MIDP 2.0 遊戲開發系列 (2)：使用 Sprite 實現動畫角色

Sprite 簡介

Sprite 專為遊戲動畫角色設計（飛機、坦克等）。MIDP 1.0 需自行實作，MIDP 2.0 內建強力支援，可建立靜態/動態、透明/不透明 Sprite。

主要建構子

• Sprite(Image)：單圖 Sprite。
• Sprite(Image, int width, int height)：動畫 Sprite，圖片依指定大小切割為多幀，透過 setFrame(int index) 切換動畫。

實作範例：移動坦克

使用 64x16 透明背景 PNG（含 4 個 16x16 幀：上、右、左、下），在 Eclipse 建立專案結構。（原文含圖片）

TankGameCanvas.java 核心代碼

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

public class TankGameCanvas extends GameCanvas implements Runnable {
    private static final int INDEX_OF_UP = 0, INDEX_OF_DOWN = 1, INDEX_OF_RIGHT = 2, INDEX_OF_LEFT = 3;
    private boolean isPlay;
    private long delay = 20;
    private int currentX, currentY, width, height;
    private Sprite spriteTank;

    public TankGameCanvas() {
        super(true);
        width = getWidth(); height = getHeight();
        currentX = width/2; currentY = height/2;
        try {
            Image img = Image.createImage("/res/img/player1.png");
            spriteTank = new Sprite(img, 16, 16);
        } catch (Exception e) { e.printStackTrace(); }
    }

    public void start() { isPlay = true; new Thread(this).start(); }
    public void stop() { isPlay = false; }

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

    private void input() {
        int keyStates = getKeyStates();
        if ((keyStates & LEFT_PRESSED) != 0) {
            currentX = Math.max(0, currentX - 1);
            spriteTank.setFrame(INDEX_OF_LEFT);
        }
        if ((keyStates & RIGHT_PRESSED) != 0) {
            if (currentX + 5 < width) currentX = Math.min(width, currentX + 1);
            spriteTank.setFrame(INDEX_OF_RIGHT);
        }
        if ((keyStates & UP_PRESSED) != 0) {
            currentY = Math.max(0, currentY - 1);
            spriteTank.setFrame(INDEX_OF_UP);
        }
        if ((keyStates & DOWN_PRESSED) != 0) {
            if (currentY + 10 < height) currentY = Math.min(height, currentY + 1);
            spriteTank.setFrame(INDEX_OF_DOWN);
        }
    }

    private void drawScreen(Graphics g) {
        g.setColor(0); g.fillRect(0, 0, getWidth(), getHeight());
        spriteTank.setPosition(currentX, currentY);
        spriteTank.paint(g);
        flushGraphics();
    }
}