J2ME GameCanvas 實戰：打造流動星空特效完整教程

前言

MIDP 2.0 引入 javax.microedition.lcdui.game.GameCanvas，內建雙緩衝、按鍵狀態查詢、遊戲迴路支援，是手機遊戲開發利器。本文通過實現「流動星空」特效，詳細講解 GameCanvas 核心用法。

GameCanvas 關鍵特性

• 繼承 Canvas，保留 showNotify()/hideNotify() 生命週期，適合初始化/釋放資源。
• 內建雙緩衝：getGraphics() 取得離屏緩衝，繪製完成後 flushGraphics() 一次性刷新到屏幕，無閃爍。
• getKeyStates() 即時查詢按鍵狀態，適合遊戲輪詢模式。

遊戲迴路標準範式

protected void showNotify() {
    random = new Random();
    thread = new Thread(this);
    thread.start();
}

protected void hideNotify() {
    thread = null; // 結束迴路
}

public void run() {
    Graphics g = getGraphics(); // 離屏緩衝
    while (thread == Thread.currentThread()) {
        int keyState = getKeyStates();
        // 1. 處理輸入
        // 2. 更新邏輯
        // 3. 渲染幀
        g.setColor(0xFFFFFF);
        g.fillRect(0, 0, getWidth(), getHeight());
        // 繪製精靈等
        flushGraphics(); // 刷新到屏幕
        // 4. 幀率控制
        try { Thread.sleep(sleepTime); } catch (InterruptedException e) {}
    }
}

流動星空實現邏輯

1. 啟動執行緒，每幀呼叫 graphics.copyArea(0, 0, w, h, 0, 1, Graphics.TOP | Graphics.LEFT) 將畫面向下平移 1 像素。
2. 最上方騰出一行，填黑背景 g.drawLine(0, 0, w, 0)。
3. 在新行隨機繪製白色像素點（星星）：if (Math.abs(random.nextInt()) % 100 < 5) g.drawLine(i, 0, i, 0);。
4. 上下鍵動態調整 sleepTime 控制流速快慢。

完整源碼

// 主 MIDlet
public class GameCanvasTest extends MIDlet implements CommandListener {
    private Display display;
    private static final Command EXIT_CMD = new Command("Exit", Command.EXIT, 1);

    protected void startApp() {
        if (display == null) {
            display = Display.getDisplay(this);
            GameCanvas canvas = new StarField();
            canvas.addCommand(EXIT_CMD);
            canvas.setCommandListener(this);
            display.setCurrent(canvas);
        }
    }
    // ... pauseApp/destroyApp/commandAction 標準實現
}

// 星空畫布
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); // 啟用雙緩衝
        graphics = getGraphics();
        graphics.setColor(0, 0, 0);
        graphics.fillRect(0, 0, getWidth(), getHeight());
    }

    public void run() {
        int w = getWidth();
        int h = getHeight() - 1;
        while (thread == Thread.currentThread()) {
            int state = getKeyStates();
            if ((state & DOWN_PRESSED) != 0) {
                sleepTime = Math.min(sleepTime + SLEEP_INCREMENT, SLEEP_MAX);
            } else if ((state & UP_PRESSED) != 0) {
                sleepTime = Math.max(sleepTime - SLEEP_INCREMENT, 0);
            }

            // 滾動星空
            graphics.copyArea(0, 0, w, h, 0, 1, Graphics.TOP | Graphics.LEFT);
            graphics.setColor(0, 0, 0);
            graphics.drawLine(0, 0, w, 0); // 清空新行
            graphics.setColor(255, 255, 255);
            for (int i = 0; i < w; i++) {
                if (Math.abs(random.nextInt()) % 100 < 5) {
                    graphics.drawLine(i, 0, i, 0); // 畫星星
                }
            }
            flushGraphics();

            try { Thread.sleep(sleepTime); } catch (InterruptedException e) {}
        }
    }

    protected void showNotify() {
        random = new Random();
        thread = new Thread(this);
        thread.start();
    }
    protected void hideNotify() { thread = null; }
}

核心 API 速查

總結

GameCanvas 以雙緩衝、按鍵輪詢、生命週期掛鉤三大特性，極大簡化了 J2ME 遊戲開發。星空特效展示了 copyArea 實現無限滾動背景的經典技巧，可延伸至跑酷、飛行射擊等多類遊戲場景。