J2ME 連續按鍵事件處理：旗標法跨平台解決方案

MIDP 1.0 中 Canvas 提供 keyPressed/released/repeated 三大按鍵事件，但部分裝置不支援 keyRepeated（可用 hasRepeatEvents() 檢測）。本文提出「旗標法」：在 keyPressed 設 boolean pressed = true，keyReleased 設 false，遊戲主迴圈中輪詢旗標持續觸發移動邏輯，完美解決跨裝置相容性問題。

核心邏輯

private boolean leftPressed, rightPressed;
private int px; // 角色 X 座標

public void keyPressed(int keyCode) {
    int action = getGameAction(keyCode);
    switch (action) {
        case LEFT:  left();  leftPressed  = true;  break;
        case RIGHT: right(); rightPressed = true;  break;
    }
    repaint();
}
public void keyReleased(int keyCode) {
    int action = getGameAction(keyCode);
    switch (action) {
        case LEFT:  leftPressed  = false; break;
        case RIGHT: rightPressed = false; break;
    }
    repaint();
}
// 若裝置支援 repeated，可保留作為優化
public void keyRepeated(int keyCode) {
    int action = getGameAction(keyCode);
    if (action == LEFT) left();
    else if (action == RIGHT) right();
    repaint();
}

// 遊戲迴圈 (Runnable)
public void run() {
    while (true) {
        if (leftPressed)  left();
        if (rightPressed) right();
        try { Thread.sleep(50); } catch (InterruptedException ignored) {}
    }
}
private void left()  { if (px > 0) px--; }
private void right() { if (px < getWidth()) px++; }

完整 MIDlet 範例

public class KeyActionMIDlet extends MIDlet {
    protected void startApp() {
        Display.getDisplay(this).setCurrent(new MainCanvas());
    }
    // pauseApp/destroyApp 留空
}
class MainCanvas extends Canvas implements Runnable {
    private String btnText = "";
    private boolean leftPressed, rightPressed;
    private int px, py;
    public MainCanvas() {
        px = getWidth()/2; py = getHeight()/2;
        new Thread(this).start();
    }
    // keyPressed/Released/Repeated 如上
    public void paint(Graphics g) {
        g.setColor(0xFFFFFF); g.fillRect(0,0,getWidth(),getHeight());
        g.setColor(0x000000);
        g.drawString(btnText, 20, 20, Graphics.LEFT|Graphics.TOP);
        g.drawString("J2ME", px, py, Graphics.HCENTER|Graphics.TOP);
    }
}

優缺點分析

進階擴充

• 加速/減速：引入 velocityX，keyPressed 疊加加速度，run() 整合位移並施加摩擦係數。
• 斜向移動：同時檢測 UP/DOWN 旗標，向量合成。
• 按鍵去抖：在 keyPressed 記錄時間戳，忽略 50ms 內重複事件。
• 虛擬鍵盤支援：觸控裝置對應 pointerPressed/Dragged/Released 同步設定旗標。