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();
}
}
