使用MIDP2.0开发游戏(2)使用Sprite

Sprite，精灵，顾名思义，专用来代表游戏中的动画角色，比如飞机，坦克等等。在MIDP1.0中，我们必须自己写专门的类来实现Sprite，幸运的是，MIDP2.0为Sprite提供了强力支持，可以创建静态，动态，不透明和透明的Sprite，下面我们预备在上次的GameCanvas基础上添加一个Sprite并让它动起来。

Sprite的主要构造方法有：

Sprite(Image)：构造一个单幅图案的Sprite；

Sprite(Image, int width, int height)：构造一个动画Sprite，图片将按照指定大小被分为N个Frame，通过setFrame(int index)就可以让Sprite动起来。我们用了一个有透明背景的png图片创建坦克的Sprite：

（注重这个图是放大的JPG格式，你需要用Photoshop之类的软件处理成有透明背景的png格式，大小为64x16）

我们在Eclipse中建立如下工程和目录：

以下是画出Sprite的TankGameCanvas.java：

package tank.midp.core;
import javax.microedition.lcdui.*;
import javax.microedition.lcdui.game.*;

public class TankGameCanvas extends GameCanvas implements Runnable {
    // 控制方向：
    private static int INDEX_OF_UP = 0;
    private static int INDEX_OF_DOWN = 1;
    private static int INDEX_OF_LEFT = 3;
    private static int INDEX_OF_RIGHT = 2;

    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

    private Sprite spriteTank; // our sprite!

    // Constructor and initialization
    public TankGameCanvas() {
        super(true);
        width = getWidth();
        height = getHeight();
        currentX = width / 2;
        currentY = height / 2;
        delay = 20;
        // init sprite:
        try {
            Image image = Image.createImage("/res/img/player1.png"); // 注重路径
            spriteTank = new Sprite(image, 16, 16); // 大小是16x16
        }
        catch(Exception e) { e.printStackTrace(); }
    }

    // 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);
            spriteTank.setFrame(INDEX_OF_LEFT);
        }
        // Right
        if ((keyStates & RIGHT_PRESSED) !=0 ) {
            if ( currentX + 5 < width)
                currentX = Math.min(width, currentX + 1);
            spriteTank.setFrame(INDEX_OF_RIGHT);
        }
        // Up
        if ((keyStates & UP_PRESSED) != 0) {
            currentY = Math.max(0, currentY - 1);
            spriteTank.setFrame(INDEX_OF_UP);
        }
        // Down
        if ((keyStates & DOWN_PRESSED) !=0) {
            if ( currentY + 10 < height)
                currentY = Math.min(height, currentY + 1);
            spriteTank.setFrame(INDEX_OF_DOWN);
        }
    }
    // Method to Display Graphics
    private void drawScreen(Graphics g) {
        g.setColor(0); // black
        g.fillRect(0, 0, getWidth(), getHeight());

        // 画一个Sprite非常简单：
        spriteTank.setPosition(currentX, currentY);
        spriteTank.paint(g);

        flushGraphics();
    }
}