.lcdui.*;
import javax.microedition.lcdui.game.*;

public class MyGameCanvas extends GameCanvas implements Runnable {
    private boolean isPlay; // Game Loop runs when isPlay is true
    private long delay; // To give thread consistency
    private int currentX, currentY; // To hold current position of the 'X'
    private int width; // To hold screen width
    private int height; // To hold screen height

    // Constructor and initialization
    public MyGameCanvas() {
        super(true);
        width = getWidth();
        height = getHeight();
        currentX = width / 2;
        currentY = height / 2;
        delay = 20;
    }

    // Automatically start thread for game loop
    public void start() {
        isPlay = true;
        new Thread(this).start();
    }

    public void stop() {
        isPlay = false;
    }

    // Main Game Loop
    public void run() {
        Graphics g = getGraphics();
        while (isPlay) {
            input();
            drawScreen(g);
            try {
                Thread.sleep(delay);
            } catch (InterruptedException ie) {}
        }
    }

    // Method to Handle User Inputs
    private void input() {
        int keyStates = getKeyStates();
        // Left
        if ((keyStates & LEFT_PRESSED) != 0) {
            currentX = Math.max(0, currentX - 1);
        }
        // Right
        if ((keyStates & RIGHT_PRESSED) != 0) {
            if (currentX + 5 < width) {
                currentX = Math.min(width, currentX + 1);
            }
        }
        // Up
        if ((keyStates & UP_PRESSED) != 0) {
            currentY = Math.max(0, currentY - 1);
        }
        // Down
        if ((keyStates & DOWN_PRESSED) != 0) {
            if (currentY + 10 < height) {
                currentY = Math.min(height, currentY + 1);
            }
        }
    }

    // Method to Display Graphics
    private void drawScreen(Graphics g) {
        g.setColor(0xffffff);
        g.fillRect(0, 0, getWidth(), getHeight());
        g.setColor(0x0000ff);
        g.drawString("X", currentX, currentY, Graphics.TOP | Graphics.LEFT);
        flushGraphics();
    }
}


// GameMIDlet.java
// 编写MIDlet
import javax.microedition.midlet.*;
import javax.microedition.lcdui.*;

public class GameMIDlet extends MIDlet {
    private Display display;
    public void startApp() {
        display = Display.getDisplay(this);
        MyGameCanvas gameCanvas = new MyGameCanvas();
        gameCanvas.start();
        display.setCurrent(gameCanvas);
    }

    public Display getDisplay() {
        return display;
    }

    public void pauseApp() {
    }

    public void destroyApp(boolean unconditional) {
        exit();
    }

    public void exit() {
        System.gc();
        destroyApp(false);
        notifyDestroyed();
    }
}

下
面是一个最简单的GameCanvas的例子，出自《J2ME  & Gaming》：

编
译后就可以在模拟器中运行了，一个X在屏幕中心，可以用上下左右键移动它。