J2ME 低階 UI API 深度解析：Canvas 繪圖、事件與命令完整實戰

MIDP 定義兩套 UI API：高階 API（Form/List 等，面向語意、自動排版、跨裝置可移植）與 低階 API（Canvas，掌握完整繪圖與事件控制權，適合遊戲開發）。兩者可在同一應用中混用（如撲克牌切換），但同一畫面只能二擇一，唯 Command 物件可共用。

低階 API 核心：繼承 Canvas 實作 paint()

import javax.microedition.lcdui.*;
import javax.microedition.midlet.*;

public class MyCanvas extends Canvas {
    private MIDlet midlet;

    public MyCanvas(MIDlet midlet) {
        this.midlet = midlet;
    }

    protected void paint(Graphics g) {
        // 清背景
        g.setColor(0xFFFFFF);
        g.fillRect(0, 0, getWidth(), getHeight());
        // 繪製文字
        g.setColor(0x000000);
        g.drawString("Hello there!", getWidth()/2, 0, Graphics.TOP | Graphics.HCENTER);
    }
}

啟動 Canvas

public class MyMIDlet extends MIDlet {
    private Display display;
    private MyCanvas canvas;

    public MyMIDlet() {
        display = Display.getDisplay(this);
        canvas = new MyCanvas(this);
    }

    protected void startApp() {
        display.setCurrent(canvas);
    }
    // pauseApp, destroyApp 略...
}

鍵盤事件處理：覆寫 keyPressed/keyReleased/keyRepeated

protected void keyPressed(int keyCode) {
    // 直接結束應用範例
    ((MyMIDlet) midlet).exit();
}

按鍵代碼映射：正值為 Unicode 字元，負值為功能鍵。為跨裝置相容，Canvas 定義遊戲動作常數：UP, DOWN, LEFT, RIGHT, FIRE, GAME_A~D。可用 getGameAction(keyCode) 轉換，getKeyCode(action) 反查。

建立遊戲基礎類封裝按鍵映射

public abstract class GameCanvas extends Canvas {
    protected MIDlet midlet;
    protected int fireKey, leftKey, rightKey, upKey, downKey;

    public GameCanvas(MIDlet midlet) {
        this.midlet = midlet;
        fireKey = getKeyCode(FIRE);
        leftKey = getKeyCode(LEFT);
        rightKey = getKeyCode(RIGHT);
        upKey = getKeyCode(UP);
        downKey = getKeyCode(DOWN);
    }
}

// 具體遊戲畫布
public class MyCanvas extends GameCanvas {
    private String message = "Press any key";

    public MyCanvas(MIDlet midlet) { super(midlet); }

    protected void paint(Graphics g) {
        g.setColor(0xFFFFFF); g.fillRect(0, 0, getWidth(), getHeight());
        g.setColor(0x000000);
        g.drawString(message, getWidth()/2, 0, Graphics.TOP | Graphics.HCENTER);
    }

    protected void keyPressed(int keyCode) {
        if (keyCode == fireKey) message = "FIRE";
        else if (keyCode == leftKey) message = "LEFT";
        else if (keyCode == rightKey) message = "RIGHT";
        else if (keyCode == upKey) message = "UP";
        else if (keyCode == downKey) message = "DOWN";
        else message = getKeyName(keyCode);
        repaint();
    }
}

指標事件（可選支援）

// 檢測是否支援指標
if (hasPointerEvents()) {
    // 可用 pointerPressed/Dragged/Released
}

protected void pointerPressed(int x, int y) {
    // 處理觸控/滑鼠點擊
}

命令事件：提供標準化退出/操作入口

// MIDlet 實作 CommandListener
public class MyMIDlet extends MIDlet implements CommandListener {
    private Display display;
    private MyCanvas canvas;
    private Command exitCmd = new Command("離開", Command.EXIT, 1);

    public MyMIDlet() {
        display = Display.getDisplay(this);
        canvas = new MyCanvas(this);
        canvas.addCommand(exitCmd);
        canvas.setCommandListener(this);
    }

    protected void startApp() { display.setCurrent(canvas); }

    public void commandAction(Command c, Displayable d) {
        if (c == exitCmd) { destroyApp(true); notifyDestroyed(); }
    }
    // 其他方法略...
}

• Command 含 label、type（BACK/CANCEL/EXIT/HELP/ITEM/OK/SCREEN/STOP）、priority（1 最高）
• 裝置依 type 映射實體鍵，衝突時依 priority 排序
• 同一 Command 可在多個 Canvas 間共用

繪圖方法速查

Graphics 提供：drawArc、drawLine、drawRect、drawString、fillRect、setColor、translate 等，座標原點可平移。

掌握 Canvas 繪圖、事件、命令三大核心，即可構建高效能的 J2ME 遊戲與低階應用。