在android

public static class MyView extends View {
    private Bitmap  mBitmap, bg_bmp;
    private Canvas  mCanvas;
    private Path    mPath;
    private Paint   mBitmapPaint;

    public MyView(Context c, AttributeSet attrs) {
        super(c, attrs);
        Bitmap bmp_canvas = BitmapFactory.decodeResource(getResources(), R.drawable.bg_fun_game_draw_canvas);
        mBitmap = Bitmap.createBitmap(BitmapFactory.decodeResource(getResources(), R.drawable.bg_fun_game_draw_canvas), 0, 0, bmp_canvas.getWidth(), bmp_canvas.getHeight());
        mCanvas = new Canvas(mBitmap);
        mPath = new Path();
        mBitmapPaint = new Paint(Paint.DITHER_FLAG);
    }

    @Override
    protected void onSizeChanged(int w, int h, int oldw, int oldh) {
        super.onSizeChanged(w, h, oldw, oldh); 
    }

    @Override
    protected void onDraw(Canvas canvas) {
        canvas.drawColor(0xFFAAAAAA);

        canvas.drawBitmap(mBitmap, 0, 0, mBitmapPaint);

        canvas.drawPath(mPath, mPaint);
    }

    private float mX, mY;
    private static final float TOUCH_TOLERANCE = 4;

    private void touch_start(float x, float y) {
        mPath.reset();
        mPath.moveTo(x, y);
        mX = x;
        mY = y;
    }

    private void touch_move(float x, float y) {
        float dx = Math.abs(x - mX);
        float dy = Math.abs(y - mY);
        if (dx >= TOUCH_TOLERANCE || dy >= TOUCH_TOLERANCE) {
            mPath.quadTo(mX, mY, (x + mX)/2, (y + mY)/2);
            mX = x;
            mY = y;
        }
    }

    private void touch_up() {
        mPath.lineTo(mX, mY);
        // commit the path to our offscreen
        mCanvas.drawPath(mPath, mPaint);
        // kill this so we don't double draw
        mPath.reset();
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        float x = event.getX();
        float y = event.getY();

        switch (event.getAction()) {
            case MotionEvent.ACTION_DOWN:
                touch_start(x, y);
                invalidate();
                break;
            case MotionEvent.ACTION_MOVE:
                touch_move(x, y);
                invalidate();
                break;
            case MotionEvent.ACTION_UP:
                touch_up();
                invalidate();
                break;
        }
        return true;
    }
}

<LinearLayout
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:background="@drawable/bg_fun_game_draw_bottom"
android:orientation="vertical" >
    <view xmlns:android="http://schemas.android.com/apk/res/android"
    class="mypackage.android.DrawGame$MyView" 
    android:id="@+id/linedraw"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:scrollbars="vertical"
    android:fadingEdge="vertical"
    android:layout_gravity="center_horizontal" />
</LinearLayout>